Ingemar, Sorry, but you will get an answer. Here is mine.
In past jobs that I worked in, we screened (automatically) as many of the BGA balls as possible. With most of the newer X-ray machines, the software typically has the capability to set a nominal ball volume with a tolerance range, and can screen voids and check them with an algorithm to determine if the void is greater than 25% of the diameter and volume and reject them and mark their location so a rework operator can rework the component, if necessary. We were not able to screen 100% of the BGA balls due to shadowing problems from components on the back side, etc., but we can capture at least 90%. The X-ray screening also picks up solder bridges, missing balls, and other defects. A typical board required 3-5 minutes to go through the X-ray screen, but it was very effective in capturing the defects prior to test.
The discovery of many solder ball voids is what drove changes in solder paste, slightly longer TALT's, and DOE's to obtain the minimum amount of voiding. We were able to nearly eliminate process-caused voiding. So there are a number of process changes that can be made.
Presence of voids in the as-received BGAs limited us from totally eliminating the voids. If the parts you get have voids, then you will still have them after reflow onto the assembly. I have not found a way of getting rid of voids of this type.
However, I did find out that with the optimized TALT, those that were present in the component as received were reduced a small amount in both quantity and size.
There were other voids that are caused by soldermask defined pads, where the outgassing from under the solder mask went directly into the solder ball, and those caused by via-in-pad where the via was not properly ablated, or more accurately, not cleaned properly after ablation. Or not plated up to the surface completely.
There is not a lot you can do as far as "process modification" when you run into these types of issues.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Ingemar Hernefjord
Sent: Thursday, October 20, 2005 8:12 AM
To: [log in to unmask]
Subject: [TN] SV: [TN] Voids in BGAs, again
Ioan,
First: the Intel report was great, whoever sent the link.
Some reflexions on IPC 7095A:
1. What is the difference between A1 and A2, B1 and B2 etc?
2. <20% of the balls have voids: use as is. >20% use with process modification. What modification, if 50% of the balls have voids?
3. The whole table, with percentage hither&dither, doesn't that demand for inspection of hundreds of thousands of balls in a mass production?
Do you do that? Is it based on sample or 100% inspection?
4. Ditto B1 and B2. All B2 to be returned to supplier. Won't that end in a lot of discussions with the package supplier? And judging the percentage, mustn't that take a lot of time?
5. Ditto the rest of the categories.
6. If to be followed strictly, won't that need best X-ray equipments, but still give some interpretation problem when you have to do with very complicated and large packages with thousands of balls and a thick metal cover etc?
7. Are any of you at all following the std daily? No..kidding. Of course I'll not get an answer on that.
8. We accept 'no voids at all'. With this std we may relax a little, finding 'right' category...
Best regards
Ingemar Hernefjord
Ericsson Microwave Systems
-----Ursprungligt meddelande-----
Från: TechNet [mailto:[log in to unmask]] För Tempea, Ioan
Skickat: den 17 oktober 2005 15:03
Till: [log in to unmask]
Ämne: [TN] Voids in BGAs, again
Hi Technos,
I know this has been debated at least once a month, but I will not ask what is the acceptability.
What I want to know is if anybody had the ocasion to actually see failures related to voiding. How did you realize they were due to voiding? Also, do you have any dirty pictures of the issue?
Thanks,
Ioan
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