First full PCB reflow - mixed results. - Page 1
Forum section: electronics / general

author: paulca
author_rank: Super Contributor
timestamp: July 22, 2025, 12:02:53 pm
content: So for the first time I bought a paste stencil with the PCB, applied my paste, place all SMD components and gave it a full heat profile cycle on the hot plate. Soak heat to 150C for a few seconds, then ramp to 230C as per the datasheet. 2 minute timer on reaching 185C or "flow". Timer expires right about the 230C mark on the hot plate. Initially cooled in place with a light fan to 180C and then removed. No poking or intervention.Mixed results.Every passive, crystal, LED, regulator perfect. No rework required.STM32F411 LQP48 thin pitched? Trashed. Bridged on all sides, required about 4 or 5 applications of solder wick to clean it up. I think it had too much paste.A similar result on the USB socket, one bridge due to too much paste that luckily came out with the iron and flux as I had no chance of getting the wick in there. It had two attempts and if it failed on the second attempt it was coming off and a replacement hand soldered.So what did I do wrong with the paste applications?Board was lined up okay. I put a good thick line of paste down one side and smeered across. I think the failure is here and it's just a matter of "skill and/or technique". I was hoping a single pass each way with the card would do it and the slots would auto matically fill up like magic. That isn't what happened. It took me half a dozen swipes to figure out you need to "smeer" the card down almost flat against the stencil, not just drag the edge across.I think what happened is that by the time I had finished covering all holes the IC pin holes had had tool much solder "pressured" into them and so it has squeezed out underneath.I do have enough components to repeat the entire board, but I don't have a need, nor want for two circuits.Instead I might see if I still have one of those cheap SMD IC soldering kits in a box somewhere and see if I has a LFQP48 footprint and throw away chip I can use the stencil on.Don't want to use $6 MCU for solder practice!

author: jmelson
author_rank: Super Contributor
timestamp: July 27, 2025, 11:31:25 pm
content: First rule is that stencil aperture is not equal to pad size. This is especially important for fine pitch parts. You can get away with this on 0805 passives and above, and for SOIC parts with 0.050"/1.27mm lead pitch. But, anything with finer pitch needs to have the apertures cut way down. I often cut them down to 50% smaller area than the pad. I also cut the pads on the board thinner that the component libraries in the CAD software have, often cutting the width of the pads down to just a LITTLE wider than the component leads. I have developed these rules over many years of making boards. Also, I usually get the thinnest stencils they provide, such as 0.75 - 0.8 mm thickness. This reduces the amount of solder applied to the board. Finally, GOOD solder paste is critical! I am a HUGE believer in GC10 paste, there is some real "secret sauce" in their flux recipe that vastly reduces solder bridging.Solder smeared on the underside of the stencil will cause excessive solder on the board, as you guessed. I also "wipe" the top of the stencil with the squeegee vertical to remove excess paste on the top of the stencil. I have no idea if that affects the amount of solder on the pads, but I just do it anyway.Jon

author: SMTech
author_rank: Frequent Contributor
timestamp: July 29, 2025, 08:25:52 am
content: One single pass with a squeegee the right width for the board, not overhanging, angled at 60degrees with even firm pressure. The paste should roll in front of the squeegee and the stencil should be pretty much clean after it has past over. LQFP48 should be pretty easy, even with 1:1 pads:aperture, 1 or 2 easy to clear shorts for a poor print.You start with a well mixed strip of paste at one end the width of your board. the stencil needs to be big enough that you start and end away from your apertures and it must not be able to move, at all. You also need to be able to lift it way cleanly in one nice movement. If you have floppy stencil, good luck and stay small.

author: paulca
author_rank: Super Contributor
timestamp: July 29, 2025, 09:42:29 am
content: So like all soldering, it's about refining the technique. No surprise then, no 'free' wins, just easier in some ways harder in others, horses for courses.Thanks for the tips.When you say "Squeegee" is that a sponge "carrier" or just a plastic spreader/spatual/credit card?

author: meshtron
author_rank: Regular Contributor
timestamp: July 29, 2025, 04:29:41 pm
content: Quote from: paulca on July 29, 2025, 09:42:29 amPlastic or metal spreader - straight, rigid edge.

author: jmelson
author_rank: Super Contributor
timestamp: July 30, 2025, 01:01:26 am
content: Quote from: meshtron on July 29, 2025, 04:29:41 pmI generally use a metal (brass or aluminum) sheet as a squeegee. I think it better presses the stencil hard against the PC board to prevent paste from leaking between the stencil and PCB. I have used an Ikea shower stall squeegee, too. See pic below.https://pico-systems.com/images/Stencil.JPGJon

author: Kean
author_rank: Unknown
timestamp: July 30, 2025, 01:26:25 am
content: I sometimes use a plastic "credit card" for small PCBs, but normally I use a stainless squeegee.Here is an example of what I use - I have several different sizes for different and tend to use the smallest one that suits the stencil apertures.https://vi.aliexpress.com/item/1005008373875924.htmlI can't vouch for this seller, but there are many other sellers selling a similar design.

