QFN bridging tips

[JoeWright]

Hi all, I'm looking for advice on how to increase my margin of error for bridging vs too little solder, on things like 0.5mm QFN chips and similar.

I'm home prototyping using a toaster oven so of course its all slightly bodged.  Tbf, I'm having quite a lot of success, and my goal is to be good enough for prototyping because eventually I will be paying to have my designs converted into market ready versions (I just need the electronics to be functional, to work out the whole system, as the electronics are just part of it).

I found reducing the amount of solder paste has helped eliminate bridging, but in doing so I'm finding it a bit fiddly, as sometimes when applying the paste through the stencil the deposits are too small/non-existent for the smallest holes.  That means I have to do some manual touch up of the paste before putting the components on.  Ideally I'd like to make my stencil holes a little bit bigger so I'm wondering if I could do something to reduce bridging.  My two thoughts are:

1) reducing pad width so that there is enough gap between pads for solder mask to be applied.  The pcb maker I've been currently using (smart prototyping) has too wide a pad gap spec for soldermask anyway (0.352mm) but other places are specifying smaller gaps, such as 0.2mm.  However, I did spot this comment: https://www.eevblog.com/forum/manufacture/allpcb-global-free-shipping!/msg1343315/#msg1343315 saying that although you can get the soldermask inbetween, sometimes it is too thin and comes off in reflow, causing problems.

2) increasing pad length so the extra solder can spread over it.  Does that make sense?


Thanks for any help,
Joe

[asmi]

In my experience I found that having proper solder stops between pins of 0.5 and 0.4 mm pitch QFNs drastically reduces chances of getting bridges. But so far I've found that only one fab that is able to consistently manufacture ultra-narrow solder stops that don't come off if you accidentally touch it, while not breaking the bank in the process - ourpcb.com . Here is an example of 0.1 mm wide solder stops manufactured properly for 0.4 mm pitch QFN:


Their marketing focus seems to be the quality, so they don't seem to be interested in competing with cheap-ass manufacturers, as a result the boards will come out somewhat more expensive than typical "$5/10 boards" fab, but this is where you have to decide if much higher quality (and somewhat longer lead time) is something you're willing to pay for.

Another factor is solder paste printing quality. if solder paste is dispensed uniformly and there are solder stops, chance of getting a bridge is very small. But this usually require using flamed stencils, or some kind of jig to hold frameless stencil in place and firmly pressed to the board during printing. If you do it all in a "hobbyist-like" fashion, there will most likely be issues with uniformity.

Now, from practical standpoint, bridges between outer pins of QFN is not the end of the world, as you can easily fix them by using a lot of liquid flux and soldering iron, so for prototyping - as long as there are no bridges between exposed pad and outer pins - it's not that big of a concern, even if somewhat annoying - and reliable solder stops will help with this as well. I found it somewhat ironic, but I'm yet to encounter a soldering problem of BGA packages, while I still do get solder bridges on QFNs every once in a while despite my best efforts.

[JoeWright]

Thanks asmi.  Great to learn about ourpcb.com.  As you say, I guess I have to now decide whether the increased cost vs reduced hassle is worth it for me right now.  But it's nice to have the option now.

[darrell]

In addition to what's been said, a good stainless stencil (OSHStencils or similar) and quality fresh paste makes a big difference. I have good luck with an 0.1 mm / 4 mil thick stencil for QFNs.

[JoeWright]

Thanks darrell, I think for the revision after next I'll have a go with the better pcb production and stencil quality.

[asmi]

Thanks darrell, I think for the revision after next I'll have a go with the better pcb production and stencil quality.

I mostly use electropolished stencils from allpcb or pcbway. They are slightly more expensive, but still quite cheap.