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电阻、电容、PCB:无源元件并非真的“无源”

2018-5-31 15:19 3441 1 分类: PCB 文集: 基础
无源元件并非真的“无源”:第1部分——电容

摘要:晶体管、集成电路等有源器件利用来自电源的能量对信号进行转换,而电阻、电容、电感以及连接器等无源元件则不消耗电能——或许是我们的假设。由于无源元件均具有寄生参数,它们实际上会以不可预知的方式改变信号。本文分为3部分,这里为第1部分,讨论寄生电容的影响。


引言

有源元件和无源元件——在工程设计领域真的是非白即黑吗?

晶体管和集成电路由于利用来自电源的能量改变信号,所以被认为是有源元件。基于这个依据,我们将电容、电阻、电感、连接器,甚至是印刷电路板(PCB)称为无源元件,因为它们看起来不耗电。然而,由于无源元件均具有寄生参数,它们实际上也会以不可预知的方式改变信号。所以,许多所谓的无源元件并非真的“无源”。本文分为3部分,这里为第1部分,专注于讨论电容的有源特性。

并非完全无源的电容


无源可定义为惰性和/或不活跃,但无源电子元件会以不可预知的方式成为有源电路的一部分。所以,纯容性电容实际上是不存在的。所有电容在本质上都存在一定的寄生成分(图1)。
Figure 1. The capacitor (C) and its largest parasitic components.
图1. 电容(C)及其最大的寄生元件。

我们进一步观察图1所示寄生元件。标有“C”的电容是我们的考察对象,其它所有元件则是不希望存在的寄生元件1。并联电阻RL引起泄漏,从而改变有源电路的偏置电压、滤波器的Q因子,并影响采样-保持电路的保持能力2。等效串联电阻(ESR)则会降低电容抑制纹波和通过高频信号的能力,因为等效串联电感(ESL)形成谐振电路(即自谐电路)。这意味着,在自谐频率以上时,电容呈现为电感,不再具备电源与地之间的高频噪声去耦作用。电容介质可能是压电介质,增加振动产生的噪声(AC),就好像电容C内部嵌入了电池(未绘出)。冷焊应力造成的压电效应可以改变电容值。压电电解电容也具有等效的串联寄生二极管(未绘出),这些二极管会对高频信号进行整流,改变偏置或增大信号失真。

较小的电池SB1至SB4表示塞贝克(Seebeck)结3,是由不同金属(寄生热电偶)在此形成的电压源。当我们连接测试设备时,需要考虑共用连接器的塞贝克效应。Jim Williams在参考文献4中指出,BNC和橡胶插头连接器对的热电势范围为0.07µV/°C至1.7µV/°C (附录J,图J5)。这一变化只适合我们日常在实验室内部的简单连接。将看起来较小的失调增益乘以1000,就达到1.7mV——这是我们尚未实际开始操作就存在的。

SB2和SB3可能是电容内部连接引线的箔,或连接至焊盘或表贴元件焊料的金属化物。SB1和SB4表示器件通过焊料到PCB铜线的结。以往的焊料是63%的铅和37%的锡,但现在使用的符合RoHS标准的无铅焊料成分变化很大,会影响电容附近的电压,所以必须查询合金成分。

可对介质吸收(DA)或Bob Pease所称的“渗透”进行建模,等效为无数个RC时间常数:DA1至DAINFINITY,其中每个时间常数由电阻RDA和电容CDA组成。Bob Pease列举了一些“渗透”非常重要的实例,本文附录中介绍了一段关于吸收的有趣经历。

“如果您关闭彩色电视机,然后打开后盖,那么在您开始操作之前首先必须要做的是什么?在螺丝刀上连接一条地线,然后接触高压插头上的橡胶垫圈下方,对CRT放电。那好,现在电容已经放电了,如果让这一过程持续大约10分钟,那么有多少电压将“渗透”回显像管的“电容”?当您第二次放电时,足以造成可见的电弧....这就是我所说的介质吸收5。”

由此可见,电容会随着作用电压的改变而改变。然后再加上老化、温度的影响,以及其它可能造成电容器物理损坏的众多因素6,这种简单的无源元件就变得非常复杂。

现在,我们应该讨论一下与自激有关的因素,这是去耦电容以及接地不良的电容最常见的问题。如果接地不良,任何电容都不能正常工作。电容自激主要受图1所示ESL的影响,当然,PCB过孔也会产生一定的影响。工作在射频频段时,这些过孔将影响小电容的自激点。以图2为例,讨论了1µF电容的曲线。
Figure 2. Self-resonance (lowest point on the graph) of three capacitors. Graph shows that capacitors do not all perform identically. On the left side as the traces (impedances) are moving downward, the capacitors act as capacitors. However, when they reach their lowest point and start upward, they become inductors (ESL) and are no longer effective as decoupling capacitors.
图2. 三个电容的自激频率(曲线的最低点),图示表明,电容的性能并不完全一致。在左侧,当曲线(阻抗)向下移动时,电容表现为电容。当达到其最低点时,电容呈现为电感(ESL),不再是有效的去耦电容。

1µF曲线在4.6MHz时达到最小,高于该频率时,ESL占支配地位,电容的工作特性表现为电感。由此,去耦电容在高频下称为一个双向导体:对于电源总线上的高频信号而言,电源线与地短接,反之亦然。电容模糊了电源和地之间的差异。

随着对信号频率和电容的深入考察,我们可能忘记了所产生的谐波或边带。例如,一个50MHz方波的SPI时钟,具有无限次的奇次谐波。大多数系统(并非所有系统)会忽略5次以上的谐波,因为这些谐波的能量已经非常低,在噪底以下。如果谐波在半导体器件中经过整流,仍可造成负面的影响,因为它们会转换成新的低频干扰。

