Reflow soldering, adding components - Page 1
Forum section: electronics / beginners

author: davegravy
author_rank: Regular Contributor
timestamp: December 23, 2021, 02:22:03 pm
content: I'm looking to manufacture my first pcb using the reflow technique I've seen some youtube videos demonstrating. I've hand soldered boards before but this is my first design that uses a bunch of DFN and QFN packages that have exposed pads so it seems reflow is the way to go.From what I've seen you order a stencil which you place over your pcb to apply paste just to the pads, place your components on top, then bake in a toaster oven until solder flows.Can this be used to easily add components after the initial bake in the oven? Some of my components have a couple month lead time due to supply chain issues but enough are available now that I can do some testing and add new components as they arrive?If my board needs redesign is it practical to flip the board upside down in the oven and cause all the components to drop off so they can be resoldered on a new pcb?

author: DavidAlfa
author_rank: Super Contributor
timestamp: December 23, 2021, 02:33:53 pm
content: Flipping the board will be ok for small smd parts (resistors, capacitors, qfn/soic/bga packages) as long as you handle it with care.Reflowing isn't hard, however if the heating goes too fast, the paste might boil and move the parts.If your parts have some age, bake them first at 100ºC for few hours to remove moisture, or they might crack (Pretty much like popcorn). Same for the boards.2-3 minutes reflow time is usually enough.If it's your first time with this method, it's easy to cause a little mess, but don't worry, eventually you'll get it done.Check for solder shorts, specially with QFN packages, these love the bridges!Thorughly mix the solder paste before use, the solder tends to separate from the flux and pour to the bottom, you'll get either too "fluxy" or too dry paste if not taken care of.

author: Siwastaja
author_rank: Super Contributor
timestamp: December 23, 2021, 03:05:15 pm
content: Adding components is basically impossible as the already placed components prevent the stencil contact and thus, paste printing. Just wait until you have all the parts you need and do it in one go. Easily hand-solderable parts of course can be added later.Nothing prevents you from practicing, though, just use easily available parts to do that. Any part with the same footprint works.Avoid double-sided load unless you have to do that. Small components stay in place with surface tension of molten solder. Still, if you have just a few random bypass caps on the bottom side, you can just hand solder them in place.

author: rstofer
author_rank: Super Contributor
timestamp: December 23, 2021, 08:31:11 pm
content: I often put a bunch of decoupling capacitors on the back side with hand soldering but that's about it.I have never used a stencil, they haven't been available through ExpressPCB until recently. Even so, I get by with just dabbing solder on the pads and cooking the board in either my converted toaster oven or on a flat top hot plate. The hot plate is faster! The toaster oven controller makes an attempt at controlled ramping but, realistically, there isn't enough heat available even when the walls of the oven have been stuffed with high temperature insulation.In the end, it all works out.I don't mind drag soldering so sometimes I avoid reflow.Of the 3 methods, I like the hot plate best. I can get right down in there and watch the solder flow. This only works because the ExpressPCB MiniBoard is fairly small (3.8"x2.5") and will fit easily on the plate. Some folks add a circular saw blade as a heat spreader. I haven't tried this.

author: thm_w
author_rank: Super Contributor
timestamp: December 24, 2021, 01:00:08 am
content: Quote from: davegravy on December 23, 2021, 02:22:03 pmYou can add components after the initial bake yes, but you will want to use a hot air gun.Depending on whats already on the board you may want to preheat or shield surrounding areas.The initial stencil will have already laid down solder on the pads, clean area off with isopropyl, apply fresh flux, and reflow the new component.

author: JustMeHere
author_rank: Frequent Contributor
timestamp: December 24, 2021, 02:29:37 am
content: I started with hot air reflow gun and paste in a syringe. This let's you work a section of the board at a time. It's much easier to troubleshoot.Get a good loupe. You will need it for inspection.

