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solder balls and flux

Robert Hartmann

#4785

solder balls and flux | 29 February, 2000

My company is just getting into the flip chip packaging arena. We are putting in a prototype/small volume wire bond PBGA line. We have some questions with regard to choosing solder balls and cleaning of flux.

1) Is there a formula for the shrink of solder balls during reflow. We have a drawing which shows a .022" ball after reflow. Could some tell us the general rule of thumb what size that solder ball is before reflow. Someone told us that the ball will shrink .010".

2) We are using an aqueous flux for ball attach. The material is Alpha Metals WS600. They do not have much information about the disposal of the flux (along with the lead from the balls during cleaning). We are using a batch cleaning system. Could anyone give us some advise on what we need to do to be sure that we are clean coming out the drain?

Thanks in advance for any help!!!

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Kelvin Chow

#4786

Re: solder balls and flux | 29 February, 2000

Dear Robert,

On PBGA assembly, BGA sphere shrinkage number depends to the following factors,

a. BGA Sphere size (before reflow) b. Solder mask opening diameter

As far as I know, no formular has been established yet. Most of my friends just collect experimental data as reference. For a .022" ball diameter after reflow, I suppose that the solder ball should be 20mil (0.508mm) before reflow. The solder mask opening would be 0.5mm

For the waste lceaning water treadment, I am sorry that I do not know the local drainage law in your country. You may want to contact a local Cookson/Alpha guy to help.

Regards

Kelvin

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Robert Hartmann

#4787

Re: solder balls and flux | 1 March, 2000

Kelvin,

Thanks for the help. The actual opening size is .440 mm, but I think what you said makes sense. After discussing with the customer, it looks like it is the height which matters, more than the diameter. We have learned that the height of the ball should compress about 25%. Could you confirm that? Thanks for your help.

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#4788

Re: solder balls and flux | 1 March, 2000

Robert:

On your balls: Solder suppliers make several "standard balls" and can advise you.

On your cleaner: Recycling water / direct to drain? Most batch cleaner suppliers can advise you on recycling and filtering systems to keep you out of trouble with you local PWTF. Recognize that reporting an analysis of your discharges periodically is a good way to stay out of trouble. And since few have established reporting requirements, your recommended scheme will probably be accepted.

Good luck

Dave F

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#4789

Re: solder balls and flux | 1 March, 2000

Robert: Expect a 0.033" raw ball to be within 0.022" of the pad, after reflow. To be clear, that�s not a matter of shrinking by 33%. The volume of solder on the pad is the same as the volume of solder in the raw ball. It�s just that the solder is no longer a sphere after reflow, due to the wetting force between the pad and the solder.

Ta

Dave F

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