控制生产误差

从图2可以看出,电容在生产过程中存在不一致的问题。一般而言,高质量电容的重复性非常好,而一些廉价电容则会受成本控制而存在较大的生产误差。有些厂商按照严格的误差等级或标准筛选电容(图3),并收取高额费用。这对用于设置系统时间或频率的电容并不适合。
Figure 3. Binning, or sorting, of manufacturing tolerances affects capacitor performance in different ways.
图3. 生产误差等级或筛选,会以不同方式影响电容性能。

图3中的实线(黑色)为一个好的生产过程的标准方差,尽管该图在Maxim Integrated应用笔记4301“零晶体管IC,IC设计领域的又一里程碑”中用于表示电阻特性,但也同样适用于电容。当生产误差变化时,每个“盒子”内的器件数量也随之变化。误差曲线可向右移动(绿色虚线),结果是没有符合1%容限的元件;统计概率也可以是双峰曲线(灰色虚线),得到较多的符合5%和10%容限的元件,而符合1%和2%容限的元件数量很少。

从分布特性看,“似乎”能够保证2%容限的元件只有-1到-2,或+1到+2 (没有满足1%容限的器件);“好像”从5%容限的“盒子”里移除了1%和2%容限的器件。我们之所以用“看起来”和“好像”是因为销售量、人为因素也会影响分配比例。例如,工厂经理可能急需发货5%容限的电容,但又没有足够的产品满足本月的需求。而库房又存放了过多的2%容限元件。于是,他将这些元件划分到5%容限的“盒子”里,然后发货。很容易解决了上述问题,人为干预(也确实这么做了)会“歪曲”统计数据和方法。

这样做对于无源电容意味着什么?我们必须了解所预期容限,比如±5%,其统计分布可能在±2%中心位置有一个缺口。电容用于控制关键频率或定时,我们需要预先考虑到这点。这也意味着我们需要规划,通过校准来修正较宽变化范围。

焊接对无源器件性能的影响

焊接会对电容造成应力,尤其是表贴元件。应力将随着振动产生压电电压,甚至损害电容,存在系统故障隐患。

大家对回流焊流程并不陌生,液体焊料的表面张力使元件整齐排列滚动,好像被磁铁吸住一样。如果焊料的温度特性较差,则有可能损坏器件。您可能在现场看到过,电容像墓碑一样单脚直立?如果焊料温度变化出现问题,既有可能引发这种情况。请务必遵守制造商的焊接建议。有些元件对温度更为敏感,所以可能需要用两种或多种不同温度的焊料进行焊接。首先用高熔点焊料对电路中的大多数元件进行焊接,然后再用低温焊接“敏感”元件。必须以正确的顺序使用焊料,避免前期焊接的器件不会随后“溶化”掉。

总结

当我们讨论电容等无源元件时,必须注意这些元件均具有寄生效应,从改变了信号。当然,这种影响取决于信号强度。当测量微伏级信号时,需要谨慎考虑以下因素:接地(星形连接点)、屏蔽去耦电容、保护线、布局、塞贝克效应、电缆结构,以及连接器。我们的原理图上往往忽略了这些因素,但当我们排查微弱的噪声干扰或信号时,将不得不考虑这些因素。
注意,无源电容不仅仅是一个无源元件,要比表面看起来“活跃”得多,寄生成分、误差、校准、温度、老化,甚至组装方法和操作规范都会对电路产生微妙的影响,从而影响器件性能。了解到这一点,我们还需要理解电容器的累积误差。在本文的后续部分,我们还将讨论其它类型的无源元件:电阻、电位器、开关,甚至是不引人注意的PCB。

参考文献
  1. For information on distortions caused by capacitors, see “Capacitor Distortion Mechanisms,” TWEM (The Electric Web Matrix of Digital Technology), www.co-bw.com/Audio_Capacitor_Distrotion_Mechanisms.htm. Note: the author realizes that the word “distrotion” in this URL is misspelled, but the URL is correct as shown.
  2. Bob Pease, “What's All This Capacitor Leakage Stuff, Anyhow?,” Electronic Design, March 29, 2007, http://electronicdesign.com/analog/whats-all-capacitor-leakage-stuff-anyhow.
  3. Jim Williams used an “x” to indicate a Seebeck junction. He would count the junction in parallel paths and purposely cut the PC trace and solder them back together to make equal numbers of junctions. See Jim Williams et al, application note 86, “A Standards Lab Grade 20-Bit DAC with 0.1ppm/°C Drift,” http://cds.linear.com/docs/en/application-note/an86f.pdf. See also Bob Pease, “Understand capacitor soakage to optimize analog systems,” www.datasheetarchive.com/files/national/htm/nsc03883.htm. For more general information on the Seebeck effect, you can start at http://en.wikipedia.org/wiki/Thermoelectric_effect.
  4. Williams et al, “A Standards Lab Grade 20-Bit DAC.”
  5. Bob Pease, “What's all this soakage stuff, anyhow?,” Electronic Design, May 13, 1998, http://electronicdesign.com/analog/whats-all-soakage-stuff-anyhow.
  6. John Maxwell, “TECHNICAL INFORMATION, CRACKS: THE HIDDEN DEFECT,” AVX Corporation, www.avx.com/docs/techinfo/cracks.pdf.
  7. Spice tools for Kemet® can be found near the bottom of the page at www.maximintegrated.com/cal.
  8. Keith Snook, “WHAT’S ALL THIS TRAPPED CHARGE AND DIELECTRIC COMPRESSION STUFF ANYHOW?,” www.keith-snook.info/capacitor-soakage.html.
附录
无源电容的介质吸收、渗透和电压放电

