Wayne,

 I would try a thinner stencil foil first. It would help with release on small apertures and reduce the problem with too much paste.

We use a MY600 paste printer (solder jet printer) and at .4mm it does not have "Perfect Prints". I have never seen perfect prints a .4mm. But there is less paste between the pads and it helps the paste to reflow evenly onto the pads. With continuous strips of paste across the pads you don't get as even divide of solder reflowing to each pad. 

I would try 2 or 3 mil foils first.

Bob K.

-----Original Message-----
From: TechNet <[log in to unmask]> On Behalf Of Wayne Thayer
Sent: Thursday, April 23, 2020 1:05 PM
To: [log in to unmask]
Subject: [TN] QFP stencils

Hi Gents-

I just did a prototype for a fairly simple board which had reasonably sized parts. Several 0402's and everything else was significantly larger.
Except....

Except for a 100 lead 0.4mm pitch QFP. Based on most of the parts being pretty sizable, I selected a 125 micron stencil. I found I could get solder down into the apertures for the QFP, but it was WAY too much solder, and it was clear that if I made the apertures any narrower, they wouldn't print.
Also, with the 125 micron deposit, the deposits tended to short together anyway with that pitch.

I recall from somewhere that I saw someone use apertures that went crosswise to the QFP leads, so a QFP would only have 4 apertures, shorting all of the pins together. But my recollection is that still worked because surface tension pulled the solder deposits away from each other. (I'm sure you have to make sure the volume is not on the high side, which would result in shorting.) But when I searched for advice on QFN apertures on the web, nothing came up on this.

Anyone with helpful experience in this area?

Thanks,

Wayne Thayer