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What is the Reliability of Reballed BGAs?
What is the Reliability of Reballed BGAs?
Will the process of re-balling lead-free BGA components with tin-lead solder balls yield quality connections better or worse than soldering using tin-lead paste?
Production Floor

Authored By:
M.H. Biglari, A. Nazarian, R. Denteneer, M. Biglari, Jr.
Mat-Tech BV, The Netherlands

A.A. Kodentsov
Laboratory of Materials and Interface Chemistry, Eindhoven University of Technology, The Netherlands
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Transcript
The use of lead containing solders for low temperature soldering in electronics has been commonplace for many decades.

Due to the toxicity of lead, the use of lead in electronic manufacturing has been regulated.

Tin-lead solders have primarily been replaced by tin-silver-copper based solders.

The reliability of these tin-silver-copper solder connections have not been proven to such an extent to satisfy the requirements for some critical application including aerospace, military and medical industries.

These industries therefore are still exempt from the R.O.H.S. regulations and continue to use tin-lead solders for the production of electronics.

The use of tin-silver-copper solders for consumer electronics and tin-lead solders for exempted industries has created some particular issues.

One issue is the obsolescence of components compatible with the tin-lead soldering process due to the bulk of the electronic producing industry requiring lead-free compatible components.

This has pushed many component manufacturers to cease production of tin-lead compatible components for economic reasons.

In particular, Ball Grid Array components can be affected as the balls of some BGA components will have tin-silver-copper balls while the solder paste used will be tin-lead.

Some products are not able to withstand the necessary life cycling requirements as stresses arise from different thermal expansion coefficients of the tin-silver-copper solder balls and tin-lead solder leading to cracking of the connection.

The stresses might be avoided by either of two processes. The first process is to re-ball the BGA components with tin-lead solder balls if the only version of a BGA component is lead-free.

The second process is to ensure complete mixture of the tin-silver-copper BGA and tin-lead solder by adjusting peak temperature and dwell time. Both processes have their advantages and drawbacks.

The re-balling process creates additional process steps, which could have a detrimental effect on reliability. It requires two additional heat processes and could cause extra growth of the brittle intermetallic layer between the component and the solder ball.

In addition because the original finish layer applied for wettability was dissolved in the old ball and therefore removed during de-balling, it will no longer protect the surface from oxidation and can decrease wetting properties for the new ball.

The application of flux used for re-balling can also give rise to more and larger voids. Finally reballing will incur extra costs due to the extra process steps.  

The main advantage is that if the reballing is done properly and the quality of the BGA is maintained, the remainder of the soldering process would stay the same and the quality of the connection could be assumed to be as good as original tin-lead BGA components.

Adjusting peak temperature and dwell time to ensure complete mixture of the tin-silver-copper BGA component and the tin-lead solder paste during reflow may result in solder connections with different mechanical properties compared to a standard tin-lead solder connection. The different reflow profile could also contribute to more oxidation and cause problems commonly associated with oxidation during soldering.

The adjusted profile could also contribute to a thicker intermetallic layer. The main advantage of using mixed alloys is the relative ease and low cost of this process. It only involves a minimal adjustment of the reflow process.   The goal of the research reported in this work was to investigate the quality of re-balled BGA components compared to the connections produced with an adjusted reflow process.

So what are the conclusions?   The reliability of a lead-free, or tin-silver-copper, balled BGA components compared with the reliability of a reballed tin-lead BGA components revealed the following.

Large voids were observed when using a mixed alloy connection.   Kirkendall voids were present at the component/solder interface when using a mixed alloy connection.

The thickness of intermetallic layer between the solder and the component was comparable for both samples.

The thickness of intermetallic layer between the solder and PCB was comparable for both samples.

The re-balled BGA component samples exhibited good quality.

It can therefore be concluded that re-balling lead-free BGA components with tin-lead solder balls, if done properly, should yield better quality connections than soldering lead-free BGA components with a tin-lead solder paste.
Summary
Due to the obsolescence of SnPb BGA components, electronics manufacturers that use SnPb solder paste either have to use lead-free BGAs and adjust the reflow process or re-ball these components with SnPb balls. The reliability of Lead-Free and Lead-Containing solder joints for BGA's has been investigated after re-balling using optimal microscopy. The goal was to compare the quality of the connections for both options. For the lead-free BGA, voids produced by the release of volatile species in flux during soldering were present. Large voids have been observed at the interface component/solder.

Using components that were re-balled did not show the amount of voiding observed for the lead-free BGA. Kirkendall voiding has been observed for the lead-free component at the component/solder interface. It has been therefore concluded that the use of the reballed components is to be preferred to adjusting the reflow profile and using lead-free components.
Conclusions
The reliability of a SAC ball/SnPb solder BGA connection has been investigated and compared with the reliability of a re-balled SnPb ball/SnPb solder BGA connection.
  • Large voids are observed for the mixed connection
  • Kirkendall voids are present at the component/solder interface of the mixed connection
  • A thoroughly mixed connection was achieved for the mixed connection (i.e. there was no visible distinction between a SAC region and SnPb region of the solder connection)
  • The thickness of intermetallic layer between solder and component is comparable for both samples
  • The thickness of intermetallic layer between solder and PCB is comparable for both samples
  • The re-balled sample was of good quality
It can therefore be concluded that re-balling lead-free BGAs with SnPb balls, if done properly, should yield better quality connections than soldering lead-free BGAs with SnPb solder paste.
Initially Published in the IPC Proceedings
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