PCB007 Magazine

PCB-Jun2017

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68 The PCB Magazine • June 2017 The big challenge with active components is getting reliable interconnects, and they're do- ing it. I think that's the next big thing to get de- vices off the surface of the board. I would also like to add that, while we're talking embed- ded, sometimes we use the word buried mean- ing within a multilayer structure, an HDI. For these cellphone MEMs modules, a lot of them use surface resistors where the only thing over them is solder mask. Basically, it's like solder mask on bare copper except they're over the re- sistor element. One of the challenges for the board shops that also drives up the cost is when you're doing a surface resistor or an embedded resistor on a board that has buried vias. Then you panel plate because you have to metalize the holes, then pattern plate and etch through more copper for the second etch to define the resistor elements. Not only is it difficult, but it adds cost. Before, we thought surface mount resistors were inexpensive and all we were doing is put- ting solder mask over them; in terms of fine- line etching they can be just as much a chal- lenge as anything else. But that's old news. The new news is the buried vias and sequen- tial-build type PCB constructions where the re- sistors are in the subassembly of the sequential build and are essentially buried surface resis- tors; so there isn't solder mask over them, but prepreg or whatever they're using to build it up with. The challenge, again, is using direct imaging to etch through not only the copper cladding or the OhmegaPly copper, but etch through the plating as well. There may be a 25-micron mini- mum on the hole wall that turns out to be what you're adding to the traces and then etching be- comes more difficult. People do it all the time, but you're adding cost to it. Figure 9: RF power divider application in Globalstar satellite antenna. A DEEP LOOK INTO EMBEDDED TECHNOLOGY

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