>I  agree with John and Charles on this.   Use a 6 mil SS stencil (laser
>cut), Stainless steel blades,

 Rubber squeegees and thick stencils (0.007")
>bring always a poor soldering result for the QFP (0.5 or
>0.4mm pitch??).  Recommend the following:
>
>- Use metal squeegees
>- Increase stencil aperture aspect ratio, recommend
>0.006" or 0.005" stencil thickness(I prefer 0.005").

>        -maybe a .006"thk, should be OK for a .020" pitch
>
>     6. Definitely move away from using a polymer squeegees.  Forget using
>        a durometer to measure the squeegee hardness and go with a
>        stainless steel squeegee.  Unless you have stepped stencils, which
>        I do not care for, there is no need to have the rubber.
>



These are some of the responses to this problem of unsoldered QFP leads.
How? How? How? Can reducing the thickness of the stencil from 7 thou to 6
thou increase the amount of solder on a QFP lead? We use 7 thou stencils all
the time with brilliant yields. I in fact argue for 7 thou over 6 thou.

The Metal squeegee is FAR, FAR, FAR, over rated. There is no replacement in
my mind for HARD PINK POLYMER SQUEEGEES. Try getting these puppies (English
vernacular), to scoop out paste from the apertures. What's more the metal
squeegees fail to put down paste in behind the leading wall of the aperture.
Yes, they give absolutely the correct height but if they fail to fill the
aperture with paste completely it is in fact less paste volume. If the Hard
Polymer squeegees don't scoop, give the correct height and pack solder down
into the aperture, including behind the leading aperture wall then you add
up to more paste.
More paste is the answer to UNSOLDERED leads not less.

The aspect ratio issue is closer to the problem though. The stencil
dimensions and the pad geometry are the key to cracking QFP soldering. I
have written E-mails before on this so if you look up the archive on IPC you
should find it. If you E-mail me directly I can foward one to you directly.

Regards,

[log in to unmask]

##############################################################
TechNet Mail List provided as a free service by IPC using LISTSERV 1.8c
##############################################################
To subscribe/unsubscribe, send a message to [log in to unmask] with following text in the body:
To subscribe:   SUBSCRIBE TECHNET <your full name>
To unsubscribe:   SIGNOFF TECHNET
##############################################################
Please visit IPC web site (http://www.ipc.org/html/forum.htm) for additional information.
For the technical support contact Dmitriy Sklyar at [log in to unmask] or 847-509-9700 ext.311
##############################################################