我对第一次看到介质渗透的经历印象深刻,与我第一次测量功率变压器场景大不相同。

在我十几岁时,当地一位“火腿族”(20世纪中期的一个称呼,指业余无线电爱好者——糟糕,我可能暴露了我的年龄。)在他的车库中维修电视机。我从他那里学到了很多东西,有些是手把手教的。在他的工作台上有一个断开的功率变压器,引线裸露在外。我说我能够用欧姆表测量出电阻,于是,我非常幼稚地抓住两个探头,然后将每个探头按到裸露的引线上。嗖!即使欧姆表仅由3V电源供电,电感产生的反冲也足以使我牢记这次教训。

他同情地看着我(希望我牢记教训,并非要我死)。于是,他像Bob Pease所说的那样,把CRT接地,然后向我展示电荷仍会停留几分钟。我照样子做了,急于弄清电荷到底能够停留多久——结果发现电荷似乎无休止地保持着(直到我觉得无聊,停止了试验)。Keith Snook 8对DA理论进行了深入讨论,这是值得关注的一个好课题。

答案就在我们学过的知识中:我们不可能对电容完全充电,除非我们等待无限长时间。实际应用中,对于大多数电路,我们认为达到时间常数的5倍之后,即充电完毕,此时电压达到所加总电压的99.3%。电容放电的过程亦如此。就CRT而言,从高压开始,在较长的时间内都能产生令人痛苦的电击。

无源元件并非真的“无源”:第2部分——电阻

电阻可不是简单的角色

Passive components don't draw power but even resistors can, and do, modify signals in unexpected ways. A resistor's reaction to temperature, voltage, and signal frequency can often catch the inexperienced engineer by surprise. Tolerances may not be as they seem and simple resistors may provide nonlinear signal response, introducing harmonics where there were none.

Capacitors, resistors, inductors, connectors, and even the PCB are called passive because they don’t have gain or control power like semiconductors or other active devices. But these apparently passive components can, and do, change the signal in unexpected ways because they all contain parasitic portions. In Part 1 of this series on “passives”we talked about capacitors. Now in Part 2 we look at resistors. Indeed, resistors are simple, benign, passive devices—right? Wrong. As we will see, resistors really do some unexpected things. In Part 3 we discuss how PCB flaws and errors that are usually hidden, or at least disguised, can introduce passive errors into IC performance.

The Simple Resistor, Really Ain’t that Simple

How many times have we walked down a street and seen concrete that is lumpy, bumpy, and horrible. It reveals someone’s inexperience and overconfidence because pouring concrete looked so simple. Resistors have the same basic issue: they seem simple until one looks closely. There is a superb book on resistors1 and the authors Dr. Zandman et al. have the same lament, “This work [book] attempts to demonstrate that the design and fabrication of resistive components require the application of particularly complex physical phenomena and are no longer based on the traditional empirical methods generally associated with the ‘kitchen recipe’ approach.”2

Ahhh, the kitchen. While cooking, so many of our mothers use to say, “a little of this and a pinch of that.” Fine for cookies, but this kitchen-recipe approach to manufacturing resistors is a serious issue. There are vendors that prioritize price over quality. Other vendors accept a large variation in tolerances as if they were formulating batches using a kitchen recipe. A little difference in food can add variety and interest, but the kitchen recipe has no place in manufacturing of close tolerance parts.

The late Dr. Zandman, inventor of a zero-tempco resistor and founder of Vishay Intertechnology, certainly underscores his work with mathematics and material science. His book delineates the formulas and reasons behind the many variations in resistors. He devotes three wonderful chapters to Ohm’s law, first to Ohm’s law itself and its limitations, then to reversible and irreversible phenomena associated with the law. Reversible conditions include a rise in temperature that changes the resistance, but the resistance returns to the starting point when the temperature is reduced. An irreversible effect means that the resistor’s change becomes permanent like those caused by diffusion or oxidation.

Set Tolerances to Match the Application

Let’s admit an important fact at the outset: resistors introduce error. Our initial reaction may be to ignore resistor inaccuracy as “too small to matter.” After all, a pinch of salt in an Olympic-sized swimming pool does not make it salt water. True, but adding a ton of salt would be a different issue. Obviously, an application dictates the acceptable error. The more precision required, the tighter the component tolerances must be. Knowing this, we should define what magnitude of error is acceptable.

We will examine an example system with 12-bit resolution, one-half the least significant bit (LSB) is one part in 8192, or 0.012%, or 122 parts per million (ppm).3 A quick look at Digi-Key® and Mouser® catalogs finds thousands of 1% tolerance resistors with typically a ±100ppm/°C temperature coefficient (tempco). Consequently, barely more than a 1°C temperature change causes more than one LSB deviation. Not so good. So let’s try 0.1% tolerance resistors with a ±25ppm/°C, which means if the temperature changes 5°C, our error is one LSB. Remember that this is just one resistor and most systems have many resistors.

We can draw some important conclusions from this example. To bracket the resolution numbers:

A. For 8-bit (1 part in 256) resolution, one-half LSB is 0.195% or 1953ppm; and
B. At 16-bit resolution (1 part in 65536), one-half LSB is 0.0015% or 15ppm.

Clearly the need for smaller tolerances and tempco is more important at higher resolutions.

That is actually true for many systems, but there are two extreme cases to note. First, a completely open-loop use, such as an arbitrary waveform generator, needs the DAC output and amplifier to have nearly perfect linearity. Second, a system with a feedback loop, such as a mechanical movement generated in a process controller, has servo action that is always driving the action toward the center to null out any error. As long as the servo is directed in the proper direction (the system is, by definition, monotonic), small nonlinearity errors will be ignored.

Resistor Parasitic Components

Figure 1 illustrates the parasitic components that are present with resistors. Inside the dotted Resistor box is the resistor. The inductors and capacitors on either side are the PC board (PCB) connections and traces. The R is what we want; the additional factors inside that box are unavoidable parasitics. To illustrate the effects of these parasitics, we drive the left side of the resistor with a low-impedance signal generator. That will swap out the left capacitor to ground, CG.