author: davegravy
author_rank: Regular Contributor
timestamp: December 24, 2021, 05:51:33 pm
content: Quote from: Siwastaja on December 23, 2021, 03:05:15 pmUh oh, I have a DFN-8 package and a MSOP-E-8 on the bottom, not just small passive components. I did this only because it was in the Analog Devices reference design I copied from. Is this going to be tricky enough I should consider adjusting the layout?

author: Siwastaja
author_rank: Super Contributor
timestamp: December 24, 2021, 06:03:04 pm
content: Well, if it was me, despite giving opposite advice, once you are at this point, I would just take the challenge and try it anyway. These parts have low mass in proportion to the surface area of all the pins, so maybe they are designed to be held in place with the surface tension of molten solder, i.e., without gluing, I don't know. Reflow the bottom side parts first, then do the top side. While reflowing top, be very careful not to touch the board at all.Also tinning all pads with just solder (not paste), or applying paste with a syringe, and then local reflowing is always a possibility like mentioned by others, but I do this only when I have to, i.e., for debugging / repair, when already existing parts prevent paste printing. Obviously you can use the same pattern to add component later. But I won't recommend it because using proper stencil gives the optimum result and best reliability because a well controlled amount of paste is correctly placed at the pads, this is why I prefer to get everything done in one go.

author: Peabody
author_rank: Super Contributor
timestamp: December 24, 2021, 06:46:10 pm
content: I did a video on the hotplate with circular saw blade method mentioned by rstofer. Parts total about US$20, and they may be available to you in Canada. It's the $10 Walmart hotplate, a saw blade from Home Depot, and some screws from Ace Hardware. It operates strictly based on time - on/off/on/off for specific time periods, with no thermocouple. The time periods shown in the video are for leaded solder paste. The picture below shows the profile produced. This would be for single-sided parts only.Edit: As will be obvious in the video, you should use a stencil. I've found it's almost impossible not to put too much paste on a pad.

author: Siwastaja
author_rank: Super Contributor
timestamp: December 24, 2021, 06:56:25 pm
content: Contactless IR thermometer is a good investment, you can easily look at the PCB surface temperature. Luckily, the green solder mask material gives decent IR readings (unlike the shiny pads, but their area is usually negligible).Do a few dry runs with bare PCB to get an approximate idea when you have to turn the heater on/off. Learning to do this by hand makes more sense than investing time to build and tune a PID controller, IMHO.Or, you can do a combined plate + hot air like I do, with hot plate offering "base level" of heat which is never too much, and the hot air gun gives the more controllable fast rise when you want to go from preheat to soak and finally reflow. I use a 3D printer heat bed at around 130degC for this purpose. Hot air is what does the job, but it's made easier by the preheating coming from bottom.

author: davegravy
author_rank: Regular Contributor
timestamp: December 24, 2021, 07:48:14 pm
content: Quote from: Siwastaja on December 24, 2021, 06:56:25 pmGreat I have a good IR thermometer already.Quote from: Siwastaja on December 24, 2021, 06:56:25 pmHow does this work, when you're doing the second side using a hotplate the bottom will be supported by the already soldered components so it won't be making consistent contact with the hotplate and I imagine you get very uneven transfer of heat to the top side of the board you're wanting to solder with hot air. Or do you elevate the board above the plate somehow so the underside is being heated by convection rather than conduction?

author: Siwastaja
author_rank: Super Contributor
timestamp: December 25, 2021, 08:19:31 am
content: I use temperature resistant white silicone-glassfiber sleeve, you know that kind that is used for additional insulation of wires in hot environments etc., 5mm diameter, cut in pieces supporting the PCB on the top of the heat plate. The idea on this for me is that air can circulate a bit under the PCB thanks to convection. I don't know if this is truly necessary. But I very rarely do a 2-sided load (or if I do, I just hand-solder the bottom side parts later). But you could use the same idea, still; cut the spacers small enough and place in spots where there are no components. For example, four pieces, one diagonally at each corner would work if there is even 1 sq cm with no components there.In the professional world, assembly panels always have some extra edges around the actual PCB for mounting, connected with "mouse bites"; even if it's just one PCB per panel. Then, after everything's built and tested, the board(s) is/are cut out of the panel.