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September 2010

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Subject:
From:
Guy Ramsey <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Guy Ramsey <[log in to unmask]>
Date:
Tue, 7 Sep 2010 08:56:14 -0400
Content-Type:
text/plain
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text/plain (120 lines)
I believe Bob was referring you to the way solder mask can act as a shim. It
lifts the stencil off the copper effectively creating a non-contact solder
print. Solder mask thickness can vary depending on the features being
masked. Tented vias can be covered with a blob of mask. This might shim your
stencil off the board more at that location, causing print variation. 
There are DFM concerns. Silk screen on the board surface can do the same
thing.
If not given proper consideration these conditions and easily double the
thickness of the print. 

If the stencil is not properly designed or created for the application, it
is possible for print volume to be less than optimal, leaving poorly formed
solder deposits on the board. The same can be caused by bad paste, improper
printer settings, malfunctioning printers, very humid environments . . . use
a fishbone diagram to explore the possibilities, manpower, materials,
methods, machines, environment. How might they interact to produce variation
in solder paste deposits? 

Guy

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Eric Christison
Sent: Tuesday, September 07, 2010 4:24 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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