We may only want a resistor, the R above, but we also have all the other unavoidable parasitic components. We can minimize some parasitics, but they are always present.

Figure 1. We may only want a resistor, the R above, but we also have all the other unavoidable parasitic components. We can minimize some parasitics, but they are always present.

To the right of the resistor we see the composite of all the network components. A frequency sweep of a sine wave shows the dominant RC high-frequency rolloff caused by resistor R and the right-side capacitor, CG. The series inductors cause additional, but minor, high-frequency attenuation. Capacitor C and inductor L inside the resistor cause a minor frequency peaking. Yes, each of the parasitic components is small. Still, we need to consider them when designing circuits so we can decide whether or not to ignore them. For example, at audio frequencies we can choose to disregard the parasitics, but at radio frequencies we may have to adjust for them.

The piezoelectric element, P, is interesting as it influences performance during stress and vibration. (It also could represent a magnetostriction response to a magnetic field.) Stress can change the resistance depending on the resistor chemistry, and vibration can be converted into small AC voltages, which then add to the electrical noise. The solder stress is probably dominant and is important especially with surface-mount parts. Older designs with through-hole resistors allowed the leads to twist to absorb and mitigate most of the stress. Surface-mount parts, however, are held against a relatively rigid PCB. As the solder solidifies, these parts capture the change in thermal expansion between the resistor and PCB. To minimize the stress we must carefully follow the manufacturer’s recommendations for solder time/temperature profile.

Let’s now talk briefly about wire-wound resistors, often chosen because they have very low temperature coefficients (tempcos). These resistors also have an important, unique characteristic: its structure can react to magnetic fields. Because they are essentially a coil of wire, they magnify a magnetic field that a single conductor might pick up. As a coil, they also have more inductance than other types of resistors. We have seen circuits with transformers, inductors, and wire-wound resistors cross talk over small magnet fields. To mitigate these effects, careful layout, rotating components 90 degrees, increasing the spacing, and shielding may be necessary.

Finally, do not forget our friend Seebeck. Any dissimilar metal connections such as at the solder-to-board interface can cause small temperature-dependant offset voltages.

Manufacturing Tolerance, Power Rating, and Temperature Coefficient

Two other parameters, manufacturing tolerance and power rating (wattage), also impact resistor operation. In our last article4 on capacitors we explained how sorting and binning can distort manufacturing tolerances. This can also happen with some kinds of resistors. As a general rule, binning can cause performance problems for both manufacturers and customers if there is a process shift and then there is a large demand for the most precise item. A manufacturer can always ship more precise parts in place of low-precision parts, but the reverse is not true. For example, a 5% tolerance resistor could actually contain resistors with a tolerance between -5% to -2% and +2% to +5%. This is clearly not the full range between -5% through +5% that one might expect. If not enough high-precision devices are binned or if the customers only want the high-precision parts, then the manufacturer faces parts shortages.

Power rating is simple, right? Voltage times the current (V × I) tells you what wattage rating to choose so the resistor does not burn up from self-heating. Right? No, wrong (or, maybe)? The answer, of course, depends on the application. A series resistor to limit current in a light-emitting diode (LED) can be a plain “vanilla” circuit where little additional concern is necessary. If the resistor has a negative tempco the resistance is reduced as the temperature increases. This, in turn, causes the resistor to draw more current at higher temperature, and that can contribute to overheating. At the other extreme, bias and modulation currents are critical in radio and laser communications systems.

Many systems including radio and laser communications systems need to remain stable over the operational temperature extremes. Centering the feedback loop over temperature and voltage changes requires deeper study. How much power is dissipated and how each of the components reacts, including the resistor, are important. In such circuits the laser must be cooled to keep it on frequency; the heat of the surrounding components (self-heating) must also be removed. What do you do? There are questions to ask and answer before your design goes any further.

  • Is there air flow across the circuit?
  • What is the temperature of the air at the critical circuit (not just the air entering the enclosure)?
  • Was the air heated by another circuit?
  • Many times the air is first routed to the circuit boards and then exits through the power supply. Now does the power-supply voltage change with temperature?
  • Are there other systems racked together in the same enclosure?
  • Are there fans? How does dust and dirt collect, and what happens if one or more fans fail?

Most resistors have a negative tempco, meaning that the resistance is reduced at higher temperatures. This also means that the resistor draws more power when heated. Each of us needs to carefully read a resistor’s data sheet because the different chemistries and manufacturers may have different ways of specifying the tempco. The tempco curves can be just about any shape and they may be specified by the “box method,” which is common for integrated circuits (ICs).5 Even a factory-trimmed part with thousands of transistors will show a family of curves over temperature and process variation. Simulation and correlation allow us to define a box that contains all the possible curves. The box “x axis” is the total operating temperature and the “y axis” the total magnitude of the error. Statistically we guarantee that the error of all the parts is within the box, but we do not know the shape of the curve for any individual part.6 Specialized resistors called thermistors can have negative (NTC) or positive (PTC) tempcos, and the curves tend to be very nonlinear.

Basic Chemistry and Voltage Coefficient of Resistance (VCR)

What is inside IC resistors, i.e., the chemistry, is very important for understanding resistance. Designers and process engineers need to understand how chemistry in the manufacturing process affects resistor performance.7

In chemistry there are two broad classifications for things made of one or more chemicals. A compound is two or more chemicals that react to make something new. A mixture is multiple mixed chemicals that retain their original properties. Remember that the resistors with color bands on a brown shell are carbon composition resistors, CC. The CC resistors are mixtures and some of the contact points inside form semiconductors. They change resistance with heating, cooling, vibration, and applied voltage.

