Inspecting Printed Circuit Boards and Assemblies

Before going into PCB design, my employer was in the telecom business. I started out putting PCBs into anti-static bags and then individual boxes with appropriate labels. A group of eight distinct boards went into a larger box to form a die-group. The big box labeling reflected the part-dash number and revision for each board. This was called "Final Prep" and was the last step prior to shipping.

Figure 1. Dial calipers are essential for accurately measuring features on a PCB. Measuring outer or inner edges including hole sizes is possible using one end of the jaws or the other. Depth measurements use the other end of the calipers. Image Credit: Author.

A Few Selected To Represent The Whole Shipment

There was someone who would audit the shipping boxes, looking for incorrect information on the labels and any other obvious defects. If they found a problem with the ones they sampled, the entire lot would have to be gone through again by Final Prep. If a bag didn't have an anti-static awareness label, the whole shipment could be in jeopardy. It could happen, especially towards the end of the quarter when we're sprinting to make our quotas.

The QC inspector and the QA auditor were the last line of defence before submitting the die group to AT&T or one of the "baby bells" that came out of the antitrust settlement of 1984. That's when the phone company was broken into several regional companies. After a few months, I transferred over to assembling card shelves and heat shields for the telecom equipment racks.

Inspectors would check the card guides for burrs. They looked at everything from correct labeling to bent pins among and even finger prints among other things. Each assembler had their own rubber stamp with their unique number. I was "ASSY68" surrounded by a square in black ink. Traceability matters.

It wasn't long before I got to move over to the Rack Wire and Test group where we assembled entire equipment racks. You've probably seen IT racks where the wiring looks like a plate of spaghetti and others that looked so clean and orderly. I strove for cleanliness.

Eventually, I moved back to a sit-down job building the fuse/alarm panels that were installed at the top of the racks. The fuse panels had a 46 point wiring harness (23 wires) that wrapped around the inside of the two RU (rack-unit) enclosures. The wires were for signals while bus bars strapped all of the fuses to power and ground.

I had my own way of dressing the wires into an S-turn so that the service loops to the fuses didn't overlap. When they made me the Lead Assembler, I wanted all nine (plus temps) of us to take the time to form the 18 gauge wires just like mine so that it was easier to spot a mis-wire. Aside from the ASSY-number, there was no way to tell who built the fuse panel.

One day, my manager asked me if we were on "tightened inspection" - where the sample size was much higher than normal inspection. I answered that if the lot that was just submitted to the AT&T source inspector passes, we would be eligible for reduced inspection. He shook his head and said, "You're the only one" (in the TransMUX department that is not on tightened inspection).

A sampling plan is derived from MIL-STD-105E. The document has several tables that specify how many units out of a lot or batch must be inspected based on the AQL (acceptable quality level) and the total number of units in the lot. A track record of rejects leads to greater scrutiny. A control chart would be the basis for keeping track of a specific failure point if it becomes an issue.

Figure 2. A loupe with a built in light enables inspection of smaller objects. Note the graduations shown in the eyepiece. There is an open square with different hash marks for alternate units of measure. Image Credit: Author

One day, a job opening came up in the Radio division. Point to multi-point radios were like a precursor to cellular networks that we know today. One of the use cases was off-shore oil rigs where the land based managers stayed in touch with the array of drilling platforms at sea. This job was in-process inspection. I sat at the end of a row of fine-fingered ladies who stuffed the boards and put them through the drag solder machine before soaking them in freon to clean off the flux.

To Reject or Not To Reject?

If I couldn't find anything wrong with a PCBA, I pressed my triangular QA23 stamp into a red ink pad and applied it to the board. If placement or workmanship was off, I had little red arrows to stick to the board and a red tag to describe the defect. One thing the ladies didn't want was a red tag on their work.

There would be an issue for letting them fix a non-compliant solder joint without first rejecting the board. I found that out the hard way when I asked for a touch-up on a borderline solderjoint. Ruth, my Manager said that "if it's good, accept it and if it's not, then reject it. It's on you to decide what is acceptable."

To be fair, the company needs that information so that they know where to focus their process improvement efforts. The better course of action would have been to get in touch with Henry, the QA inspector and get his opinion. He would audit the lot, choosing a random selection of units for visual and electrical qualification. The source inspector would do the same with their chosen samples.

Analog Circuits Can Be An Unusual Case

One of the things I found to be unacceptable was when the meniscus of the mica caps were jammed down to the surface of the board. There was no room for a top-side solder fillet. Even though that was a clear violation of the workmanship standards, these were analog boards and any length of wire acts as an antenna. As a practical matter, we accepted the defect on the radio boards where we wouldn't have on the digital boards.

One day, Ruth had the four of us inspectors in a meeting and said that if any of us wanted to move from in-process inspection over to receiving inspection, they could. The only requirement was that we would have to enroll in two concurrent Quality Assurance classes at a local junior college. My hand shot up immediately. One of the things that lifted my hand was a reluctance to put a red arrow on a PCBA with assemblers sitting right there. Rejecting something from a vendor seemed much less personal.

Studying the works of Dr. Ed Demming (statistical process control) and Dr. Joseph Juran (quality assurance, author of the Pareto Principle) was indeed a life shaping experience. We learned the theory about the standard deviation of a Poisson Distribution along with practical aspects of Quality Assurance.

Figure 3. Today's PCBs begin to resemble substrates while substrates incorporate silicon in 2.5D construction borrowed from chip technology. We can expect this tech-drift to continue into the future. Image Credit: Author.

Receiving Inspection: The First Line Of Defence

Running the Receiving Inspection lab meant checking bare boards before they went into the stock room. Of course, all of the components also had to be compared to the source control drawings. Looking for absolute perfection wasn't the thing. What mattered is that the PCBs and their components "met the requirements". Our field guide in these matters is the IPC-TM-650 Test Methods Manual.

When a new part number came into the RI lab, it was given a First Article Inspection. That meant that every dimension or feature was thoroughly examined. As an example, if soft gold is called out on the fab drawing, we use a nondestructive method to verify the hardness using a specially cut diamond on the tip of an indentor. A fixture holds the board or a cross section and the indentor drops down making a tiny indent in the metal. A bigger indent registers a lower number on the Knoops scale of hardness.

Pure gold is the softest and rates between 50 and 90; graded as ASTM B488 Code A. The next level of hardness is medium gold which tests between 91 and 126 on the knoop scale; classified as Code B. Hard gold is rated between 130 and 200 which would fall under ASTM Code C. For reference, diamonds are rated at the high end of the knoop scale at 7000.

The Receiving Inspection lab also included a machine for checking the copper thickness in the via barrel. Another one measured the thickness of gold on the edge fingers. An optical comparator could measure tiny objects projected onto a back-lit glass with graduation marks. The shadow of the part was projected at 10X actual size to facilitate the math. We also had a binocular microscope to find cracks in the PTH barrels.

There was a set of Mitutoyo pins calibrated to check board warpage and hole sizes. We used plain old Scotch tape to see if any ink, mask or copper could be peeled from the boards in accordance with the Test Methods Manual.

These days,we would add automated optical inspection (AOI), X-ray and other technology to look deeper into the PCBs. A reliability lab would perform destructive testing whereas all of the other inspections are non-desctructive. Weeding out the defects is one thing. Improving the process so that there are fewer defects is the ultimate goal of the inspection team. This is the Inspector's value-add that goes beyond a rubber stamp.