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PBGA vias throwing solder balls

Carol

#30513

PBGA vias throwing solder balls | 14 September, 2004

During processing, the solder in the vias reflows and forms solder balls. PCBs have solder in random PBGA vias from the board manufacturer.The manufacturer says this is a result of the HASL process and is unavoidable. We use HASL finish, multilayer, FR4 circuit boards with vias solder mask plugged from the bottom side (opposite to placement side). The BGA is placed and reflowed with a rework machine, not with the reflow oven. We thought that was the problem, but put a blank board through the reflow oven, and the solder balls formed.

We understand why it's happening - it's the solution that's the problem. We have to use HASL finish. We have considered the following: 1) Reduce via hole size 2) Use blind vias (not sure this is possible) 3) Button fill from the pad side - mask can not get into pad area (not sure Suppliers can manufacture reliably) 4) Allow solder balls as long as they can not short between BGA balls or components (7095 doesn't give an allowable size)

The rest of the world seems to be processing with no problems...

Any recommendations?

Much appreciated,

Carol

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dan

#30517

PBGA vias throwing solder balls | 14 September, 2004

What is your cure time for the mask?

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#30520

PBGA vias throwing solder balls | 14 September, 2004

Your board fabricator is correct. Your HASL boards cause the problem. So, that leads to the solution to the problem => use something other than HASL.

"The rest of the world seems to be processing with no problems..." [Hello!!! Earth to Carol. This is a test.]

Things you are considering could be more trouble than they�re worth. "1) Reduce via hole size" [If you reduce the via drill size below 18 thou, you see additional charges and probably see no improvement.] "2) Use blind vias (not sure this is possible)" [This sounds daunting, without seeing your board.] "3) Button fill from the pad side - mask can not get into pad area (not sure Suppliers can manufacture reliably)" [This has potential. Eliminate the plugging from the bottom-side if you do this. Why aren�t we doing this in the first place? Regardless, you�ll still see solder balls.] "4) Allow solder balls as long as they can not short between BGA balls or components (7095 doesn't give an allowable size)" [A-610 gives the stand for accepting solder balls. 7095 is not an acceptance document.]

How did you determine that your use of the rework station was not the source of the problem? [We think all you know is that both the rework station and the reflow oven cause solder balls. It�s possible that in both cases, you caused the problem by using similar thermal recipes.]

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#30549

PBGA vias throwing solder balls | 15 September, 2004

Just to help convince those on the fence that we don't know dip: 2221A, 4.5.1 tells you the vias have to be tented on both sides.

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pdeuel

#30551

PBGA vias throwing solder balls | 15 September, 2004

We had same problem. Solder balls appeared on top side of board and led to damaged second side stencles. We worked around problem by running blank boards thru oven and inspecting and removing solder before processing boards with parts.

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RDR

#30561

PBGA vias throwing solder balls | 16 September, 2004

You may want to either open up the bottom of the vias so they will not "volcano". Or you will want to seal both sides. It never works having one side of the vias masked and the other open with HASL.

Russ

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Carol

#30565

PBGA vias throwing solder balls | 16 September, 2004

Thanks Everyone, the information was very useful. We use 7095 as a bible. A-610 6.5.3 doesn't allow for any solder balls not entrapped/attached etc. so we will look again at plugging/tenting both ends of the via. We ran into the problem of the mask interfering with the solder pad on the placement side. If the mask is offset, the solder ball could rest on the edge of the mask and cause micro cracks.(So I've been told). Also, maintaining the amount of mask deposited is an issue - could cause the pads to be below the mask.There goes manufacturability again. Our finished product receives a lot of vibration, although the completed board is conformally coated. Wish us luck.. Have a good one...

Carol

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#30595

PBGA vias throwing solder balls | 18 September, 2004

Carol: If you would have problems both covering the via and keeping mask from the BGA pads, maybe you via are too close to the pads. Maybe some of the IPC pad design guidelines would help.

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