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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