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Design-Feb2018

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FEBRUARY 2018 I DESIGN007 MAGAZINE 55 copper is the largest potential driver of loss. This leads me on to the mythical concept of the "perfect material library." The Mythical Perfect Material Library "If I could specify every material precisely and draw the material from a library, I would never have to rely on the fabricator not to mess with my designs," says the PCB designer. Whilst there is an element of truth to this, it overlooks the fact that PCB manufacturing is not an assembly process where raw materi - als are simply assembled. In order to make a robust, reliable, manufacturable PCB, the fabricator has to process the raw materials. Copper layers are an excellent example of how manufacturing processes get in the way of the designer's dream of "locking down the spec so tight this fab will have to make it my way." Let's start with foils, which can be made with a variety of profiles. These will either be supplied to the fabricator or the base mate- rial supplier, who will laminate the foils to the base material to create a core. The laminate side foil stays much as received and has as little chemical treatment as possible to ensure bonding. What happens to the outside of the foil is partially in the hands of the fabricator and partially dictated by the stackup. For a start, any outer layer which is drilled and requires plating will have plating copper applied to the whole surface; this is also true for blind or buried vias. So for HDI and sequen- tially laminated boards, designers should be aware that any layers with drill ends, laser or conventional, will have additional copper plat- ing and the plating roughness will be depen- dent upon the fabricator's process. You need to consider this for low-loss designs—the copper foil you specify may not end up on the critical surface because of the stackup design. This is not anyone's fault. It is simply an inherent necessity of today's PCB fabrication process. Likewise, the non-drilled end layers will also be treated to promote adhesion. The chemistry suppliers are well aware of the drive for smoother copper, and generally the trend is towards less aggressive pre-treatment for bond- ing, but bonding really smooth surfaces, whilst possible, is expensive and still in its early days. PCB fabricators have long memories and are Figure 3: Showing 3 mil lines again, but with a mid-loss material. Figure 4: Taken to the extreme, with a low-loss material.

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