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44 PCB007 MAGAZINE I JULY 2019 How reliable is this order in the long run without verification? It's a risk that, unfortu- nately, is taken due to production pressure and high revenue deliverables. Sure, in most cases, there may be no negative results from this de- cision, but it only takes one failure at assembly to cause a line stoppage and a costly return to the manufacturer. This also results in the OEM and/or CM possibly questioning the reliabili- ty of the manufacturer, and in today's market, reputation is key! From a reliability standpoint, we need to quickly assess what risk we may have uncov- ered when faults are detected during electri- cal test. What are the showstoppers, and what statistically shows a low latent risk at the as- sembler and beyond? In most cases, isolated inner layer defects, random shorts, or surface solder tails are not statistically significant on the overall long-term reliability of the product. These are usually either reworked (if allowed) or scrapped. Many OEMs no longer even allow repairs on their product, so the latent risk is fully removed as the board is scrapped. The significant defect that requires much more scrutiny is the void. This can be even more important when microvias and blind and/or buried vias are involved. These are the defects that may hide in a "passed" board on- ly to manifest during assembly due to thermal stress and high temperature during solder flow. Once the void is identified, it is crucial to iden- tify its type and what substructures may be in- volved (in the case of sub-part stack lamina- tion). If the void is determined to be a bubble or air entrapment type circumferential void, it could be an isolated case. This may be isolated to a specific flight bar on a plating line, and some sampling of other board serial numbers processed on that flight bar may be indicated. However, if a taper plate or thin copper void is determined, it may indicate a wide range of risk on the entire load that was processed. Failed bonding on a microvia can also indicate an un - determined plating issue or even an anomaly in sub-part lamination. Now, the risk on the over- all long-term reliability of the product has be- come very high. Shipping product when defects like these are found, even in a small percentage, blows the statistical curve of ET yield. A 96% yield does not bode well when that 96% may be hiding 100% of potential field failures. We must determine what risk is present, which requires high-resolution resistance test- ing of the barrels or, alternately, 4-wire Kel- vin testing. In most cases, this is done on the smallest holes as the higher the aspect ratio, the greater the risk. Studies have been done to calculate the theoretical resistance that can be expected based on the copper weight and aspect ratio. With this test, you can determine if you have a risk within the remainder of the product that "passed" electrical test. We must remember that standard electrical test is mea- suring continuity resistance at 10 ohms and above even for Class 3, Level C product. Ta- per plate and thin copper will not be detected under these test conditions. The fluctuations in resistance of a good barrel versus a suspect barrel will be in the milli-ohm range. This is undetectable with standard ET, which is why we can have a 96% yield in ET with a hidden train wreck just waiting to happen. The problem arises here that now we have an entire order that must be screened but the di- rect access to the barrels is not available due to solder mask and/or via plug. The best results in 4-wire Kelvin testing is the direct probing of the opposing sides of the barrel. Probing at the first opportunity from the barrel introduces more copper and thus increases the mean re- sistance of that given barrel. What happens is that the resistance master developed from that longer circuit now becomes too large to accu- rately detect the small changes in resistivity that thin copper or taper plate may cause. From a reliability standpoint, we need to quickly assess what risk we may have uncovered when faults are detected during electrical test.

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