Of the .1 mm paste deposit, about 50% by volume is metal. The rest goes
away, leaving about .05 mm of solder spread across the other BGA land
patterns. The solder mask protecting the land that you want to remain a
no-connect will be about .05 mm thick. The solder balls should collapse
about 25% from .2 mm to .15 mm. I expect the ball will contact the land
pattern an you will be relying on the solder mask to create the no-connect
condition.
In prototype stages this may be tolerable. In production, it will be a
problem. Remember Murphy was an optimist.
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-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Upton, Shawn
Sent: Thursday, March 15, 2007 8:08 AM
To: [log in to unmask]
Subject: Re: [TN] Chip Scale Packagine (CSP)
I visited our assembly house yesterday, and they indicated that they didn't
think this would be a problem. They will be using around 5mil of
solderpaste, so the unused solderball won't be sitting on the PCB anyhow
(the unused solderball wouldn't have solderpaste beneath it).
Shawn Upton, KB1CKT
Test Engineer
Allegro MicroSystems, Inc
[log in to unmask]
603.626.2429/fax: 603.641.5336
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Eric CHRISTISON
Sent: Tuesday, March 13, 2007 10:10 AM
To: [log in to unmask]
Subject: Re: [TN] Chip Scale Packagine (CSP)
Presumably the balls are too close together to get a track through to the
centre? Why don't you ask the manufacturer how you're supposed to do it?
It might make them come up with a better layout next around (this wasn't
made by my company I hope?).
Regards,
Upton, Shawn wrote:
> I am looking at a part with 0.2mm balls on the bottom, in a 3x3
pattern.
> I need to get a trace to the center ball, but one of the outer balls
> is not in use. I'm thinking of just running the trace under the
> solder ball and counting on the soldermask to prevent the ball from
> attaching to the PCB. Good/bad? I'm a bit worried that when the
> solderballs reflow, the lack of a pad under the unused solderball
> might cause that side of the part to sit higher, as there wasn't a pad
> to wick away that solder.
>
Eric Christison Msc
Mechanical Engineer
Home Personal, Communication Sector
Imaging Division
STMicroelectronics (R&D) Ltd
33 Pinkhill
Edinburgh EH12 7BF
United Kingdom
Tel: +44 (0)131 336 6165
Fax: + 44 (0)131 336 6001
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