Are you using Type 4 solder paste, or Type 3?
Type 4 is a much smaller grain size and prints into smaller apertures better.
You can even go to Type 5 particle size, but that is a special order and may take a little longer to get.
Type 4 used to be a special order, but with today's ever-shrinking electronics, it is now the standard solder paste, at least for the companies I work for.
But don't just jump into it. If you are currently using the "old" standard Type 3, and you want to switch to Type 4, you need to have your printing process extremely well dialed in, ie, extremely repeatable.
Else you will have small fines wedged in between the edges of the SMT pads and the edge of the masking, down in the via holes, etc.
The reward, however, is much better print definition, better wetting, fewer blowholes and voids, etc., etc.
dean
-----Original Message-----
From: TechNet <[log in to unmask]> On Behalf Of Wayne Thayer
Sent: Thursday, April 23, 2020 1:14 PM
To: [log in to unmask]
Subject: Re: [TN] QFP stencils
----
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Thanks to both you and Joyce for the advice. Since I have lots of big parts on this one maybe the answer is a stepped stencil.
Wayne T.
On Thu, Apr 23, 2020 at 10:37 AM Robert Kondner <[log in to unmask]>
wrote:
> 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
>
>
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