Remember to leave soldermask openings for testability.

----- Original Message -----
From: "Mitch S. Morey" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Thursday, September 30, 2004 12:18 PM
Subject: Re: [DC] Via protection beneath a fine pitch BGA...


> Ditto that.
>
> Soldermask cover the top side (under the BGA component), and relieve the
> backside soldermask at the vias, so the vias won't blow open during
> assembly. I'm not a proponent for 'plugging' vias, since it adds
> additional processes to the task, and I haven't personally done plugging
> (always just the simple soldermask overlay). But often times people think
> it's necessary for reliability (?). One thing you can do when relieving
> the backside vias is to only partially open them. You can adjust the
> opening so it still partially covers the vias but leave the finished hole
> exposed. This gives you added coverage at the vias for protection, but
> still allows the vias to 'breathe'.
>
> Good day.
>
>>
>> I was told by one of our populators that the vias should be open on the
>> bottom side and closed on the top. If both sides are sealed then one
>> will blow out during heating of the PCB in reflow. This could be the
>> top side and throws debris under the BGA. I seal the top and leave the
>> bottom open to 'breathe'
>>
>> Jim Thornhill
>
>
>
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