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50 PCB007 MAGAZINE I NOVEMBER 2020 In materials, there is a different mix of glass and resin. Previously, it has been the practice that the properties for laminate and prepreg stated in the datasheet are for the type with the thickest glass, type 7628, but this is now beginning to be nuanced a little more. This is good because, typically, the most advanced PCBs use prepreg with the lowest glass con- tent—glass styles 106 and 1080. The CTEz expressed in ppm is approximate- ly 25% higher for a 106 (thinnest) prepreg than for a 7628 (thickest). For Ez, which is mea- sured in megapascal (MPa), then the typical value for 106 prepreg is around 4300; for 7628 prepreg, it is approximately 5,800. Resin con- tent in a 106 prepreg can be well over 70%, while in 7628, it is down in the range of 42%. For an EMS or others who will assemble components and solder them, it is important that they also understand a stackup and know what challenges they may have to solve. I am thinking in particular that a number of copper- filled buried vias directly into BGA footprint is something that they should know about, and it will help them determine a good soldering profile. Early Involvement, Lower Cost I have been in the industry of printed circuits for ages and deal with layouts and PCB design every day. Vias play an important part of the PCB, so it's important to know what they are, how they work, and how to solve any chal- lenges involving them. My job is to advise on design, solve EQs, and ensure all parameters are covered early in the design process. The earlier we talk and discuss, the fewer the mis- takes and the lower the cost for avoiding ex- pensive redesign. Here are the top five things to remember about vias: 1. Be sure to stay updated with the latest IPC standards relevant for your design and need. 2. Make sure you have the AR in place. 3. Make sure the pad diameter is sufficient. Remember that the unprocessed hole is drilled around 0.1 mm larger than the nominal diameter. For laser vias, the unprocessed hole is <0.1 mm. 4. Use stacked laser vias only if needed. Otherwise, they should be staggered. 5. Only use stacked laser vias on top of buried holes when needed. The last advice from me might sound sim- ple but it's crucial: It is important that you dis- cuss design requirements with your PCB sup- plier and make sure that you understand each other and agree on the way forward. PCB007 John Steinar Johnsen is senior technical advisor at Elmatica. To read past columns or contact The PCB Norsemen, click here.