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September 2000

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From:
"Ingemar Hernefjord (EMW)" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Thu, 7 Sep 2000 14:24:59 +0200
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Walter, this is of interest also for us, as we do have SMD problems with one board. For mass production, we avoid SMD, but this is a small scale one with special requirements, as such we use superBGAs and PBGAs and other creeps together.The BGA maker said that SMD should be best for safe registration, and we swallowed the bate. I have put some questions to guru Klasek, but he is not active on TN anymore (or..?), hopefully other brilliants will horn in his place. So, SMD can (snatched from official articles):
- cause hanging balls (I still talk BGA)caused by wrong proportions of aperture diameter vs. ball diameter vs. mask height and more vs.

- cause questionable Au/Ni quality if you have selective plating processes, and that will cause wetting/life solder joint problems. Selective platings are said to be (oftenly) aggressive and can interact with the mask and create strange contaminations.

- cause contaminations from platings to fasten in the interface (pockets sometimes)between mask and pad, and these can creep out on pads and make wetting troubles.

- cause cracks starting in the region where ball and mask meet.

- and more issues that Werner and others have been talking about for years.

My interest is to see if there are still people who stick to SMD and if they do, why?

SMD stands for Solder Mask Definition, of course

Regards

Ingemar Hernefjord
Ericsson Microwave Systems



-----Original Message-----
From: Horaud, Walter [mailto:[log in to unmask]]
Sent: den 6 september 2000 18:43
To: [log in to unmask]
Subject: [TN] Solder mask problems


Thechneters,

I'd like to know if there are special design rules for the solder mask on
PCB.
Most of PCBs I have seen have pads surrended with a gap and solder mask.
In our case I have got a PCB with solder mask on the periphery of BGA pads
(to prevent bridging according to the customer).
It seems to me that the solder mask on the periphery of pads is not so
efficient as expected to prevent bridging and it seems to be also
responsible of the poor mechanical properties of the joint.
Does anybody have experience with this weird technique (solder mask on the
periphery of pads)?

Thanks in advance,


Walter Horaud

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