author: Psi
author_rank: Super Contributor
timestamp: July 30, 2025, 01:38:32 am
content: My advice is1) Press down really had with the Squeegee and move slow, around 10-20mm/sec2) You want to get it done with 1 wipe of the Squeegee. Don't keep going over it again and again or you risk pushing the paste that's already in the stencil holes under the stencil and that will destroy the nice crisp and clean solder paste transfer.3) Use cold solder paste. Once it warms up it gets runny and makes a mess. I always got much better paste transfer with paste I had just gotten from the fridge than paste that has warmed up to room temp. This is backwards to what some people say but it's just my experience.4) If you're getting bridges you are applying too much paste or it's not an even amount across all the pins.5) The paste quality matters, *Cough* Chipquik sucks *Cough*. Well.. i shouldn't say it sucks outright, I just found it very temperamental and random in quality from one batch to another. When i bought Kester it just worked much better.It takes a bit of effort to optimize everything to get good results with zero bridges. Best I could do was 1 or 2 bridges on every 4th board or so (on average) usually just requiring a bit of flux and touching the pads with an iron to clear the bridges.But it will depend on how fine pitch your parts are. Also if you're doing really fine pitch stuff you may need paste with smaller ball size. (The T number, eg T3 T4 T5)

author: SMTech
author_rank: Frequent Contributor
timestamp: July 30, 2025, 07:28:55 am
content: Quote from: Kean on July 30, 2025, 01:26:25 amThese are fine, the blades aren't as nice as the ones designed to go in a printer (bit rough and scratchy on the edges) but they get the job done.

author: SMTech
author_rank: Frequent Contributor
timestamp: July 30, 2025, 07:39:21 am
content: Quote from: Psi on July 30, 2025, 01:38:32 amIf your country flag is accurate, you have 3 great paste options made right here in the UK. Warton P2010, Solderking P2-5 & BLT LFS-UFP-T4. All of these cost us ~£50gbp/per 500g pot direct, and we are not a volume buyer. 3rd party stockists such as Somerset solders or Kaisertech charge about double. I'm less of a fan of Qualitek pastes which leave a more noticeable yellow post reflow residue and don't last as well. None of these are quite as ambitous as GC10 for storage but I can have a pot in daily use for 3-4 weeks if I need to.

author: Psi
author_rank: Super Contributor
timestamp: July 30, 2025, 08:13:33 am
content: Not sure what you mean. My country flag is New Zealand.I assume you meant to quote paulca.

author: SMTech
author_rank: Frequent Contributor
timestamp: July 30, 2025, 09:02:21 am
content: meant to quote you and tag PaulcaYou can get little hints from plenty of online sources such as pages like these @ Tecanhttps://tecan.co.uk/smt-stencils-and-and-squeegee-blades/The Youtube manual printing examples from Tech "influencer" types are universally terrible. I can only assume many of them have never been anywhere near the process in a commercial setting unless it was with their eyes shut.

author: Psi
author_rank: Super Contributor
timestamp: July 30, 2025, 10:33:02 am
content: Quote from: SMTech on July 30, 2025, 09:02:21 amI have no problems getting paste, I can order lead based paste from Digikey. It's not like the EU were everything lead based is illegal.

author: paulca
author_rank: Super Contributor
timestamp: July 30, 2025, 10:38:48 am
content: Quote from: Psi on July 30, 2025, 10:33:02 amI did get some. I had to order it from Digikey and get it sent from Texas though.I bought QuickChip though.I have 2 year old "Mechanic" leaded. QuickChip or some random Aliexpress stuff to pick from now.I even bought 2 tiny tubs of the Mechanic stuff while it was available, specifically for stencil use, then forgot and left it in the fridge.

author: Kasper
author_rank: Frequent Contributor
timestamp: August 04, 2025, 04:52:26 pm
content: Quote from: paulca on July 22, 2025, 12:02:53 pmI usually try to build at least 2. Helps determine if problems are with the design or the soldering.Practice chips help. Can also practice squeegy with no chip. Do one pass, lift stencil, if it doesn't look good, clean it off and redo it.Sometimes I use an exacto knife to remove some paste after lifting the stencil. It's probably better to make smaller paste aperatures in the first place.Tape a DMM thermocouple to a blank PCB near the one you are soldering and record the temperatures to check the boards are close to the profile. With that, I turn the oven on full power, wait a bit, turn off, repeat then open door. Record your on/off times and temperatures to dial it in.Just try a few different things, you'll get it. You can do a lot of things wrong and still get decent results. I don't reflow often, maybe once a year or 2. I've had good results even with expired lead free chip quick, paste aperature = pad size = datasheet recommendation, multiple passes with leftover PCB as squeegy, in an un-modified toaster oven I bought used for $20.