The high-voltage vacuum tubes (“valves” in the U.K.) remembered from our past history (and that still “resonate” for some audiophiles today) created “resistor distortion”8 that some people actually find pleasant. The distortion is caused by a voltage coefficient of resistance (VCR), a reduction in resistance value with an increase in voltage. In an audio system with a sine-wave signal of 75V peak-to-peak (VP-P), biased at 50V, the resistor that sets gain will be at a higher resistance (gain) on the lower half of the sine wave and have a lower resistance and gain on the positive peak. This adds second-harmonic errors to the signal. This “resistor distortion” is soft and smooth in the onset of distortion, which, as mentioned above, some find pleasant. For most resistors the voltage-coefficient error only becomes measurable over 25V. Today most circuits are lower voltage so the resistor distortion tends to be ignored.

VCR is an important characteristic of high-voltage thick-film resistors.9 Typical thick-film ink consists of conductive material suspended in an insulating matrix. As the voltage across the ink is increased, new conducting paths are opened. The result is a drop in resistance. This means that the VCR is always negative in value. Thick-film resistors can be used as series-protection resistors in electrocardiogram (ECG) input circuits. These resistors help protect the ECG input from the 3kV to 5kV from defibrillator pulses.10 Obviously, we want the resistors to maintain their value and survive multiple voltage pulses. There are resistors with a VCR of < -1ppm to 5ppm over a large working voltage. With ECG, the humidity and the temperature performance of the resistor are critical. The resistor also must dissipate the heat energy of the defibrillator pulses.

Thus, while the IC designer does not define the internal chemistry of a circuitry, it is important to understand how chemistry affects the tolerance and tempco of the part. Once again, this speaks to the importance of studying the data sheet.

Thermal, White, or Johnson Noise

Thermal noise, also called Johnson noise, is present in all passive resistive elements and is caused by the random thermal motion of electrons. The thermal noise level is unaffected by DC current.

Resistors always generate noise, even when floating outside a closed circuit. This is white noise, which has a uniform spectral density and increases with temperature and resistance. Because some resistors are made of semiconductors, they can have other types of noise, such as shot, avalanche, flicker (1/f), and popcorn noise11 There is a free Thermal Noise Calculator and a User’s Guide that further explain the different noise types.12

Conclusion

Good engineering is about the details and we are fortunate to be standing on the shoulders of engineering “giants.” Pioneering engineers like Dr. Zandman have struggled while researching physics and materials science, executing the careful work that produced the understanding that we rely on every day. As he observed, the seemingly small insignificant factors in an IC are many times taken for granted. This is certainly true for resistors that seem to be benign and passive, until their performance in a circuit startles us awake. That little resistor, in fact, dominates the circuit’s error budget. Tempco and manufacturing tolerance are just the start. That passive resistor can change value with voltage and actually lowpass filter a signal. The effect is unexpected and surprising until we look closely and realize that there is more to a resistor. Ultimately, the resistor that we tried to ignore is just following the laws of physics and we need to pay it special attention.

References

  1. Zandman, Felix, et al., Resistor Theory and Technology, Vishay Intertechnology, Inc., ©2001.
  2. Ibid., Introduction, p. 3.
  3. Maxim Integrated tutorial 5060, “ADC/DAC Accuracy Calculator Tutorial,” Free “Accuracy Calculator” (ACCU).
  4. Laumeister, Bill, “Passive Components Aren’t Really So Passive (Part 1): Capacitors,” Electronic Design, online June 4, 2013, also as Maxim application note 5663.
  5. Maxim Integrated application note 4419, “Understanding Voltage-Reference Temperature Drift.”
  6. Maxim Integrated tutorial 5062, “Bandgap Reference Calculator Tutorial,”. The free calculator includes the User’s Guide in the calculator documentation zip file, and explains the theory and practical operation in detail.
  7. Thei, K. B., et al., Characteristics of Polysilicon Resistors for Sub-Quarter Micron CMOS Applications, Inst. Of Microelectronics, Dept .of Electrical Engineering, National Cheng-Kung University, Taiwan.
  8. Keen, R.G., Using the Carbon Comp Resistor for Magic Mojo, Copyright 2002 R.G. Keen.
  9. Birkett, M., “VCR Characteristics of High Voltage Thick Film Resistors,” Resistor Business Unit, TT electronics Welwyn, July 2007.
  10. Oxley, Stephen, “High reliability passive components in three broad areas: Contact, imaging, analysis,” TT electronics, Fixed Resistors Business Unit, 7 April 2011.
  11. Edwards, Steve, “Managing Noise in the Signal Chain, Part 1: Annoying Semiconductor Noise, Preventable or Inescapable?” Maxim Integrated application note 5664.
  12. Maxim Integrated tutorial 5059, “Thermal Noise Calculator Tutorial”.

无源元件并非真的“无源”:第3部分——PCB

Abstract: Active components like transistors and integrated circuits change signals using energy from the power supply. However, passive components like resistors, capacitors, inductors, and connectors actually can, and do, change the signal in unexpected ways. This happens because all these passive components contain parasitic components. This application note, the last in a 3-part series, discusses printed circuit boards and the errors that can occur because passive components aren't really so passive.

Introduction

Sometimes the best way to hide something is in plain view. Magicians use this technique along with some distraction to amaze an audience (Figure 1). It is simple actually: our experience leads us to expect certain norms and to see what we expect. Thus boxes are square, not squished parallelograms; spheres are symmetrical, not hemispheres or with elongated portions on the back where it is unseen. In that same sense, printed circuit boards (PCBs) seem straightforward. You think that you can see everything going on, but you are really only looking at the circuitry on the exterior surface. In fact, if you delve deep enough down to the board itself, you find complex layers and structures and a myriad of things that can go wrong here. When high-precision op amps and high-resolution data converters fail to perform as expected, we need to closely examine all the surrounding active and passive components, including the PCB. Into this context we also insert the PCB vendor who has an understated role that is, in fact, critical for IC performance.

