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PCB-May2018

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32 PCB007 MAGAZINE I MAY 2018 sult in impedance performance of the chosen stack-up that would be the same regardless of where it is produced. The Big Mac Index [1] provides an estimate of a particular country's price level. It was first published the magazine The Economist, Sep- tember 6, 1986. The index is based on McDon- ald's Big Mac, which is uniform all over the globe while domestically produced. For Standardization You must have a dedicated lane to follow de- pending on your product requirements. If you choose the mainstream, you have the most op- tions and the lowest price—but the more spe- cific the requirements, the fewer the options and the higher the costs. Philosophy of Toolbox The big benefit of standardization is that if you know the impact of the prepreg on the de- sign and you know the impact of copper foil and its different roughnesses you then can un- derstand the impact of PCB manufacturing process tolerances—both the one you can take as default but also how they affect your design and the manufacturing requirements you put onto it. Then you can start comparing differ- ences and design suitable resin systems with your material suppliers. The toolbox consists of three levels: 1. General properties as standard 2. Group of materials with the same properties (multiple source) 3. Specific supplier data (single source) Material Concerns When defining this we need to consider who in the supply chain controls the biggest im- pacts. We see it as follows: Conductor width gPCB manufacturer Conductor height g PCB manufacturer Dielectric height g PCB laminate manufacturer Dk/Df g PCB laminate manufacturer By making the choice of specifying standard properties from production variations, the im- pact of Dk will be less important. Basically, if the tolerances of the dielectric are within IPC B/L (Class 2) and the conductor width for 0.1 mm conductors is +/- 0.03 mm, the im- pedance will be within 10% without need for measuring. If you choose to make a test board, measure the resulting properties and re-input into your field solver, the control will go to ap- proximately 7%, without further measuring. For the raw glass material there are a few sources in the world that make it. Many of the extra features with the material make it expen- sive due to the supply chain locations. Resin is made from oil, and that equals naphtha, a pet- rochemical product that is a mainstream raw ingredient for the whole industry. Here you must decide between either FR-4.0 (halogenat- ed) or FR-4.1 (halogen-free) to start thinking of the resin's properties to fulfill its purpose and not over-design the requirements. This helped us a lot going from the 3G products to 4G with its increased requirement for speed. Here it was easy to copy the building blocks of the older stack-ups and reuse them in new materi- als with better properties. Weave is important to choose from the cloths mostly produced, and the most produced also got the best tolerances and availability. High- er frequencies require thinner, spread-glass cloths. Copper is produced by a few suppliers, each with their own treatments to reach the right properties. The most common are HTE, RTF and VLP (Figure 1). Unfortunately, there is no Copper Laminate Figure 1: Copper treatment on the foil will affect not only speed and losses but also adhesion.

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