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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