This article is Part 3 of a series on passive components in ICs. In Part 1 we talked about capacitors. In Part 2 we looked at resistors and explained that they are not seemingly simple, benign, passive devices. Here in Part 3 we are going to discuss how PCB flaws and errors that are usually hidden, or at least disguised, can introduce passive errors into IC performance.

To understand how PCBs can introduce passive errors, we must first examine the composition of a typical board. Four examples of PCB problems and efforts to solve those hidden errors will help us appreciate the contribution that a good reliable PCB vendor makes to successful products.

We admit here that our articles on passives have generated some lively discussion about the definition of “passive.” In our search for more knowledge and better-informed engineers, we are quite pleased about this. See our Sidebar: Defining Passives for some summary comments on this discussion.

A magician and his assistant provide distractions to help “sell” illusions.

Figure 1. A magician and his assistant provide distractions to help “sell” illusions.

Passive Viewing—Seeing What We Expect

Let’s see how well, how carefully we viewed Figure 1. Did you notice the PCB assembly? Yes? No? It is in the shadows just to the woman’s left side. Yes, we see what we expect to see. The same is true when we examine a PCB. When you look at a typical board directly (Figure 2), what do you see?

A PCB assembly with various components.

Figure 2. A PCB assembly with various components.

If you are like most of us, we see an Ethernet connector, another RJ-45 connector with the label “settings sensor”, “UPS data”, and “RS-232”. We see an inductor and electrolytic capacitors for a switching supply, several large-scale integrated circuits (ICs) and a bunch of decoupling capacitors. Put all this together and it is probably a digital board with several options because we can also see unstuffed components. Right? Yes, but we did not really see the bare PCB itself, and that is where this story starts.

As we said at the outset, myriad things can go wrong with something as complex as a PCB. Experience has taught us that a good, reliable PCB vendor is very important to us now. There are many choices in the materials, the density of the weave in the FR4, the polymer, via construction, minimum trace structure for a given etch method, tin plate and solder mask choices. We might specify a hard-to-find FR4 (a common fiberglass PCB) material because we prefer it, but lack of available FR4 materials could delay production and even double the board cost. Our respected PCB vendor will know about resources, what via construction methods are available, or what assembly methods are recommended for our application. There is definitely nothing passive about this relationship. When we tell the vendor that we care about board quality, he will reciprocate in like manner.

No Magic Wand Building a Board

Yes, the board—you start with fiberglass. The top and bottom layers (typically industry-type FR4) have copper on what will become the outside of the PCB. The center layer is copper with FR4 on both sides, thus comprising the two inside conductive layers. Prepreg is effectively the glue that holds the stack together; it can be just adhesive or it can be a combination of FR4 fiberglass and thermal-setting adhesive. During the fabrication process the stack in Figure 3 will be compressed under heat and pressure to bond the layers together.

Is a typical four-layer PCB stackup.

Figure 3. Is a typical four-layer PCB stackup.

The order of construction can differ depending on many things. Our favorite reference resource, the handbook that most engineers call the “PC Bible,” is by Coombs.1 He details the PCB fabrication processes, outlining literally hundreds of variations and possibilities. Just when you are thoroughly intimidated, you get to the Appendix. The knowledge in the Appendix is massive, a list of industry standards pertaining to everything PCB. It takes you from components including surface mount, general and passives, to printed boards, materials, design activities, then to component mounting and soldering, and through quality assessment, test methods, and repair. At this point we begin to appreciate and understand why we need our best board vendor to guide and advise us.

Still, mistakes do happen with boards, and it seems that they always occur just before a firm deadline. The four PCB examples below happened either before a bed-of-nails test of the bare board was available or after that test was eliminated to save time—always a bad practice that will punish us. Can you guess the errors that we found in each example?

Example 1: Over-etching
We received PCBs and assembled six boards. Oddly, the boards all had different issues. Normally when you fix one board that same fix applies to all the boards. But not this time, which was the key to understanding the problem.

We found that some of the errors were tiny shards of copper that shorted random things. Simultaneously, we were seeing a massive “passive” problem (at least we usually think that a PCB is passive) in the circuit’s performance. No circuit can function with dozens of random shorts. Because these shorts were random and different on each board, it was a troubleshooting nightmare. We sectioned the PCB and looked under a microscope. The board was over-etched, as shown in Figure 4.

PCB section with over-etched, thin copper edges that break off as long thin shards and short to adjacent traces.

Figure 4. PCB section with over-etched, thin copper edges that break off as long thin shards and short to adjacent traces.

Figure 4A has flat sides under photo resist. If the chemistry and temperature are not correct or the board is in the etching solution too long, the effect is etching “around the corner” that undercuts the copper (Figure 4B). Long thin shards can break off the top edge, stay connected on one end, and short to the adjacent traces.

Looking closely at our board, we saw two abrasive scratch-mark patterns at 90 degree angles. The vendor had used a polymer grinding wheel with an embedded abrasive. They attempted to scrub off the shards by grinding the board in two passes on each side. They did remove the majority of the shards, but then they solder-coated the board, which made the remaining shards solid random shorts. Adding solder mask hid the shorts and most grind marks.

Example 2: Orientation
We received a two-sided PCB with solder mask and top silkscreen and assembled a board by hand with through-hole parts. Nothing worked. We had a serious problem with the so-called passive PCB: all three power supplies were shorted in multiple places. Nothing made any sense; not one circuit block out of dozens functioned at all. The technician tried, but finally called the engineer for help.

