BGA PAd Soldermask

Kurt: Lots of choices, not many of them good, yano.

1 So LPI has taken over the market pretty nearly 100%, but it won't "tent" reliably, because it is not very viscous when applied and drops into the hole. So then, tenting/plugging with LPI is bad news, because the UV cure process for LPI solder mask, which is aimed at surface curing; traps solvents, solder mask, and what not in the mask in the via. Heat that stuff in HASL, reflow, wave, rework ... who knows ... blamo??? 2 Film type mask is very expensive to use. Vacuum-type sheet lamination panel-by-panel is necessary. So, it�s slower than hell and very troublesome for the fab people, who claim that it is expensive, wasteful panel-wise, and has adhesion problems. So to get the dry film to adhere better, they are forced to do another process (oxide treatment). Probably end-up costing you an arm and leg directly or indirectly. I never liked the stuff. It seems to me solder sticks to the developed margins of dry film mask, and I figure solder mask to which solder will stick is less than useless. There, got that out!!! I feel better. 3 Fab working the film, opens the via pads to about 0.016" dia. On both sides. When printed normal, the LPI mask covers most of the via pad on both sides. After HASLing the boards, "plug" on the "solder" side the with screened liquid mask (yano, just conventional thermally cured "SR1000") as a post HASL, secondary process. This'll keep solder from coming through the vias at wave solder; which sometimes causes shorts to SMT pads located 0.021" from the hole center, worst case; and permits the HASL tin-lead to coat the copper in the via. 4 Use a two step solder mask process to tent vias. First, put a layer of dry film down and print it with via image. Then coat the board with liquid solder mask. Then print that image. This produces a part with tented via's and liquid photoimageable to ensure that there is no entrapment in the via holes. The issues there are the additional standoff, especially those close to the BGA pads and LT 0.025" SMT ... that definitely affects the solder print resolution/process ... and it it�s expensive. 5 Solder plugged holes are MORE reliable than the unfilled yet solder coated holes, but then you can get those mini-volcanoes and they never do that eruption thing when and where you want it to do them, same as their geological cuz. 6 Put a piece of Kapton tape over the via holes under the BGA when wave soldering. 7 Print a WS temporary mask over the via holes under the BGA when wave soldering.

There was a session on "Plugged, Filled and/or Tented Vias" at IPC Printed Circuits Expo '99. You might check the proceedings (and report the findings.)

My2�

Dave F

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Kurt,

When I was working in a captive manufacturing house we had been to migrate toward tenting all vias under the BGA to eliminate not only shorts but insufficient (Starved) joints.

Tony

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