If we do not ³smash² the solder we see cracks in the solder joints at
approx. 5000 cycles. Steve

On 2015-02-13, 3:30 PM, "Steven Kelly" <[log in to unmask]> wrote:

>Hi Dave,
>Good question ­ I am assuming both but I have not seen any parts back
>from my customer to analyze which is giving up first. I will do so I can
>look at the joint as well as the thermistor. The joint I know how to look
>at ­ so I guess my question is how do I look at the thermistor? Regards
>Steve
>
>From: David Hillman
><[log in to unmask]<mailto:[log in to unmask]
>m>>
>Date: Friday, February 13, 2015 at 3:11 PM
>To: IPC <[log in to unmask]<mailto:[log in to unmask]>>, "Steven A. Kelly"
><[log in to unmask]<mailto:[log in to unmask]>>
>Subject: Re: [TN] Voids in solder joints
>
>Hi Steve - yes, the BGA void papers we did don't directly apply to your
>case but there are some parallels. When you say "must pass 15000 thermal
>cycles", do you mean having the thermistor electrically function
>correctly or are you describing solder joint integrity?
>
>Dave Hillman
>Rockwell Collins
>[log in to unmask]<mailto:[log in to unmask]
>>
>
>On Fri, Feb 13, 2015 at 1:31 PM, Steven Kelly
><[log in to unmask]<mailto:[log in to unmask]>> wrote:
>Hi All,
>I have some queries that is a little off the wall- at least for us. We
>are soldering an aluminum nitride thermistor onto a 2 layer flex. Under
>the thermistor there is mostly a ground plane and there are vias through
>the ground to the other ground layer (basically thermals) . We also
>reflow a .015 solid copper stiffener/heat sink on the other ground layer
>of the flex.
>I have read Mr. Hillman¹s papers on voids but in this case we seem to
>find if we ³smash² the solder paste to reduce the voids when we run the
>finished unit we pass more thermal cycles ­ I think in reading Mr.
>Hillman¹s papers that makes sense ( I hope).
>Solder paste is water soluble SAC 305.
>We also find if we glue the thermistor down and not ³smash² the solder
>paste we pass more thermal cycles- I think this also makes sense to me.
>We are going to run one more set of tests doing both the smashing and
>gluing to see what happens.
>We need to pass 15000 thermal cycles - -40 to + 85 but we think we are
>seeing deterioration in the thermistor- any thoughts on how we look for
>that ­ what kind of analysis can be done for this?
>
>Thanks. Steve Kelly
>
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