QFN on Wave Solder PCB via Castellated/Half-Hole Baby Board? - Page 1
Forum section: electronics / projects

author: TimNJ
author_rank: Super Contributor
timestamp: July 11, 2020, 02:54:56 am
content: Hi all,Hit a bit of a snag at work. I've been consulting with our factory in China but just looking for some more ideas. I'm working on an AC/DC power adapter design. Cost is a priority. Just about everything we make is 100% wave-soldered, as we have a high mix of SMT and THT parts, and the SMT parts are always SOIC, SOT-23, 0603-1206 sized. But, for this design we have no choice but to incorporate a QFN package IC.So far the options are:1. Two pass process: First, reflow QFN and surrounding parts. Then, pass through wave. This requires a special wave solder mask/pallet to block the QFN circuit from the wave. This may wind up being a fairly economical approach.2. Daughter board with pin headers: Transplant the QFN circuit to a small daughter board, and wave solder the board to the main board. Unfortunately, due to tricky space constraints, it doesn’t look like this will work.And finally...3. Daughter board with castellated/half holes: This is something I’ve been thinking about but am not sure the practicality.Reflow solder QFN circuit to daughter board with half-holes, like those commonly seen on RF modulesRed glue the daughter board to the solder side of the PCB.Pass through wave-solder to connect half-holes to main PCB. No mask/pallet used. Is wave soldering a compatible process for half-hole PCBs?The idea is to save on tooling since we’re not sure about production volumes yet. If we go this route, I think the daughter board’s components will need to be red-glued down, to prevent them from falling off in the wave. Can red glue go through more than one process, i.e. reflow then wave, reliably? Anyone ever tried to glue down a QFN…4 dots, 1 in each corner maybe?Does it sound ridiculous? Just tool up for the proper masking pallet?Thanks!

author: TimNJ
author_rank: Super Contributor
timestamp: July 11, 2020, 03:14:01 am
content: ThQuote from: blueskull on July 11, 2020, 03:03:47 amThanks for the idea. Apparently before I started working here, they tried this on some other designs, placing the QFN at a 45 degree angle with respect to the wave. Maybe the soldering process just wasnt set up perfectly but apparently we had both poor yield and a bunch of latent field failures from thermal cycling and borderline solder joints. I suppose it’s worth considering again, but pretty sure our policy is “no” since then. Thanks though. QFP seems a little more reasonable...I’ve seen Sony and Yamaha do this many times on their consumer A/V equipment.

author: TimNJ
author_rank: Super Contributor
timestamp: July 11, 2020, 03:41:57 am
content: Maybe passing the castellated board through the wave will just cause the QFN’s joints to reflow anyway? Perhaps the good initial reflow soldering will just be undone...or at least have the potential to be altered.

author: voltsandjolts
author_rank: Unknown
timestamp: July 11, 2020, 09:07:05 am
content: What is the QFN part number?Just wondering about alternatives.

author: TimNJ
author_rank: Super Contributor
timestamp: July 11, 2020, 02:04:15 pm
content: The part is a a Weltrend WT6636F USB PD controller. If you check the datasheet, you'll see there is a SO-14 version! In fact, our original design used this version, and life was good. But, we learned recently that the SO-14 version has a 16-20 week lead time...which does not work with our material sourcing system.There are plenty of other USB PD controllers out there. However, we already use this controller on a few other products, and we have a good relationship with the FAEs at Weltrend. (They write the firmware for the IC.) On the other products, we use a right angle daughter board, and it works well.Without getting into it too much, the right angle daughter board approach simply does not work for this new design. It is mostly an issue of space, and trying to make every work in an existing injection molded housing that we already own the tool for. Thanks!

author: voltsandjolts
author_rank: Unknown
timestamp: July 11, 2020, 03:19:14 pm
content: Well, if you are keen to stick with Weltrend, I think the best option would be to hold plenty stock yourself of the SO-14 version.That seems better than having potential production problems with QFN wave soldering.Holding stock would quickly become a very cheap fix compared to production stoppages and recalls!

author: TimNJ
author_rank: Super Contributor
timestamp: July 11, 2020, 04:16:06 pm
content: Quote from: voltsandjolts on July 11, 2020, 03:19:14 pmI agree with you. There are many parts these days which have longer than "normal" lead times. Ultimately, our sourcing is handled by my Chinese colleagues. Since they are much closer to the actual manufacturing, they tend to understand the component procurement and manufacturing process better than I do. So, I tend to follow their lead on issues like this...Sometimes, though, with issues like these, things can get a little irrational. I'm still curious (in general) about the process I described above. But, I will also work on some sort of analysis to help show why the SOIC version is still the best option.