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

Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.


BGA,s Storage

Hany A. Salam

#18098

BGA,s Storage | 7 November, 2001

Dear All :

1- We assemble mixed technology boards. For some reasons, sometimes we store them semi assembled (only SMT). Knowing that they have plastic BGA,s , can the wave solder m/c affect or damage those BGA,S on continuation their production?

If yes, then how to avoid ?

2- Also when repairing boards with BGA,S , how to avoid damaging the BGA,s while removing ? I mean some damages like buckling & I am interested because we have to reball them again for reuse.

Best regards Hany

reply »

#18107

BGA,s Storage | 7 November, 2001

1 CAN WAVE SOLDER AFFECT OR DAMAGE BGA? Sure.

Since you are vague, assuming: * Partially assembled board was not stored to prevent moisture absorption. * BGA is in a plastic package. * Partially assembled board was stored long enough to exceed the moisture absorption allowance of the BGA. * BGA is on the topside when waving. * Vias near the BGA pads are not plugged.

So, here are a couple of possibilities: * Topside [and bottom side] temperature is ~200�F out of the preheater. Maybe that�s not hot enough to flash absorbed moisture to steam, but it�s hot enough to damage to package when the moisture expands. Yano 1 gm of water is 1.0018cm^3 at 68�C and is 1.0434cm^3 at 212�F. * Say the solder pot temperature is 460�F. Solder flowing through the vias conduct 460�F directly to the BGA balls and could easily reflow those connections.

2 HOW TO AVOID DAMAGING THE BGA DURING REWORK?

BGA rework guidelines are: * Set the assembly and components free of moisture. * Support board well. * Preheat board and component. * Reflow to avoid thermal shock. * Site dress, so not to damage the board. * Apply paste properly. * Position the part properly. * Reflow according to the paste manufacturer�s recommendations.

Earl Moon wrote an excellent article on BGA rework that was published in the SMTnet Newsletter. Continuing, consider searching the fine SMTnet Archives on the topic.

reply »

#18108

BGA,s Storage | 7 November, 2001

Hany,

The guidelines for proper handling and storage of moisture sensitive components, like PBGAs are clearly documented in the joint IPC/JEDEC standard J-STD-033 (free download at http://www.jedec.org).

In short you need to keep track of exposure time from the time these parts are removed from their dry bag, all the way until the final reflow. The risk is when the parts have absorbed too much moisture from the ambiant air prior to high temperature reflow this will cause internal damage like cracks and delaminations. These defects are very hard to detect and can easily escape to the field.

The IPC/JEDEC standard specifies that the risk for damage is only above 200C as measured on the body of the package. This includes mass reflow and localized hot air rework for BGAs. This should not be a concern when going through wave solder since the SMT components on the top side should never reach that temperature.

In short, if you want to re-use these BGAs after removal you must make sure that they are still within their floor life or you need to bake the complete boards before removing them.

Francois Monette fmonette@cogiscan.com 450-534-2644

reply »

Hany A. Salam

#18139

BGA,s Storage | 11 November, 2001

Mr Davef:

Thanks Dave, but what is your opinion about what fmonette hase sent, telling me that I will never reach 200^C on top side (In wave soldering) which is the danger threshold according to the JEDEC recommendations.

reply »

#18151

BGA,s Storage | 12 November, 2001

There is no doubt that we reflowed topside BGA on our wave solder machine. We run our wave pot at 450�F and that�s plenty hot enough to reflow solder on topside SMT components. When the solder balls on the BGA melt, the package is over 200�C, trust me.

Tough to say what fmonette�s talking about. Maybe he can explain how topside temperatures cannot reach 200�C over the wave.

reply »

#18164

BGA,s Storage | 13 November, 2001

Dave, Hany,

I was actually surprised to hear that topside SMT components can reach such a high temperature during wave soldering. If this is the case then the concern for component damage is very valid.

This morning I discussed this issue with Jack McCullen from Intel who is the chairman of the joint IPC/JEDEC committee on moisture/reflow sensitivity. He says that based on some studies that were made many years ago, it was determined that the actual body of the SMT components did not exceed a range of 140-150C during wave soldering. He thinks that even if the leads or balls achieve reflow temperature it does not mean that the actual body temperature will be above 200C.

Dave, did you ever measure the actual body temperature of the BGAs during wave soldering by placing a thermocouple on top of the component ? If anyone has any similar data I would present this to the J-STD-033 committee for review and this concern would be highlighted in future revisions of the standard.

reply »

#18167

BGA,s Storage | 13 November, 2001

No. I have never measured the temperature of the top of a BGA package during wave soldering. Mr. Kelly�s "actual body temperature" of 140-150�C during wave soldering is probably accurate [most of the time]. It�s the utility of this measurement that is in question.

BGA do not vent through the plastic cap [where this �actual body temperature� is measured]. Most often, BGA vent through the interposer. Although sometimes, BGA vent through the seal between the interposer and the plastic cap.

Prove this to yourself by: * Filling a tempered glass bowl with optically clear liquid for visual inspection. * �Humidifing� a BGA. * Placing humidified BGA in the bowl. * Putting the bowl on a solder pot. * Observing no bubbles emitting from the plastic cap, only from the interposer.

reply »

PCB Soldering Tools

JTAG Live PCB Debug Software