Trick question?
If the AAR of the aperture is poor (on a 0.1mm nominal foil, it'd have to be pretty small-- maybe a 0.2mm round or less), you can get zero height on the board. Literally all of the solder paste stays in the aperture.
It probably goes without saying, but that can also happen if you have so much of a gap that the paste deposit doesn't touch the board. I've seen it on flex circuits with integrated stiffeners.
And yes, you can get a localized height above that of the stencil depending on the rheology of the paste, but it won't be uniform. The deposits I've seen affected in that manner are usually tallest in the center. You could also conceivably get an "extra-thick" solder deposit height by using a couple mils of snapoff and enough squeegee pressure to inject the paste through the aperture and onto the pad. I'd expect print quality to be really poor with no gasketing, though.
The 3D paste inspection guys will tell you that's why it's critical to measure total printed volume on the pad, rather than height or area.
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-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Eric Christison
Sent: Tuesday, September 07, 2010 3:24 AM
To: [log in to unmask]
Subject: Re: [TN] Stencil thickness v solder paste height
Thanks Bob,
A quick look tells me that solder paste volume can vary by +/-50% which
seems a lot. Is it reasonable to assume that the blob height will never
be more than the height of the stencil or can the edges be pulled
upwards when the stencil is lifted?
Regards,
On 06/09/2010 19:20, Robert Kondner wrote:
> Eric,
>
> Good question, if you look at the web sites for folks that vend 3D paste
> inspection system I bet you can find some good data.
>
> What I have seen is the surface of the PCB is not flat, solder mask and
> silk screen ink act like hold offs at time. Tented vias for big high blobs.
>
> And, of course, there is scoping of paste from plastic squeegees.
>
> Bob Kondner
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Eric Christison
> Sent: Monday, September 06, 2010 11:02 AM
> To: [log in to unmask]
> Subject: [TN] Stencil thickness v solder paste height
>
> I guess this is an easy one for most you...
>
> If solder paste is applied to a PWB with a 0.10+/-0.01mm thick stencil
> what is the minimum height of paste I can expect to get on the PWB?
>
> Sn95.5Ag3.8Cu0.7 paste , DAP (20 - 38 ìm) or Sn95.5Ag4.0Cu0.5, IPC type
> 3 (25-45 ìm) if composition makes much difference.
>
> Thanks,
>
>
--
Eric Christison
Consumer& Micro group
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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