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no contact on BGA



no contact on BGA | 25 June, 2001

We experienced a random no contact at ATE on our board. The board I'm building consist of 9 BGAs, all of them are OK except for one, intermittently the tester will prompt no-contact message. It's happened on a diff. I/O, but only happened on that particular PBGA, 50% of the failed board will pass the test if I were to reflow them for the second time. Assuming there is nothing wrong with the components, what would be the potential cause. The profile were confirmed to be OK-ed.

Anyone with similar experience, pls help. Thanx.

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no contact on BGA | 25 June, 2001

Well, the paste must be OK, because other components don�t have this problem. The board must be OK, because other components don�t have this problem. So, it must be either the component or the profile.

I�d speculate that you need to do some work tailoring the profile to the peculiarities of your paste, the board, and maybe the component. When you say "The profile were confirmed to be OK-ed." Did you profile from one of the balls on this problem BGA that doesn�t solder well?

So, what do you do with the other "50% of the failed board" that you can�t fix by reflowing?

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no contact on BGA | 26 June, 2001

Thanx for the reply Dave.

We actually had a few thermocouples attached from underneath to see if there is any delta in temperature within the same BGA's ball. They looks OK, with the slope of 1 -2 Deg C,soak at 120 Deg C - 150 Deg C at about 90 sec, time above reflow is maintained at 60 -70 sec an dpeak at 220 deg C.

The other 50% of the failure brd was sent to rework, with a fresh BGA, and reflowed using the SRT machine.

Could the coplanarity of the components be a culprit ? When solder paste printed on the brd with a height of 6 mils, what would be the height of the cure solder, will it be reduced by 50%? If that so, will BGA with 4 mil coplanarity will have a contact problem ? ( just throwing some stupid thought that I have in mind ).

thanx in advance

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no contact on BGA | 26 June, 2001

Yes a 6 mil solder deposite will reduce to approx 50% of it's original size.

Does this happen on only 1 BGA? Is the BGA that its happening on have smaller balls than the others, what I'm getting to is, are you sure you are depositing the proper amount of paste? Can you measure it? Are the balls eutectic? if not you are relying 100% on the solder deposit and the planarity of the pads.

What does the bad (or unsoldered) joint look like? does the solder make any contact to the ball?

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no contact on BGA | 26 June, 2001

Your profile seems reasonable, as you say.

As CPI says: Sure it could be planiarity issues with the balls, but that is not something that makes me want to do a Pete Rose belly-flop, head-first slide into first base that this is the source of your problem.

I'm not going to barrel into Ray Fosse, in the 12th inning of the 1970 All-Star Game, to dislodge Amos Otis' throw and score the winning run for the NL on this one either, but I wonder if you're seeing warping of the BGA substrate.

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no contact on BGA | 29 June, 2001

Dear Danial,

The only stupid thought is the one left only as a thought and not asked. The size of the board and the placement of the BGA on the board could play a big factor during reflow. If you are processing boards on an edge conveyor and the board is wide or thin, and BGA is in the center of the board the board may be sagging as it heats up in reflow. This could cause the balls on some of the outlying edges to make contact and the balls in the center will lift away from the pads. Just a stupid thought on my part to think about.


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no contact on BGA | 29 June, 2001

Thanx for the reply guys!!

The problem happened on only one PBGA, the balls are eutectic and the paste volume height is at 5.96 mil min. The stencil opening has been increased to 30 mil which initially was 20 mil.

You guys are right!! One probable caused could be the warpage, of the PCB during reflow process. This BGA is located at the center of teh board and the board width is at 14 inch running on edge conveyor without center support. I didn't checked teh warpage degree of the PCB at post reflow yet but that will be my first assignment when we run the next lot ( I can't persue my investigation due to low demand lately ).

Unfortunately, I don't have the capabilities in house to check the ball's copanarity, but one thing that I should do is to look at the joint quality of the failed board.

Thanx again guys!!!

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