The odd shape pad design intend to pump more solder to the BGA I think.
See some simple design before and works well.  Star shaped may have
benefit over the triangular and oval pads.  It can pump solder more
effectively due to surface tension effect.
                                               ;-) jk

>-----Original Message-----
>From: TechNet [mailto:[log in to unmask]] On Behalf Of Tempea, Ioan
>Sent: Thursday, October 17, 2002 8:40 AM
>To: [log in to unmask]
>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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