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78 SMT Magazine • November 2017 HDI boards in the fabrication level of the bare- board. Obviously, you are driving up the com- plexity of the board, the cost of the board, with the different lamination cycles and drill passes and so forth. From the assembly standpoint, we are mainly dealing with the top and the bottom of the board—what's going on in the middle of the board doesn't have a huge effect on us. The number one concern we would have, because of the HDI boards, is that designers are going to put the vias in the pads, so you've just got to make sure that they are filled and plated over to facilitate a smooth assembly process. Las Marias: Are there key considerations or pa- rameters to ensure successful assembly with HDI boards? Maxson: Yes. There are a variety of different vias, and IPC-4761 specifies the seven types of via plugging process. Obviously, there's always a trade-off. The assembly house wants you to fabricate the boards with the most assembly op- tions to make their lives easier; and obviously the customer wants to use the cheaper option, so there's kind of a tradeoff there. You try to find somewhere in the middle where the cus- tomers will be willing to pay for a middle-of- the-road option. Las Marias: Has there been an increase in the use of HDI? Maxson: There has certainly been an increase. The industry is getting smaller and tighter, and they are trying to fit more and more into small- er boards, which leads the designer to having to use these new via topologies and high-den- sity interconnects just to be able to fit every- thing into the board that the electrical engi- neers would like to have specified on the layout. Weight is another thing. The smaller and lighter the board, anything in flight applications where poundage is money, correlates there. Las Marias: With HDI, will customers wind up with lower cost? Maxson: Your bareboard will cost more there. Anytime you do an HDI, you got to pay for it to get smaller and tighter. There's a couple dif- ferent options when you're going to HDI board, whether it's buried vias or blind vias, or micro- vias. In blind vias, you typically are doing, for instance, when you need more routing room and there's too many signals on the board, and frees up routing channels on the opposite side of the board. The buried vias are what you typ- ically see employed with microvias on a dense placement where you are trying to fit too many parts on the board. What that will do is let you stand up the parts without interfering with what's on the opposite side of the board. If we have a dense placement board, we end up with buried vias; with dense routed boards, we end up with blind vias. Las Marias: How often do designers discuss the design with the assemblers to have the best layout for assembly when dealing with HDI boards? HDI CONSIDERATIONS: INTERVIEW WITH ACDI'S GARRET MAXSON Garret Maxson

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