Reflow soldering QFN package without stencil (have paste, hot plate & hot air) - Page 1
Forum section: electronics / general

author: wb0gaz
author_rank: Frequent Contributor
timestamp: September 27, 2025, 01:18:22 am
content: Need get a handle on how I can solder QFN device (the two QFN devices have contact pads on 0.5mm centers and center square ground/thermal pad), the PCB finish is ENIG, available paste is 63/37 (and have BiSnAg as option), paste is dispensed from a syringe with 20 gauge blunt tip needle, and the heat sources are a thermostatically controlled hot plate and a hot air tool (air flow and temperature settable).The task is only to solder these two parts to a single board - this is not production nor commercial.The problem is this - I don't have a stencil, and the contact pitch - 0.5mm - is too fine for me to dispense small dot of solder paste on each PCB contact individually (I've done pad-by-pad dispense and reflow with a few other parts, but the number of contacts was far less and the pitch was more relaxed.)I'm looking for advice (or even a "how to resource") that will let me tackle this soldering task with a reasonable chance of success (all pads soldered, no inter-pad shorts/bridges.)Happy to answer any requests for clarification, of course.ThanksDave

author: Smokey
author_rank: Super Contributor
timestamp: September 27, 2025, 01:41:29 am
content: This is my method for soldering/reworking QFNs. Works great for small quantities. This is assuming you have hot air and a decent soldering iron already. No stencil needed. This is all much easier if you have a microscope, and if you don't have a microscope then get a microscope (optical, around 0.7-3x zoom)!1) Put down some tacky flux on the PCB. This actually helps a lot with getting an even layer, and with the hot air reflow later.2) Take the soldering iron and manually tin the pads of the PCB with an even layer of solder. Try to get all the pads about even without excess solder. As long as they have minimum solder, it will squirt out the side later if you have too much. Don't go crazy on the center pad if your part has that.3) If there isn't enough tacky flux left, add more.4) Drop the part into the tacky flux and hot air reflow.4a) If you want a little extra confidence all the pads got soldered, including the exposed center pad, poke the part straight down with your tweezers once it's reflowing. This will also squirt out all the extra solder if you put too much. If that happens, make note that you used too much and use less next time.5) ...6) Profit!7) Inspect the sides of the DFN. If there are any solder bridges, add more flux and swipe with a small soldering iron tip. Should leave nice fillets.

author: wb0gaz
author_rank: Frequent Contributor
timestamp: September 27, 2025, 02:01:04 am
content: Thank you, Smokey, for the detailed instructions!Of course I failed in my initial posting to indicate that I have a small optical stereo microscope (an AMSCOPE KIDS with 5X eyepieces, which has worked well for a long time for my SMD work), and the PCB silkscreen has two concentric squares (0.5mm and 1.0mm larger than the package outline itself) and fine tipped tweezers that will help guide part placement.I'm assuming that no flux is applied separately to the device underside itself, that the flux on the PCB pads (once tinned) is sufficient? The two parts each have a center square pad underside; the PCB's corresponding pad has a grid of vias to help shuttle heat from the device down into the PCB itself.Thanks again for the quick reply!Dave

author: Smokey
author_rank: Super Contributor
timestamp: September 27, 2025, 02:13:52 am
content: Quote from: wb0gaz on September 27, 2025, 02:01:04 amThe puddle of flux on the board is enough. I used to also tin the pads on the part itself, but I stopped doing that and I didn't see any difference in solderability.If there is a huge thermal mass that needs to be heated before the part reflows, then one way to check is to poke the part with the tweezers once some of the solder reflows. If it moves like it's floating, then you should be good to go. If it doesn't move or looks like it "sticks" in some position that's not in the middle, then you probably have to heat it more to get the exposed pad hot enough.Careful with the via-in-pad. Unless they are plugged or something the solder will want to wick through and starve the pad. And this isn't something you can compensate for because if you just put more solder it can float the part if it doesn't wick and the edges won't solder well.I try to avoid via-in-pad as much as I can.

author: wb0gaz
author_rank: Frequent Contributor
timestamp: September 27, 2025, 02:26:28 am
content: I appreciate the heads-up about vias in the center pad - I hadn't thought of this... I can see where the solder would find a path of least flow resistance and drain away from the top of the board. The parts (a pair of RF power devices) both expect to use the center pad as a thermal exit, so I am probably stuck with needing vias (which the datasheets specify), however, a workaround for the solder flow-away issue might be to put kaptan (polyimide) tape on the bottom side of the board (and remove it when done) so that solder couldn't go any further than that needed to plug the vias.

author: Smokey
author_rank: Super Contributor
timestamp: September 27, 2025, 02:37:12 am
content: Quote from: wb0gaz on September 27, 2025, 02:26:28 amYou can try, I've not heard that working before. Get the boards made with plugged vias if it's a problem.This might not be possible with a QFN depending on how close the exposed center pad is to the outer edges, but I typically put thermal vias around the perimeter of the pad and keep them out of the pad.There was a thread recently about via thermal impedance. My own empirical intuition is that vias suck pretty hard at transferring heat to the other side of the board anyway. You get most of the transfer by having a big plane under the part on as many layers as possible. More copper seems to always win.

author: mtwieg
author_rank: Frequent Contributor
timestamp: September 27, 2025, 02:08:25 pm
content: IMO solder paste and stencils are not necessary for hand-soldering QFNs (unless you actually care about voids in your joints...).With regards to "poking" the part to check if the solder is at liquidus, I generally avoid it. If you are watching it closely, the transition to liquidus is usually pretty obvious (the appearance of the solder changes, the solder moves, the part itself moves, etc). Poking it in any direction just raises the probability of solder bridges. It's not such a big deal with QFN, but with BGA/LGA packages I never "poke".In extreme cases where I'm having trouble getting all pins to flow (i.e. due to corrosion on the pins due to being on the shelf for a long time) I might actually tin the pads of the chip itself. Needs to be done carefully to avoid popcorn-ing the chip though. And need to be extra careful about keeping the solder bumps even.

author: Smokey
author_rank: Super Contributor
timestamp: September 27, 2025, 08:51:23 pm
content: Agreed. Don't poke the BGAs (or the bears)

author: wb0gaz
author_rank: Frequent Contributor
timestamp: September 30, 2025, 09:52:47 pm
content: Thanks again for the discussion - very helpful - I hadn't thought of the tin-and-reflow approach at all - Dave