Issue link: https://iconnect007.uberflip.com/i/1166358
SEPTEMBER 2019 I PCB007 MAGAZINE 29 a robust general specification to bolster these baseline requirements. In addition to this, knowing your PCB manufacturer's standards are the only way to guarantee your PCBs will be built correctly and consistently every time. When moving parts to a new manufactur- er, a solid specification will help to ensure the PCBs received are consistent with what you have already been getting. Our many years of experience in the industry have taught us that unit cost is not the only concern; longevity and the reliability of the final product must also be a top consideration. Enhanced Standards While IPC standards are a solid foundation, our specifications dictate requirements that, in some instances, are above and beyond what IPC requires. However, we manage to do this in a way that adds extra layers of protection to the product. We consider this to be the lowest total cost. One of our own standards that we empha- size is that all holes will be plated per IPC- 6012 Class 3 requirements. This applies to all boards, even if they are not a medical prod- uct. This puts a strong foundation on the circuit board. When the circuit board goes through the thermal cycles, the board will want to expand in the Z-axis; a poorly plated hole will most like- ly be the weak link that will cause the board to fail. Figure 3 shows a good copper deposit through the hole, ensuring the minimum requirement of nominal 25 µm is consis - tent through the hole. Figures 4a and 4b show very thin copper through the hole, which is possi- bly reduced by micro-etch- ing before the surface finish but lower than IPC. Figure 4b shows where thin copper has facilitated barrel crack- ing; this has caused an open circuit within the via hole. Another standard we stress is no track welding or open circuit repair. We have found that a repaired circuit almost always has an impact on the finished product, whether on the impedance required or, in some cases, on the overall reliability of the repaired trace. If Figure 3: Good copper deposit through the hole. Figure 4: (a) Very thin copper through the hole; (b) thin copper has facilitated barrel cracking, which has caused an open circuit within the via hole.