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Large Voids with Via in Pad

#27995

Large Voids with Via in Pad | 12 April, 2004

We are sometimes, not always, seeing very large voids in the solder joints of BGA's with Via in Pad. Some lots of PCB will have the voids and other lots of boards will not. I feel the via's are outgassing during reflow, due to the PCB not having been properly manufactured. Baking the PCB's has no effect. If we replace one of these BGA's using the Hot Gas unit, there are NO voids at all in the solder joints of the same BGA site, after rework. Managers do not go along with my conclusion. Has anyone else seen this problem?

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#28016

Large Voids with Via in Pad | 12 April, 2004

Large voids are common in via in pad [seach the fine SMTnet Archives for earlier laments], especially in blind via.

Observations on your via out gassing theory: * If the copper plating on your via is GT 1 thou, there will be no outgassing from the glass laminate through the copper. * If the copper plating on your via is LT 1 thou, there could be outgassing from the glass laminate through the copper. * If the copper plating on your via is cracked, there will be outgassing from the glass laminate through the copper.

You can bake your boards until they turn brown and have no impact on the voiding issue. You are causing the voids by printing paste in a fashion that the air in the via is unable to escape before being covered by solder paste.

What is it about your product end-use that concerns you about these voids?

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Ken

#28018

Large Voids with Via in Pad | 12 April, 2004

Dave is correct. The void is una"void"able when printing over the via. Sorry for the pun. The smaller the via, the smaller the problem.

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#28025

Large Voids with Via in Pad | 13 April, 2004

Dont forget, there is a method for BGA via in pad that is sometimes overlooked.

Please look at http://www.sipad.net/sipadvip.htm

It works!!!!

mkehoe@sipad.net

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arcandspark

#28028

Large Voids with Via in Pad | 13 April, 2004

The PCB's we are seeing the voids on are made of some material called Thermount, not FR4. The pads are very easily pulled off if any touch up or rework is done. The voids I am seeing are (hugh) 50% to 75% of the Solder Ball volume. The last new batch of these boards we just ran had almost no voids at all. If the did the voids were very small.

Thanks to everyone for your comments.

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#28032

Large Voids with Via in Pad | 13 April, 2004

Now, that's different. We misunderstood the problem from your original descrtiption. We thought the voids were showing at the via in the pad. Now, we understand the voids appear to be in the solder balls. Suggestions are: * Search the SMTnet Archives. * Get some of the papers published by Dr. Lee at Indium Corp. He has conducted a number of investigations concerning solder joint voiding and BGA components. * Following from this previous comment, recognize that solder paste composition is a major contributor to BGA voiding. * That voids disappear when you rework them with hot air, may indicate that you should revist the recipe you use for reflow. [At least, you should understand the differences the two thermal profiles that you use.]

Based on X-ray imaging, IPC-7095 standard specifies three categories for void size for BGA solder joints. These categories are based on the percentage of joint cross sectional area occupied by the voided area. 1. Class III Small: Void area is LT 9% 2. Class II Medium: Void area GT 9% but LT 20.25% 3. Class I Large: Void area GT 20.25% but LT 36% This standard does not specify a category for voids GT 36% and assumes that is beyond acceptable limits of most products. Class III product would be for highest reliability with the smallest allowable void area.

[Yeh, Sparky, Thermount is almost "unreworkable".] [Background on Thermount: http://www.dupont.com/thermount/pdf/applied.pdf ]

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Ken

#28033

Large Voids with Via in Pad | 13 April, 2004

Threre has been speculation as to voiding as a function of solder paste liquiduous vs. ball liquiduous. Gas entrapment is thought to result if one melts before the other (I forget which order). Two profiles may yield different results as determined by voiding characteristics. This is a great example of where a Taguchi experiment could yield some quick and interesting results.

Are you saying your joints are "egg-shelled" or hollow inside? That is pretty severe.

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#28035

Large Voids with Via in Pad | 13 April, 2004

Arcandspark, I am having the exact same problem with a prototype product I ran last week, could you please give me a call about this so we can compare some notes.

S. Wagner (260) 920-6072

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arcandspark

#28036

Large Voids with Via in Pad | 13 April, 2004

These voids only appear in the solder connections that have a (via in pad). The pads that do not have (via in pad) do not have any voids. Profile is 60 seconds above 183 C and peak of 218 C. The BGA is an OMAP lead free,I am using standard eutectic solder paste and the profile matches perfect with the vendors recommendations. Since the voids only happen on pads with (via in pad) I still feel the boards are the problem. When I run this same OMAP lead free BGA on other boards that do not have (via in pad) with the same solder same profile, there is no voiding. I was looking for other that might be using Thermount, its tough stuff but T.I. loves it.

Thank everyone for your comments they have been helpful.

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arcandspark

#28037

Large Voids with Via in Pad | 13 April, 2004

David F. thanks for information about the copper plating issues, I will check out the Indium site as you suggest. You are right about Thermount being Unreworkable, but T.I. makes us do it due to the cost of these RF proto types, $250 each bard board. Also the voids put these guys out side of any IPC610 criteria. I reject all that are over 50%. After replacing the BGA there are NO VOID at all.

Ken thanks for the HTM site, the board vendor is suppose to be filliing the vias but sometimes it looks like they are not doing a good job. Also Ken this company is so cheap they will not even allow us to do a simple Matrix experiment much less a good DOE, nver a full blown Taguchi.

SWAGNER I am glad to here other people are fighting with Thermount. This stuff is strange and delicate.

Again thanks to all.

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RDR

#28042

Large Voids with Via in Pad | 13 April, 2004

I didn't know we worked for the same company (cheap)! Do you at least get to look at the boards before the ship schedule and pricing is determined and the kit is on the floor? DOE, taguchi, process capabilities? what?

Russ

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Ken

#28061

Large Voids with Via in Pad | 13 April, 2004

"Do you at least get to look at the boards before the ship schedule and pricing is determined and the kit is on the floor? "

Where I am at, the quote model is a closely guarded secret. It envolves eye of newt, liverwort and a sprinkle of pixie-dust. Sales suscribes to the thought that "if you build it they will come". There are some customers I wish had never come....

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