Dave,
Lunch time you want me to think? Oh, well, life is tough. If I say it
got nothing to do with the mask, I would get stoned by photos. If I say
otherwise, I would get papers and books through at me... So, what you
say...hmmmm, there is one thing for sure, with or without the mask, if
the material compatibility issue is not resolved, the process is out of
the window (max. thermal budget e.g. intermetallic less than x micron,
melting temperature not exceed y degree = surface tension
etc.etc.)...anything is possible...both side are right, everybody
reported correct observation, as for the root cause=TBD until you know
more...
Don't stone me ....hhaaaaaa,
jk
>-----Original Message-----
>From: TechNet [mailto:[log in to unmask]] On Behalf Of Dave Hillman
>Sent: Thursday, October 17, 2002 10:09 AM
>To: [log in to unmask]
>Subject: Re: [TN] Z-axis BGA solder joint verification?
>Addendum - Stress Riser Question
>
>
>Hi gang! I just have to stir the pot this morning and ask a
>question: Does the pad "edge" truly act as a stress riser? As
>the industry has ramped up the learning curve on the solder
>processing and reliability of BGA components one of the first
>"lessons" to hit the streets was that a soldermask defined pad
>was less reliable than a nonsoldermask defined pad. This
>conclusion was derived from the observation that the solder
>joint cracks were located at the solderball/component pad
>interface. A couple papers were published stating that the
>cracking source was due to the soldermask acting as a stress
>riser. However, if you use a nonsoldermask defined pad the
>solder joint cracks will be found in the same place! The
>soldermask/solder joint interface unfortunately happens to be
>located at the smallest solder joint cross-sectional area.
>IMHO the minimal cross-section area of the solder joint is the
>real culprit. We have even attempted to force a stress riser
>generated crack in BGA solder joints by "dimpling" the center
>of the solderballs on a BGA's outer row and then subjected
>them to thermal cycling. The solder joints still cracked at
>the solderball/component pad interface! So I have opened the
>debate - what does TechNet think!
>
>Dave Hillman
>Rockwell Collins
>[log in to unmask]
>
>
>
>
>
>"Tempea, Ioan" <[log in to unmask]>@ipc.org> on 10/17/2002
>07:39:57 AM
>
>Please respond to "TechNet E-Mail Forum." <[log in to unmask]>;
>Please respond
> to "Tempea, Ioan" <[log in to unmask]>
>
>Sent by: TechNet <[log in to unmask]>
>
>
>To: [log in to unmask]
>cc:
>
>Subject: Re: [TN] Z-axis BGA solder joint verification ?
>
>
>Hi Werner,
>
>it's unbelievable how experiments can contradict common sense.
>Thank you for being there, Werner.
>
>But I have another one, the producibility of star shaped pads,
>with all the necessary radiuses at all the corners. And how to
>determine the surface of the pads for optimum reliability?
>
>Any rule of thumb we could use for calculating the dimensions
>for triangular or oval pads? Just keep the same surface as
>circular and calculate the pertinent elements?
>
>Thank you,
>Ioan
>
>> -----Original Message-----
>> From: Werner Engelmaier [SMTP:[log in to unmask]]
>> Sent: Wednesday, October 16, 2002 10:55 PM
>> To: [log in to unmask]
>> Subject: Re: [TN] Z-axis BGA solder joint verification ?
>>
>> Hi Ioan,
>> While we have never tried star-shaped pads, we tried rectangular
>pads--and
>> found a small but significant increase in reliability,
>everything else
>> being nominally equal (oval may be better). We simply got less
>> ball-shaped but more
>> column-shaped solder joints which give less of a stress-concentration
>near
>> the SJ interfaces. I would not dismiss star-shaped pads out of hand
>> for reliability reasons; the resulting SJ may actually have
>less of a
>> stress concentration than ball-shaped SJs due to the effect of the
>> surface tension of molten solder.
>>
>> Werner Engelmaier
>>
>>
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