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PCB-June2016

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34 The PCB Magazine • June 2016 Throwing Power Measurement Methods A reliable throwing power measurement method is essential for throwing power compar- isons as performance criteria of different electro- less copper baths. There is currently no industry standard available, which is why throwing pow- er measurements and performance values of dif- ferent players in the PCB industry are typically not comparable. To overcome this issue two reliable and standardized measurement meth- ods based on cross sections and SEM evaluation are proposed which have been applied for all measurements shown in this paper. These two methods are required to measure the electroless copper thickness in different areas, namely on base materials respectively laminates (method A) and directly on copper (capture pad or copper clad on the surface; method B). For the measurement of the electroless cop- per deposit thickness directly on the base ma- terial several factors need to be considered. Conventional throwing power measurement methods—as applied in the area of thick electro- lytic copper—are not applicable for electroless copper. Etching and polishing for example po- tentially reduce the electroless copper thickness and a protection layer could form an intermetal- lic phase with the electroless copper, both leading to significant measurement errors of the actual layer thicknesses for deposits below 1.0 µm thick- ness. As a consequence, the sample preparation for the throwing power measurement method A excludes any etching or grinding and protection layer and the embedding resin must not induce any heat into the system during curing to avoid any smear of the electroless copper layer. After sample preparation, the throwing power can be calculated by thickness measurement of several different exposed spots in the BMV (Figure 4). The average electroless copper thickness per location and per image is calculated with i = 1,2 and N = 3: Equation 1: Thickness measurement at different areas in the BMV. HIGH-THROW ELECTROLESS COPPER—NEW OPPORTUNITIES FOR IC SUBSTRATES AND HDI MANUFACTURING Table 1: Impact of selected electroless copper performance criteria on IC Substrates and HDI PCB. Figure 4: Throwing power measurement (method A).

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