The technician managed to insert the parts in some strange ways. For example, the three leads of a transistor, which would normally form a triangle in the silkscreen outline, were distorted and twisted. Looking closely under the solder mask we realized that the silkscreen and the bottom side of the board were oriented properly, but the top component copper side of the board was a mirror image. The film used to make the topside image was upside down when the solder resist was exposed.

Example 3: Find Your Way
We received a four-layer board as in Example 2 above with similar issues. Again many traces were connected to the wrong things, power supplies were shorted at multiple places, and nothing (no circuit block) worked. Usually when there is a board error, at least some of the circuits function. We had implemented a complete bed-of-nails test and were confused when it did not identify the issues. Then we found out that the purchasing department skipped the bed-of-nails test to expedite board delivery. That test would have saved us days of effort. The wasted time was a costly error.

We found that the board layers were assembled in the wrong order. Many blind vias were attached to the wrong layers. As a result, we added an edge code (Figure 5) so we could inspect the boards.

The copper layers of the PCB with a staggered edge code on the right. With the code implemented, we could quickly inspect the board layer order before we wasted time by populating the board with components.

Figure 5. The copper layers of the PCB with a staggered edge code on the right. With the code implemented, we could quickly inspect the board layer order before we wasted time by populating the board with components.

The code of Figure 5 extends to the edge of the PCB. The boards are typically fabricated in larger panels made of many smaller PCBs to ease handling during fabrication. The individual boards are separated using a router, thereby exposing the Figure 5 code on the board’s edge. A microscope lets us measure the copper spacing and see that it meets the board specification. This assures us that the stripline will be the correct impedance.

Example 4: the Right Thickness, but Not the Right Answer
We received a four-layer board. Most of it worked, but the striplines had huge ringing and reflections. Striplines are the equivalent of coaxial cables embedded into the PCB. A coax is a center conductor inside an insulating dielectric, surrounded with a circular ground shield. In addition to shielding the signal from external contamination, the coax and the stripline provide a known impedance signal path that, when terminated in its characteristic impedance, does not reflect energy. If the PCB is not constructed properly, the impedance change causes reflections and ringing which destroys analog signals and can even confuse digital signals.

Sectioning the board permitted us to measure the thickness of the various layers. We found that the PCB vendor had a shortage of some thicknesses of the board material. Their untrained employee tried to meet our delivery deadline and substituted extra prepreg layers from something in stock, thus making the total thickness correct. This might sound like a good “fix,” but it definitely was not so. Look back at Figure 4. Let’s say that the center layer with copper on both sides was substituted with thinner material. The capacitance between those two layers will rise because the dielectric is thinner. To keep the layout and the final board the same total thickness, we can compensate by increasing the upper prepreg layer thickness. This will lower the capacitance between the top copper and the nearest copper layer in the center. Note, however, that this also assumes that the prepreg has the same dielectric constant in both cases, which may not be true. Thus, the change in capacitance changes the PCB and stripline impedance, and our supposedly “passive“ PCB is now ringing. You can say “Good bye” to signal integrity.

PCB Problems Cause Passive Failures

Clearly an unseen, taken-for-granted, whatever-you-want-to-call-it PCB exerts considerable influence on precision circuit performance. Moreover, we cannot take anything for granted nor assume that passive IC problems are unrelated to the PCB itself. Common IC performance problems and errors caused by a bad PCB include voltage drops and impedance in ground vias, planes or foils; leakage resistances and moisture absorption; and stray capacitance, with welcomed and dielectric absorption or soakage.

Voltage Drop
Voltage drop in ground vias, planes or foils is a commonly overlooked issue. Adding to the complexity of the problem, voltage drops at both DC and high frequencies require different remedies. Recall Coombs Handbook,2 Chapter 10 for trace versus capacitance and crosstalk and Chapter 13 for voltage and ground copper thickness versus sheet resistance. For via impedance we look to Sayre:3

L = 5.08h [ln (4h/d) + 1]

Where:
L = inductance of the via, nH
h = length of the via, inches
d = diameter of the via, inches

Using h = 0.0625 inch and d = 0.020 inch gives us a via inductance of 0.666µH. How can we reduce this inductance? Place two, four, or more vias in parallel.

This is a good first-order approximation and is useful in thinking about signal integrity below a few hundred megahertz. For more details and consideration of the current return paths, we turn to Howard W. Johnson and his “Black Magic” series.4

Leakage Resistance
The leakage resistance5 of the PCB can disturb sensitive high-impedance circuits. The sources of leakage include improper selection of a laminate material, fingerprints, skin oils, human breath, residual fabrication chemicals, improperly cleaned solder flux, and surface moisture and humidity. If this is a problem for your circuit, consider surface and subsurface contamination and moisture absorption to be everywhere, on, in, or under the solder mask; on, in, or under conformal coatings; on or in active or passive components.

When troubleshooting an existing PCB, remember an experienced engineer blowing on the board through a soda straw. The straw localizes the moisture to help identify the sensitive area. Thorough board cleaning with the proper solvents is important. The wrong solvent, for example, cleaning a water-soluble flux with a polar solvent, can leave salts on the board. If deionized water is used to clean the boards, bake the boards to dry them. Even now you may not be done. Even the cleanest board may still cause problems. A PCB with a very sensitive circuit such as an op amp with a high impedance input and high gain likely needs additional attention. It might be necessary to guard or surround the sensitive pins on all board layers with a driven low-impedance circuit matching the DC level of the guarded pins.6

Stray Capacitance
Capacitance is usually a problem when it is stray and unavoidable. It reduces bandwidth and slows high-speed signals. It is bad when dielectric absorption or soakage7 causes hooking, slew-rate errors, or under-/overshoot. However, capacitance is welcome when it is high-frequency power decoupling. We can specify a thinner than normal dielectric (even thin FR4) between the power and ground planes to increase the capacitance. Discrete capacitors smaller than 10pF (self-resonant at ~2GHz in surface mount) are easily compromised by trace and via inductance. Where the capacitance is distributed between power and ground planes, it has low series inductance and is repeatable if, yes, if we have a “golden” excellent PCB vendor.

