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Defect of the Month

Bob Willis

Getting a close-up look at board quality.

A printed board microsection is one of the best ways to examine the quality of boards and any faults or failures.

The microsection (FIGURE 1) shows a plated through-hole that has been soldered with the nickel layer and through-hole copper visible. Normally, customers would accept the plating standards offered by the fabricator, or define their own, which may or may not impact the price. The nickel layer is part of the nickel/gold surface finish with the very thin gold of less than 1µm consumed during soldering and not visible. The remaining nickel is 5µm, and the copper is around 32µm. This is generous on many circuits board produced today and soldered very easily in production.

 

 

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Bob Willis

When separation occurs, check the oven settings.

This month we look at printed circuit board delamination. As FIGURE 1 shows, delamination is barely visible on the surface of the board and confined to the area around through-holes and where the solder mask is cracking.

 

 

 

 

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Bob Willis

Is the board preheat process optimized?

This month we look at incomplete fill of plated through-holes. During any soldering operation a balance of flux and solder/paste chemistry and soldering temperatures creates good and reliable joints. In FIGURE 1 the solder has not filled the hole completely but still exceeds the requirements of IPC-A-610, class 2 of 50%; measured, it may be 75% filled.

 

 

 

 

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Bob Willis

Soldering excursions can lead to visual process indicators.

This month we look at cracks in plated through-holes around the knee of the hole. FIGURE 1 shows very small via holes that were subjected to multiple lead-free soldering steps, then underwent thermal cycling with no failures but a little cracking.

The cracks visible in the microsection were found on via holes not after the initial two reflow steps and wave-soldering test boards, but after further temperature cycling at -55o +125oC. No electrical failures were detected, just the impact of repeated stressing of the copper. It is a good demonstration of how reliable a board can be, but all that stress does have some visual impact. Care must be taken during microsection preparation to see these indicators.

 

 

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Bob Willis

When BGAs move during reflow, intermittent shorts can result.

This month we look at ball grid array (BGA) opens and solder compression. Intermittent joints and shorts can be caused by package warpage at elevated temperatures. Hence the interest in lowering soldering temperatures commonly used for SAC alloys.

FIGURE 1 was part of an experiment to chart the movement of a BGA package during reflow soldering. Using our reflow simulation, we can see solder ball compression by the package laminate in the image. In many of our video experiments, we see package warpage causes solder shorts and open connections during second reflow. Intermittent open connections have been experienced on double-sided reflow and package rework of adjacent parts. This procedure has been helpful to demonstrate why and how this problem can exist, particularly with smaller packages.

 

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Bob Willis

AOI during fabrication will catch most pad shorts caused by etching.

This month we look at etching faults on PCBs. This is no ordinary set of pads. They are for an 01005 chip capacitor, the second-smallest chip component available. (Yes, there is an even-smaller size.) The 01005 component package is approximately 0.016" by 0.008" or 0.4mm by 0.2mm and small enough for most members of your staff.

Unfortunately, we found small copper shorts between the two mounting pads on one of our test boards that were not picked up during fabrication. The defective boards were spotted during printing trials. The nickel and gold plating are also present. As with many copper shorts, this is related to bare board etching and imaging, but they should have been picked up earlier.

 

 

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