Hi Dave
Could I have a copy of that please?
Graham Naisbitt
On 17/12/05 00:57, "David D. Hillman" <[log in to unmask]> wrote:
> Hi George! I just got off a day of being on airplanes so if this email is a
> bit confusing I'll blame it on the Dramamine! Rockwell Collins published a
> paper at the May 2005 CMAP Conference on the issue of mixed metallurgies
> for BGAs. The paper documented significant reduced reliability of a Pbfree
> BGA component soldered in a SnPb soldering process. The root cause of the
> reduced reliability was solder joint cracking resulting from a reflow
> profile that did not allow the BGA solder joints to achieve a uniform
> microstructure. There has been a number of other industry studies that have
> demonstrated that if the reflow profile allows for a uniform solder joint
> microstructure to form then the solder joint integrity is much less (or not
> at all) compromised. Let me know if you want a copy of the paper. I think
> one of the major questions a process engineer needs to ask is "can I
> achieve a uniform solder joint microstructure in a consistent, repeatable
> manner?" if he or she is going to use Pbfree BGAs in a SnPb soldering
> process. The JCAA/JGPP Pbfree consortia will be presenting their testing
> results at the APEX conference in February. I will be presenting the -55C
> to +125C thermal cycle data (per IPC-9701) and will be showing that there
> is an impact of mixing Pbfree and SnPb solderballs and/or soldering
> processes. The test results demonstrate that the solder joint integrity is
> reduced - depending on your use environment the amount of solder joint
> thermal cycle life reduction could be an issue. I am eager to see how your
> mixed product reliability investigation concludes - sounds like a good test
> set for addition to the industry knowledge base.
>
> Happy Holidays!
>
> Dave Hillman
> Rockwell Collins
> [log in to unmask]
>
>
>
>
> "Wenger, George
> M."
> <George.Wenger@AN To
> DREW.COM> [log in to unmask]
> Sent by: TechNet cc
> <[log in to unmask]>
> Subject
> Re: [TN] Antw: [TN] Mixing SnPb and
> 12/15/2005 11:14 Pb free BGA's on an assembly
> PM
>
>
> Please respond to
> TechNet E-Mail
> Forum
> <[log in to unmask]>
> ; Please respond
> to
> "Wenger, George
> M."
> <George.Wenger@AN
> DREW.COM>
>
>
>
>
>
>
> David,
>
> The short answer to your question "Is it possible to get an acceptable
> solder joint if a Pb-Free BGA is put onto a board using SnPb solder
> paste and normal SnPb profiles?" The longer answer depends on ones
> definition of "acceptable". If you had asked if mixed alloy solder
> joints are reliable my answer would have been that nobody right now can
> answer that question with any degree of certainty. I know of a few
> companies who have shipped product (either intentionally because they
> didn't have an option or unintentionally because they didn't find out
> until after they shipped it, that have area array components with SAC
> alloy solder balls that were soldered using SnPb solder paste and a
> typical SnPb reflow process and they are asking the same questions.
> There are probably many other companies who have also shipped products
> with mixed alloy solder joints and don't even know it yet.
>
> There are many consortia activities currently planning or doing mixed
> alloy reliability evaluations but my concern is that the data won't be
> available early enough to avoid having to ship mixed alloy solder joint
> product.
>
> I realize that there are many right now who have indicated that the
> reliability of mixed alloy solder joints isn't "good". However, I
> believe that the published information I've seen so far doesn't support
> their believes'. The data appears to suggest that reflow solder joint
> assembly quality and PCB surface finish have a big effect on the solder
> joint integrity/reliability. There are also others that are suggesting
> that mixed alloy solder joints are or can be reliable and the only data
> I've seen that tends to support that is from NPL.
>
> At the present time I've been asked to give my risk assessment about
> models that we've built with mixed alloy solder joints. The problem
> I'm having is the similar to the problems other companies are having.
> Companies want answers now not later and they don't have the resources
> to obtain the answers and they usually don't have the samples (i.e.,
> products) to devote to a reliability evaluation. So those who have to
> do the risk assessment have to do it on limited data and the amount and
> quality of the data usually defines the "comfort" level in the risk
> assessment.
>
> In the next week or two I'm hoping to finish up finishing up an
> evaluation on a mixed alloy product. I don't want to call it a
> reliability evaluation because is doesn't follow IPC-9701 or for that
> matter any recognized reliability methodology. However, it is the only
> data I can obtain in the required time period to do a risk assessment
> and my attitude is any "Good" data is better than none. I must
> re-emphasize the "Good" data part because I've seen many cases where
> some "Questionable" data has actually been worst than none.
