How To Populate A Surface Mount PCB


Let’s face it friends, everything is moving toward surface mount components. We’ve seen quite a few features here that cover using stencils to populate boards and using ovens to reflow. [Oleg] has put together a tutorial on the process he uses to populate and reflow his own boards.

[Oleg] is the creator of the USB Isolator and therefore has a need to frequently populate the same board. He’s using an acrylic frame that fits the PCB perfectly to hold it in place so that paste and be applied right up to the edges of the board. He ordered a laser cut Kapton stencil for applying the solder. The paste is squeegeed into the stencil holes, the stencil is removed, and parts are placed with tweezers and a steady hand. For the final step, the boards go into an old toaster oven for reflow.

[Oleg] uses temperature marker on his boards to monitor the progress of the reflow. This marker is basically a crayon that begins to melt at a specific temperature. When the board has cooled, the melted mark can be scraped away or removed with alcohol.

Of course this is only really useful if you have a bunch of high-quality boards to populate. But with the relatively low cost of getting professionally made boards we think the need for this type of assembly process is on the rise.

19 thoughts on “How To Populate A Surface Mount PCB

  1. I have no clue as to how to this but would it not be possible to place surface mount components through the stencil for corect alignment, or would that be a stupid thing to do?

  2. This is the same basic way I’ve been doing this for about 12 years now. Toaster ovens work nice because with just a little bit of experimentation you can find the point where when you place the boards in the oven the temperature ramps ups slowly over about 30-60 seconds until it gets just to the point of flowing all the solder paste nicely. This helps to prevent some of the thermal shock (the same thing he is doing by baking at just under 400 for five mins) that can prematurely shorten the life of the components. Having also worked with production grade conveyor belt fed reflow ovens I can tell you that the 5 mins is a bit more than is common or necessary for warm up. This process is good enough to make very low noise small signal amplifiers (less than 1 dB introduced noise) that last for years under constant daily use.

    Advanced Circuits ( offers laser cut stainless steel stencils for a very reasonable price (I think less than $100 for a small board if I remember right from the last one I bought and they come with a small stainless squeegee too)if you are planning on making lots of a certain board (I usually make 50-100 at a time). The steel ones are nice because they do not flex an produce much more uniform pads. Sometimes the kapton ones flex and pull up some of the solder paste when you remove them.

    Nice to see someone getting out an instruction set on this method though, good job :)

  3. and for those who think a hoven is expensive, i did baught mine on ebay for about 300€, brand new with a nice grafic lcd to display the curves.

    ok the user manual is in chinese->google->english,
    but you do not need it.

  4. I agree the paste/stencil method is worthless for your average hobbyist or maker of a prototype board. If you have a really good iron, a good pair of tweezers, and a magnifier, you can hand assemble complex boards nearly as fast as you can reflow them. Only time you should have to touch reflow on a prototype or small scale production run is for BGA, and even with that you can still do it by hand a lot of times!

  5. I’ve done a lot of surface mount assembly, and even for one-off projects I’d recommend a stencil. The main reason is that it’s difficult to regulate the amount of solder paste when you’re just dabbing it on. With a stencil, you get a nice smooth layer and the risk of dry or bridged joints is greatly reduced.

    Of course, it helps if you have access to a laser cutter. I’ve been using the laser cutter at the Menlo Park Techshop to cut stencils out of 2 mil mylar sheet from TAP. I also cut holders for the PCB out of acrylic, as seen here:

    Last weekend I had to assemble 150 PCBs and technique really paid off:

    The toaster oven method is way too slow in my opinion. I used a cheap hotplate from Target, set between Low and Warm (these things get really hot). That was enough to get the solder a little ways below the phase change temperature. Then, I used a hot air reflow gun (Hakko 850) to warm up the components enough to melt the solder. After removing the hot air, the solder would harden and the PCB was safe to remove. Just using the hot air was too slow, because the PCB had a lot of heatsink area. Preheating with a hot plate works a lot better.

    We actually did the soldering live on USTREAM. For some reason we recorded three hours of it, but if you check at 00:18:00 and 01:45:00 you can get a good view of the process. There’s a microscope cam of the stenciling and soldering.

  6. On that video you can see some of the plastic on the SMD LEDs bubbling when I applied a little too much heat. That level of melting won’t affect the performance, but I did have to dial back the hot air gun and turn up the hot plate a little. Having some form of magnification really helps identify problems like that; a jeweler’s loupe, stereo microscope, or one of those magnifying visors.

  7. Sparkfun have got some excellent tutorials on smd work, iron, hot air, oven and SKILLET….

    Theres a couple of people producing small boards for the toaster ovens that give an LCD, heat curves/profiling, an array of temperature sensors for getting it right, for about £$30 (yup, thats 30quid or 30 dollars, it’ll hit £30 by the time it gets to the UK). Just add it to the right domestic toaster oven.

  8. Yeah the Sparkfun guys actually recommended the skillet/hotplate above the toaster oven and even a cheap reflow oven.

    The reason being that some ovens get too hot and can melt plastic components such as smd usb sockets etc.

    Hotplate work great in my experience, just be sure that it doesn’t get too hot.

  9. @ DaddyStop: (disclaimer:) This is complete speculation and someone with hands-on experience should comment on this, but (disclaimer end) perhaps melted solder surface tension would keep lightweight components in place even when they are upside down, if they were already soldered, that is. However, nevertheless you would have to make two runs, one per each side, new components up, previously soldered side down. Heavy components should be hand-soldered or placed on side that gets soldered last. Also, a frame to prevent down side components from touching anything should be a must.
    Alternatively, (industry’s approach) high temperature glue paste to keep down side components in place should allow you to do both sides in single run.

  10. @ crnobs:

    Thanks for the reply, I am kinda new to all this proto’ing stuff so there are still thinks like high temp glue that I don’t think about or know exist.

    I was also thinking some sort of high temp plast could be melted as a “template” and fixed to one side so that even if things did get hot enough to melt solder, the template could keep the components in one spot. Just an idea, dunno how viable it really is.

  11. As crnobs says, surface tension and high-temperature glue are used for double sided reflow. The third technique I know of is to use solder with a higher melting point for one side and a lower melting point for the other, so you can reflow side 2 without re-reflowing side 1. This requires better temperature control of your reflow setup than a basement hobbyist is going to have, though.

  12. @Wim L
    You said: “This requires better temperature control of your reflow setup than a basement hobbyist is going to have, though.”

    …Shouldn’t have to be. In the plastics injection molding biz we do precise temp control all the time. You need:
    – PID temp controller (buy it off eBay for <$50
    – thermocouple
    – any heat source you like.
    – if the heat source draws more current than the temperature controller's output relay can switch, hook it up through a relay or SSR.

    You can get very accurate control of your toaster oven, hot plate or what-have-you and it's quick and easy. I wouldn't do it any other way.

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