Missing Reference Designators

Sooner or later, you are going to come across PCBs with components that do not have their reference designators printed on the silkscreen layer, either due to congestion of space or by design. What then?

Well, you can do the following steps:

1. Take note of component designators that are available on the component and solder sides' silkscreen layers of the PCB. Note down the largest number for each of the group designators (e.g. R123, C99, U68, etc.).

2. Determine the arrangement or layout of the components, paying attention to the way the reference designators run on the PCB, horizontally or vertically, from left to right or vice versa. Then follow the flow by adopting either a row or column numbering pattern accordingly.

3. Make a photocopy scan of the PCB, grayscale and inverse it, and then segment it into grids as shown below. Start numbering the unmarked resistors based on the grid reference: R1A, R1B, R1C, etc.

The above example uses column numbering pattern due to the layout of the components which makes it suitable for this scheme. Sometimes the components' layout maybe haphazard or random, in which case you can still produce a grayscale-inverse artwork of the PCB, print it out and manually write on the hardcopy. This is the fastest way to assign designators, but is quite messy and error prone.

A better way is to wait till you complete drawing the Visio layout diagram of the PCB before assigning the designators (what this book teaches). This will save you a lot of trouble in case you miscount or miss-count the components and need to white-out or erase the errors and re-number the designators on paper.

Components Without Markings

As the size of components get smaller, it becomes impossible to label them intelligibly, or even to label them at all. 

Some manufacturers do provide laser markings on their chip capacitors to prevent surface degradation or induced micro-cracks; these codes are based on the EIA RS 198 standards which use 2- or 3-digit alphanumeric codes to represent capacitance values. With these codes, you can then look up their product datasheet reference to determine the values. For example, A5 = 100,000pF or 100nF, where N denotes the vendor NovaCap.

But what do you do with SMD components without markings?

While you're more likely to encounter chip capacitors with no markings than chip resistors, resolving the latter is a much simpler and straightforward affair. A normal digital multimeter can usually measure resistors in-circuit quite accurately without interference from adjacent components, unless there are parallel resistors or low impedance path across the resistor in question.

Unfortunately there's no way you can measure a capacitor in-circuit due to PCB track capacitances as well as induced lead and bond-wire capacitances of components around it. For the most part, you can get some hint from the usage of these SMD capacitors. You can find out more on this and other topics in the book The Art of PCB Reverse Engineering.

Component Classifications (Part 5)

Integrated Circuits

Integrated circuits (IC) are by far the most diversified and interesting of all electronic components, from small- to medium-scale (SSI/MSI) integration like the TTL/CMOS chips, to the large and very large-scale (LSI/VLSI) peripherals and microprocessors, and finally to the ultra large-scale (ULSI) containing over one million transistors, and the wafer-scale integration (WSI) that uses the entire silicon wafer to fabricate a super-chip.

And if you think you've seen enough possible varieties of component packages in the earlier sections, think again! As of April 2014, the existing list of integrated circuit packages can be grouped under eight major categories:

Through-hole SIP, DIP, CERDIP, QIP, SDIP, ZIP, MDIP, PDIP

Surface mount CGA, CCGA, CQGP, LLP, LGA, MCM, Micro SMDXT

Chip carrier BCC, CLCC, LCC, LCCC, DLCC, PLCC

Pin grid array OPGA, FCPGA, PAC, PPGA, CPGA

Flat pack CFP, DFN, QFN, PQFN, TQFN, QFP, BQFP, CQFP, LQFP, PQFP, TQFP

Small outline SOIC, SOP, CSOP, MSOP, PSOP, QSOP, SSOP, TSOP, TSSOP

Chip-scale CSP, TCSP, TDSP, COB, COF, COG, Micro SMD

Ball grid array CBGA, FBGA, LBGA, PBGA, SBGA, TBGA, μBGA

The above list is by no means exhaustive, but it gives you an idea of the extent the world of integrated circuits encompassed. 

Note:

My book's Appendix C contains sample isometric illustrations of popular IC packages which might be useful in identifying these myriads of ICs you may come across on a PCB. Of course, it does help to know that ICs are mostly designated as U or (you guess it) IC on the PCB's solder mask.

Component Classifications (Part 4)

Had been pretty tied up with my next engineering book lately, as well as family commitments. Well, here's the next installment:

Transistors and MOSFETs

The transistor—from its humble beginning as a discrete entity in 1947 at AT&T's Bell Laboratory, to its present highly integrated forms numbering in the millions—is the basic building block of all modern electronic devices.

Transistors can be broadly classified into two types: bipolar and unipolar (also known as field-effect transistor, or FET). The bipolar transistor has three terminals labeled base, collector and emitter, and utilizes a small base current to control or switch a much larger current between the collector and emitter. As such, the BJT can function as an amplifier in the linear range, or as a switch under saturated condition. The FET also has three terminals labeled gate, drain and source, but uses a gate voltage to control the current between the source and drain. The FET can be further divided into junction FET (JFET) and insulated or metal-oxide semiconductor FET (MOSFET).

Like the diodes, military grade transistors are similarly prefixed with JAN, JANTX, JANTXV, or simply JX before their usual 2Nxxx commercial part numbers. Likewise, SMD transistors are marked with 2 or 3 alpha-numeric codes. For example, the Fairchild MMBT2222A is the equivalent of a 2N2222A TO-18 NPN transistor. There are also transistor array ICs used mainly for driving/sinking high current loads, such as the ULN2803A.