| We have successfully put .016" vias in .030" pads. | There is no drain off with .030" solder balls, since | the actual volume in the .002" depression is low. | We did notice random bubbles where, apparently, the | solder reflowed and trapped the air in the pad. | Extensive reliability testing showed no adverse affects. | Steve | | | Anyone have an idea how small a via must be to inhibit significant solder volume escaping into the via? | | Typically, micro vias are drilled and plated to about | | .003" diameter. To avoid significant solder "drain | | off," they should be drilled as shallow as possible | | consistent with specified dieletric thickness. This | | usually means to a depth reaching through a dielectric | | thickness of not more than 5-7 mils. | | The solder still will drain off but the via can | | be filled as part of the printing process. Be careful | | of outgassing back up through the via causing solder | | "blow outs" effecting unacceptable solder joints. | | Characterize your solder process to address and prevent | | this problem. | | Another, less costly way to avoid micro vias all together | | is to drill "normal" holes in SMT pads, when possible, | | fill them as part of the relamination process, then | | plate over the top providing a smooth, flat surface | | that is hole free. Steve, I concur concerning such a depression in such applications. However, I have seen significant problems with HASL, micro vias with some fab suppliers. I worked with vias in BGA pads early in the 90's and have much data the solderability problems I addressed. Again, I agree the problems typically do not occurr. However, one should be aware they could arise and bite you when and where it hurts.
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