Summary

Let’s think back to our opening magician’s mysterious box with hidden tricks. We expect certain norms and see what we expect. We simply cannot be that blind when it comes to the potential problems in a PCB. The manufacture and assembly of a board is far more complex than it appears to a casual examiner and in that complexity lies the potential for PCB flaws and errors. Now, most importantly for our discussion, those flaws and errors can introduce passive errors in ICs. We only examined voltage drop, leakage current, and stray capacitance, but the list of potential passive errors is indeed longer. Solving these passive problems inevitably means fixing the PCB, and each situation will demand its own solution. Finally, within this context we can all appreciate the contribution that a good reliable PCB vendor makes to our successful products.

References

  1. Coombs, Clyde F. Jr., Printed Circuits Handbook, sixth edition, McGraw Hill, ©2008.
  2. Coombs, op cit., Chapters 10 and 13.
  3. Sayre, Cotter, Complete Wireless Design, McGraw-Hill, IBSN 978-0-07-154452-8, page 421 in first edition.
  4. Johnson, Howard, High Speed Digital Design: A Handbook of Black Magic and High Speed Signal Propagation: Advanced Black Magic and Via Inductance
  5. Maxim Integrated tutorial 4429, “Murphy's Law and the Risks of Designing ‘Off Data Sheet,’” Figure 3 and the following text.
  6. Maxim Integrated application note 3264, “Compact DWDM laser Temperature Control with the MAX8521,” Figure 2 and surrounding text.
  7. Laumeister, Bill, Passives Aren’t Really So Passive: Part 1, Capacitors, Electronic Design, June 4, 2013 article.
Sidebar: Defining Passives

When we started talking about “passives”, we stirred up a hornet’s nest? Several engineers1 inside Maxim Integrated and in the larger engineering community immediately challenged the definition of “passive.” We are still trying to find a short, accurate definition that is universally accepted. The most common definition is simply “not active.” Thus, a typical active device uses power to do something like create gain. But there are always exceptions. For example, an emitter follower is active, uses power, converts impedance, and has a gain just less than unity. The goal of these articles on passives, therefore, has been to warn people that what we think is a passive, can and does cause nonlinear responses that can change the signal. Thus resistor voltage dependence or capacitive absorption (soakage) can cause harmonic distortion. Hydroscopic PCBs can change offset.

How does one define a passive component? It is also a tough question. Engineers in a chat room had some good suggestions. The IEEE® dictionary2 defines:

Passive device, A device that does not require power and contains no active components.
Passive Electric Network, An electric network containing no source of energy.

Davor Vujatovic in the Encyclopedia of Life Support Systems (EOLSS) suggests a passive definition:3

A passive component denotes a component that is unable to deliver more energy to an external circuit than it initially stores. To determine whether the component is passive, the total energy absorbed by it must be greater or equal to zero. In other words, a component that absorbs more energy than it delivers is passive. If the total energy delivered by the component is greater than the total absorbed energy, the component is active, i.e. the active component is capable of delivering energy to the outside world.

The chat room engineers also suggested the Wikipedia entry under, “Passivity (engineering)”.4 It has an interesting perspective in the first two paragraphs:

“Passivity is a property of engineering systems, used in a variety of engineering disciplines, but most commonly found in analog electronics and control systems. A passive component, depending on field, may be either a component that consumes (but does not produce) energy (thermodynamic passivity), or a component that is incapable of power gain (incremental passivity).

A component that is not passive is called an active component. An electronic circuit consisting entirely of passive components is called a passive circuit (and has the same properties as a passive component). Used without a qualifier, the term passive is ambiguous. Typically, analog designers use this term to refer to incrementally passive components and systems, while control systems engineers will use this to refer to thermodynamically passive ones.”

Then depending on one’s engineering discipline; Wikipedia says,

Thermodynamic passivity
In control systems and circuit network theory, a passive component or circuit is one that consumes energy, but does not produce energy. Under this methodology, voltage and current sources are considered active, while resistors, capacitors, inductors, transistors, tunnel diodes, glow tubes, metamaterials and other dissipative and energy-neutral components are considered passive.”

Incremental passivity…In circuit design, informally, passive components refer to ones that are not capable of power gain; this means they cannot amplify signals. Under this definition, passive components include capacitors, inductors, resistors, diodes, transformers, voltage sources, and current sources. They exclude devices like transistors, vacuum tubes, relays, tunnel diodes, and glow tubes.”

The Wikipedia article really sums it up in the second paragraph: “Used without a qualifier, the term passive is ambiguous.”

We included the wording “seems to be passive” in our article definition in an effort to “weasel word” the definition to allow nonlinear distortion from something that we expect to be “inert” or “benign.” “Seems“ used above drew lightning for the engineers, so now adding inert or benign will probably add more fuel to the fire. We are still trying to find a short, accurate definition of “passive” that is universally accepted. The most common definition is “not active” and it is not sounding so bad after all.

A similar version of this article appeared January 9, 2014 in Electronic Design.

References

  1. www.electro-tech-online.com/threads/capacitors-more-active-than-you-think.135643/#post-1137754
  2. The IEEE Standard Dictionary of Electrical and Electronics Terms, sixth edition, IEEE Std. 100-1996; terms are ”Passive device” and ”passive electric network.”
  3. Vujatovic, Davor, Electronic Engineering Vol I –Active Networks, ©2009 Encyclopedia of Life Support Systems, EOLSS Publishers Co Ltd, ISBN-13: 978-1848269774).
  4. Wikipedia: http://en.wikipedia.org/wiki/Passivity_(engineering)

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