>
> Several months ago I was aware of a several models being built using a
> 16-Layer 0.100" thick FR370 PCB. Four out of nine area array components
> on these models had SAC alloy solder balls and all of the model boards
> were built using our standard SnPb solder paste reflow assembly
> soldering process. After the assembly process I was asked will the
> models be reliable. I could hardly stop laughing until I realized this
> question is being and will continue to be asked by many companies. As
> hard as I tried to obtain several model for temperature cycling the only
> assembled PCB board I received was one defective sample. This sample
> wasn't defective because of a failed solder joint. It was defective
> because a design engineer manually modified circuitry on the outer
> layers for design modification trails. Since this wasn't a daisy
> chained test vehicle and it was only a sample size of line as well as
> the fact that there was no time, doing a controlled ATC made no sense so
> we decided to do destructive physical analysis (DPA),i.e., cross
> sections, to determine the degree of alloy mixing and visual quality of
> the solder joints. The DPA revealed alloy mixing of 40 +/- 10 percent,
> minimum solder joint voiding, and "good" looking solder joints on the
> mixed alloys area array solder joints as well as the SnPb solder joints.
>
> Although I wasn't able to obtain an assembled PCB for temperature
> cycling one board was subjected to HALT testing and when it was finished
> with HALT testing it was used to develop a PEST (i.e., burn in)
> procedure. After HALT and PEST I was told I could have the board for
> DPA so that I could give a more informed answer to the reliability
> question. Unfortunately, this time period came when my work load was at
> it peak. Fortunately, this was just before Thanksgiving and our
> reliability organization had a temperature cycling chamber available so
> I was able to add an addition 240 temperature cycles to the assembly
> during the Holiday. So far during the DPA what I see is extremely
> encouraging. If I can find time I hope to put the result together into
> a paper and present the finding. Most certainly it won't be entitled
> "Everything You Ever Wanted or Needed to Know About Mixed Alloy Solder
> Joint Reliability" but it might be something like "Mixed Alloy Solder
> Joint Integrity/Reliability Sanity Check"
>
>
> My apology for the long answer but I'm on a six hour flight and the
> movie wasn't very interesting.
>
>
>
>
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf of Guenter Grossmann
> Sent: Thursday, December 15, 2005 10:38 AM
> To: [log in to unmask]
> Subject: [TN] Antw: [TN] Mixing SnPb and Pb free BGA's on an assembly
>
> David
>
> Lead free BGAs can be soldered with SnPb solder in a SnPb soldering
> process. Even if you stay below the liquidus of SnAgCu the liquid solder
> will solve the ball forming a lower melting alloy at the interface. The
> process of the solution of the ball into the solder ends if
> - The alloy that formes due to the continuous solution of Cu and Ag into
> the SnPb has ist liquidus at the solder temperature
> - The temperature is lowered below the liquidus of the SnAgCuPb alloy
>
> This means that depending on temperature and time different fractions of
> the balls are are solver in the solder. we made some trials and used the
> temperature- time integral approach of James Vincent to characterise the
> influence of the production parameters on the solder structure.
> Originally I was worried that the interface where the original ball
> meets the solder will be the week point where degradation will take
> place. However, looking at the slides of Dave Hillman I see that the
> rupture takes place in the SnPb solder along the pad. This makes sense,
> since SnAgCu it mor creep resistant than SnPb and thus the strain
> occurring due to thermal cycling will be dominantly in the SnPb solder.
> I have no quantitative data in hand to say how much crack growth is
> accelerated due to this material combination.
>
> A problem which is evident is the fact that you have no collapsing balls
> (or not the entire balls) to compensate for coplanarity problems of the
> balls.
>
> Best regards
>
> Guenter
>
>
>
>
>
> EMPA
> Swiss Federal Laboratories for Materials Testing and Research
> Centre for Reliability
> Guenter Grossmann, Senior Engineer
>
> 8600 Duebendorf
> Switzerland
>
> Phone: xx41 1 823 4279
> Fax : xx41 1 823 4054
> mail: [log in to unmask]
>
>>>> [log in to unmask] 13.12.2005 15:30:24 >>>
> Is it possible to get an acceptable solder joint if a Pb free BGA is put
> onto a board using SnPb solder paste and normal SnPb
> profiles?
> The lead free solder balls are SnAgCu 96.5/3/0.5.
>
> Seems nigh impossible to get DDR2 packages with SnPb balls.
>
> I'm designing a high end DSP into an industrial instrument. The DSP is
> not available lead free, nor does the application call
> for lead free (thankfully).
> There is no option to use a different memory type with the DSP. Nor is
> there any possibility of using a different DSP or FPGA
> due to time/cost/sanity.
> Don't want to explore going lead free on this one (for obvious reasons),
> and certainly do not welcome the expense of reballing
> 0.8mm DDR2 chips.
>
>
> Best Regards
>
> David Greig
> ______________________________
> GigaDyne Ltd
> Buchan House
> Carnegie Campus
> Dunfermline KY11 8PL
> United Kingdom
> t: +44 (0)1383 624 975
> www.gigadyne.co.uk <http://www.gigadyne.co.uk/>
> ______________________________
>
>
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