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MAY 2020 I DESIGN007 MAGAZINE 41 the correct approvals and likely from materi- al which is certified for the job, too. The lay- er stackup will be tightly controlled, and any changes will have to be thoroughly investigat- ed and signed off by the appropriate parties. In between these two extremes, there is far more space for PCB technologists and procure- ment teams to make intelligent decisions to gain the best commercial advantage, and this does not necessarily mean just a price negotia- tion. A well-specified PCB will help a fabrica- tor get a better yield with appropriately priced materials, there will be less waste in the pro- cess, and everyone wins. Good communica- tion can allow businesses to work together to maximise profits and to minimise scrap and waste. Here's a sample checklist: 1. Speed: Low, medium, high, or ultra-high 2. Volume: Low, medium, or high 3. PCB size: Small or large 4. Traces: Short, long, or high-speed (Shorter traces make life easier for high- speed signals for a given material type.) 5. Layer count 6. Sequential lamination 7. HDI 8. Back-drilled 9. End-use: Consumer, industrial, automotive, or aerospace 10. Single supplier or multiple sourced 11. Laser or conventional drilled 12. Environmental/regulatory requirements Taking all of the above into ac- count, you may choose to band the type of specification you pro- vide from a low-end "layer count/ thickness" to a medium "layer count/thickness/IPC slash sheet material description" to a high me- dium "layer count/thickness/ge- neric material family/impedance requirements" to a high "layer count, specific build, material ven- dor, glass style, specific base ma- terial, foil type—treatment type" depending on how "nailed down" you need the specification to be for the specif- ic application. Understanding Materials Material is fascinating, and the more you understand, the more you can work with the characteristics rather than fighting with them. In previous decades, rarely did designers have to concern themselves with such matters as glass style and resin content, but here is an ex- ample of just how this has changed. Let's take the case of spread glass cloth. You may be aware of a property called fibre weave effect, which frustrates high-speed designers. This stems from the fact that glass cloth and epoxy resin have widely different electrical properties. Park that in your mind and consider some- thing unrelated and from a completely differ- ent arena: laser drilling. When drill companies developed laser drilling for PCBs the compos- ite nature of base materials limited the preci- sion of laser drilling techniques. You can imag- ine why. You are trying to accurately ablate holes in a material, which is a composite of relatively easy to remove polymer and woven e-glass cloth (Figure 3)—not the easiest of ma- terials to ablate. It must be like mechanically Figure 3: Traditional glass cloth/epoxy resin weave. (Source: Ventec International Group)