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January 2012

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Subject:
From:
Amol Kane <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Amol Kane <[log in to unmask]>
Date:
Thu, 5 Jan 2012 09:34:59 -0500
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Richard, I agree with your assessment for like technologies, where reflow profiles are usually well within solderpaste specs. Things go haywire as you said with mixed technologies. In our case (SnPb paste with SAC305 BGA), the SnPb paste is extremely low voiding (little or no voids) at SnPb reflow temperatures. However, when this paste was used at near lead-free reflow temperature (Peak of 223C for about 100 seconds), it produced voiding, not only under BGAs (some fairly big voids on the borderline of failing per IPC), but under QFN center pads and passive components as well. Quality had to accept them as there was essentially no IPC failure criteria for mixed technology joints and the voids did not produce any test failures and the assembly passed all reliability tests. 

Amol Kane | Process Engineer
Catalyst Manufacturing Services, Inc.
941 Route 38, Owego NY 13827
Phone: (607) 687-7669 Extn 349 | Website: www.catalystems.com


-----Original Message-----
From: Stadem, Richard D. [mailto:[log in to unmask]] 
Sent: Thursday, January 05, 2012 9:02 AM
To: TechNet E-Mail Forum; Amol Kane
Subject: RE: [TN] Solder voids in BGA balls

I agree with Dave. And I can tell you that the selection of the right solder paste brand can turn off the voids like a light switch.
 
Having qualified several solder pastes in the past few years, I found that there are certain solder paste brands that have a strong tendency for inducing voids. With these pastes, the handling parameters have little or no effect on the voiding; if you use them you WILL have voids not only in the BGA solder joints, but in the rest of the solder joints as well, especially in solder joints between large flat terminations such as QFN belly pads and larger chip caps and resistors. The voids are integral to that brand of paste.
 
While I cannot tell you here which solder pastes produce this effect, I can tell you that there are at least three brands that do NOT induce voiding. When we qualify a paste, we purposely test it under the worst-case handling conditions in an attempt to reproduce any issues that may be seen during times of high humidity, or caused by operators leaving the paste exposed, or overmixing, or even from using expired paste amongst many other tests, most of which are outlined in IPC standards or test methods. When these particular brands were tested, we saw zero voids using one particular brand, even under the worst possible conditions.

None. Zip. Zero. Nada. 6 test boards with several BGAs, QFNs, large chip caps, and about 1150 total solder joints per board. Not a single void.

The other two paste candidates advertising themselves as "low-voiding" did really well also, but there were a few small voids. They were all well within the J-STD-001 and IPC-7095 specifications however.

But the one brand of paste with absolutely no voids also performed better than all other candidates for wetting, for slump resistance, for tack time, for stencil life, for viscosity, and provided the best overall score during testing. That is the one we use today, and voids are seldom, if ever, seen in our Dage X-rays.

While it is true that via-in-pad designs, paste handling controls, certain finishes, PWB bake methods, etc., (or the lack thereof) will affect the level of voids in the finished solder joint, the selection of paste has the biggest impact. However, you should not switch to another brand for that reason alone; you need to fully qualify the paste so you do not trade one issue for another.

And for many reasons, one shop's best solder paste is not necessarily another's.


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Amol Kane
Sent: Thursday, January 05, 2012 6:48 AM
To: [log in to unmask]
Subject: Re: [TN] Solder voids in BGA balls

Hello Dave,
Happy new near to you and all the technet folks! Is the paper available in the open domain? Or is there a paid means of obtaining it.

We had a similar situation with a BGA not too long ago, but it was a mixed technology process (SnPb paste + SAC305 BGA). I am assuming the voiding limits mentioned in the J-STD are for like technologies, as it can be almost impossible to get rid of voiding in mixed technologies (depending upon component mix and overall thermal mass) due to max temp limitations.

Regards,

Amol Kane | Process Engineer
Catalyst Manufacturing Services, Inc.
941 Route 38, Owego NY 13827
Phone: (607) 687-7669 Extn 349 | Website: www.catalystems.com

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of David D. Hillman
Sent: Wednesday, January 04, 2012 9:50 AM
To: [log in to unmask]
Subject: Re: [TN] Solder voids in BGA balls

Hi Wee Mei! We just published our BGA void investigation work at the SMTAI 
conference in Fort Worth. The title of the paper is " The Last Will and 
Testament of the BGA Void" which was in paper session AAT8. The current 
BGA void requirement is "a maximum of 25% of the X-ray image area" per the 
IPC-JSTD-001. Despite the rumor mill, that requirement was established by 
the IPC JSTD 001 committee based on several data sets submitted by 
industry members several years ago. We initiated and completed the recent 
investigation as component and solderball sizes have changed considerably 
and we wanted to determine if the requirement was still valid. We have 
also used our investigation conclusions to form a revised BGA void 
criteria proposal to the IPC JSTD 001 committee for their 
assessment/review.  You stated that you were considering the voids as 
"rejectable" on several factors but the industry requirement is 25% 
maximum so technically you don't have a basis for that rejection. However, 
there is no reason to have that much void activity in a area array device 
unless you have a design feature such as a via in pad causing a void 
influence. The majority of void issues are due to solder paste problems - 
either the condition of the paste or the reflow parameters. I suggest you 
pursue two actions - (1) the Process manager needs to 
investigate/establish why there are so many voids and present that 
technical detail to you for assessment; (2) Pull a copy of the SMTAI paper 
and look at the various industry studies that are referenced within it. 
Those industry studies can be used to fully understand the various details 
of BGA voids, their formation,  and how they may be part of your case. 
Good Luck.

Dave Hillman
Rockwell Collins
[log in to unmask]




Lum Wee Mei <[log in to unmask]> 
Sent by: TechNet <[log in to unmask]>
01/03/2012 07:16 PM
Please respond to
TechNet E-Mail Forum <[log in to unmask]>; Please respond to
Lum Wee Mei     <[log in to unmask]>


To
<[log in to unmask]>
cc

Subject
[TN] Solder voids in BGA balls






Dear TechNet Buddies,

My colleague was performing BGA x-ray inspection to 20pcs of the PCBA, 
each of them has a BGA on the board. She observed that all the BGA solder 
balls have high number of multiple solder voids of various sizes within 
each of them. The estimated summation of the solder voids from each ball 
ranges from 20 - 25%. As the voids are within the acceptable value of 25%, 
process manager wanted QC to consider the workmanship as process 
indicator.

My colleague approached me for advice and being a QC, I decided to 
consider them as reject base on :

(a)    When consulted, Process Manager is not able to determine whether 
such extensive solder voids will have any impact on the PCBA reliability.

(b)   Though the solder voids size/summation are within the 25%, this 
value are observed on every BGA's solder balls, across all the 20 BGAs.

(c)    The PCBAs are Class 3 and to be used on mission critical 
application.

Before QC decision to reject them was communicated, the process engineer 
recall 5 of the PCBA to perform another round of reflow.

Questions :

1.       Should the above solder voids workmanship be considered as 
"process indicator" or "reject"?

2.       Is there disposition for workmanship that is classified as 
"process indicator" such as rework or replacement? For me, it should not.

3.       Since the process engineer recall 5 of them to perform another 
round of reflow, does it not mean he also concur that the workmanship is 
not acceptable?

I am a self-learned QC, so any sharing on this matter will be greatly 
appreciated.

Thanks and regards,
~wee mei~


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