Printed Circuit Board Assembly & PCB Design SMT Electronics Assembly Manufacturing Forum

Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.


Mark Charlton

#10688

BGA collapse during reflow | 12 July, 1999

I need some advice concerning reflow of a BGA-292. Since I am not ready to change my entire process to no-clean, I decided to place the lone BGA-292 on a proto run using our BGA re-work station. The pcb is a moderately dense, single-sided, 8-layer assembly. My problem is that one side of the BGA seems to "collapse" during re-flow. The part is truly lower on one side than the other. The "collapsed" side has shorts and the balls are about 25% larger (x-ray measurement). I started by single-site stenciling no-clean paste, placing and reflowing - result = collapse on one side. I used no-clean sticky-flux and let the eutectic balls create the joint - result = collapse on same side. Other side joints look OK visually and with x-ray. Any hints, pointers, or other assistance will be greatly appreciated.

Mark

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Earl Moon

#10689

Re: BGA collapse during reflow | 12 July, 1999

| I need some advice concerning reflow of a BGA-292. Since I am not ready to change my entire process to no-clean, I decided to place the lone BGA-292 on a proto run using our BGA re-work station. The pcb is a moderately dense, single-sided, 8-layer assembly. My problem is that one side of the BGA seems to "collapse" during re-flow. The part is truly lower on one side than the other. The "collapsed" side has shorts and the balls are about 25% larger (x-ray measurement). I started by single-site stenciling no-clean paste, placing and reflowing - result = collapse on one side. I used no-clean sticky-flux and let the eutectic balls create the joint - result = collapse on same side. Other side joints look OK visually and with x-ray. Any hints, pointers, or other assistance will be greatly appreciated. | | Mark | | Mark,

As you've pointed out, all sides must collapse equally. Using a rework capability is ok if it is done right. You seem to be doing it right with paste (printed with a micro stencil with apertures about 3 mils larger than pads for your application) or paste flux.

I've used recommended (by PHD, others, and industry) off contact reflow heater nozzle settings (anywhere from 60 to 120 mils above the board surface) and achieved excellent results. Normally, I would use this process, with a proven profile (about 100 second soak plus another 100 second equalization ramp and about a 275-295 degree C topside heater tmep and 250 botom side heater temp using the solder paste profile recommendation window [mine is 100 seconds max in the 183 window and no more than 45 seconds in the liquidous window at 205 C at a peak of 220 C, etc.]).

I say normally I would do this. However, the folks in control here say no. The think, based on dubious data, excessive intermetallics are formed using this process because the heat is exhausted from the nozzle's bottom to very closely adjacent parts. Anyway, I do it all the time in other places and have yet to have reliability issues crop up.

I guess, instead of going through all the rework stuff, I'd just go for it in production. I see new parts every day and mostly go unnoticed even while using X-Ray on every prototype run. If problems do arise, as those you indicate, they usually are associated with DFM problems. No matter, all sides have to reflow equally to be acceptable.

Earl Moon

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#10690

Re: BGA collapse during reflow | 12 July, 1999

| I need some advice concerning reflow of a BGA-292. Since I am not ready to change my entire process to no-clean, I decided to place the lone BGA-292 on a proto run using our BGA re-work station. The pcb is a moderately dense, single-sided, 8-layer assembly. My problem is that one side of the BGA seems to "collapse" during re-flow. The part is truly lower on one side than the other. The "collapsed" side has shorts and the balls are about 25% larger (x-ray measurement). I started by single-site stenciling no-clean paste, placing and reflowing - result = collapse on one side. I used no-clean sticky-flux and let the eutectic balls create the joint - result = collapse on same side. Other side joints look OK visually and with x-ray. Any hints, pointers, or other assistance will be greatly appreciated. | | Mark | | Hi Mark,

I don�t know what kind of equipment you are using. I think it�s essential that there is no mechanical influence after placing the device on the PCB for the soldering. If the part is able to freely "swim" during soldering and your PCB doesn�t come up that much that it touches your equipment again there shouldn�t be any problem (assumed your profile is allright) If it�s not a mechanical influence during soldering I would blame it on the part cause I believe in my process as long as it works well with other parts but that�s normally hard to proof.

I hope some others can give you more help, Wolfgang

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#10691

Re: BGA collapse during reflow | 12 July, 1999

| I need some advice concerning reflow of a BGA-292. Since I am not ready to change my entire process to no-clean, I decided to place the lone BGA-292 on a proto run using our BGA re-work station. The pcb is a moderately dense, single-sided, 8-layer assembly. My problem is that one side of the BGA seems to "collapse" during re-flow. The part is truly lower on one side than the other. The "collapsed" side has shorts and the balls are about 25% larger (x-ray measurement). I started by single-site stenciling no-clean paste, placing and reflowing - result = collapse on one side. I used no-clean sticky-flux and let the eutectic balls create the joint - result = collapse on same side. Other side joints look OK visually and with x-ray. Any hints, pointers, or other assistance will be greatly appreciated. | | Mark | | Mark

I have had this problem in the past/present (because I am forced to use POS rework equipment). It usually comes from heating the BGA up to fast/hot. Depending on the rework station I would try to direct more heat to the bottom of the PCB and minimize the top heat. 2nd choice, since you want to run No clean Flux on the BGA alone I would suggest after you reflow the rest of the board add the no-clean past flux (no need to add past), place the BGA and send the board back through the reflow oven. This will fix your problem.

Good Luck

Mike

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KT

#10692

Re: BGA collapse during reflow | 14 July, 1999

| I need some advice concerning reflow of a BGA-292. Since I am not ready to change my entire process to no-clean, I decided to place the lone BGA-292 on a proto run using our BGA re-work station. The pcb is a moderately dense, single-sided, 8-layer assembly. My problem is that one side of the BGA seems to "collapse" during re-flow. The part is truly lower on one side than the other. The "collapsed" side has shorts and the balls are about 25% larger (x-ray measurement). I started by single-site stenciling no-clean paste, placing and reflowing - result = collapse on one side. I used no-clean sticky-flux and let the eutectic balls create the joint - result = collapse on same side. Other side joints look OK visually and with x-ray. Any hints, pointers, or other assistance will be greatly appreciated. | | Mark | |

Mark, Turn your borad around 180' and try reflowing it. If it happens on the same side again, check for heat sink in the board around the BGA. If not change your profile. Like Eral's warm up and soak zone recommendation.

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