PCB007 Magazine

PCB007-July2019

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60 PCB007 MAGAZINE I JULY 2019 and create a conductive opportunity. We're still figuring out how we go from the anode to the cathode without the normal pathway. You have dendrite growth that transports through the metal salt, which collects on the surface. With a CAF, you always seem to have the pow- er effect; the dendrite is thicker on the power side. So, it is plating to that crystal area and then putting to a ground plane and finding a pathway through that weave—that structure— or at micro-separation where you have two lay- ers that can separate. In my presentation, I showed both of the open weave exposures, which are very vis- ible in cross-section. Then, I showed the mi- cro-separation at the prepreg level where the wrong material was used for low-flow or no flow resin. Because it was the wrong material, there was a gap between the two layers. The plating chemistry fit into that space between the via and the ground plane. I had beautiful dendrite photos showing that. It worked out extremely well. We also have inner layer shorts that occur at the power trace to ground due to poor rinsing of the prepreg itself or poor control. And when we split a board apart and use a localize C3 extraction where we can still extract the sur- face area of the inside of the board and com- pare the microvias to areas where there are no microvias, we see a difference between 3 µg/ in 2 of sulfate to 74 µg/in 2 sulfate where we see dendrites. Areas that have low levels of sulfate resin from the methane sulfonic acid plating have very low levels of contamination. Areas where we see dendrites growing around the microvias come from very high doses of con- taminants from the trapped residue from the plating. Now that we've split the board open, we're able to then take a look at the physical. Most people are surprised you can cleave it and look at the inner layers. It splits relatively easy, especially if there is a weak layer; then, it will split along that interface. We have cre - ated a tool to allow us to do that with a press and a special blade that we have. We can go in and cleave a board open and see any weave that has nearly no resin flow into it. We can see the impression made into the resin and confirm that it did not flow into it. We can compare that to other areas of a board where we have good resin flow and show there is no problem. Figure 1: Cross-section of two vias with a measurable short showing CAF shorting in a layer of resin starved prepreg Figure 2: A back-lit close-up showing CAF shorting in a layer of resin-starved prepreg.

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