Small-scale surface mount reflow

While building the mPrinter, we’ve assembled a lot of prototypes.  While it’s certainly possible to solder 0402 and 0603 surface mount components by hand, it’s not a lot of fun.  If you don’t want to hand solder, there are two main ways to do small-scale reflow: an oven or a hot air rework station.  We went with a small oven, and this post goes over a bit of the experiences we had.

Picking an oven


In concept, reflowing is simple – you want a controlled temperature profile over a set period of time.  Manufacturers of solder paste and components generally have information on this profile in their datasheets.  There are two basic ways to go: a commercial oven, or building your own.  A lot of people swear by building their own, but with the price of small desktop ovens available from overseas I found it hard to justify fiddling with building one.  The oven we ended up using is the venerable T-962, made in China and available from various eBay suppliers.

I’ve read a lot of mixed reviews on this oven.  In short, my experience was great.  Every single board I’ve done has come out perfect.  The only issues have been caused by variations in paste approval, and are generally limited to solder bridges on fine-pitch components.

Circuit boards

I use a mix of making my own boards and ordering them for prototyping.  I have the luxury of having a CNC mill, so it’s trivial to drill and etch accurate double-sided boards at home.  The largest inhibiting factor is vias.  There is no easy and fast way to quickly make a large number of vias at home without a plating process.

When you have the time, there are a number of great services to get boards made.  I generally use one of two: OSHPark for small-batch quality boards in about two weeks, and Advanced Circuits BareBones service for next-day service.  This article features boards from the later.

A circuit board from Advanced Circuits BareBones service.

In exchange for quick turnaround from Advanced Circuits, the boards have no solder mask and no silkscreen.  I was happy to find this didn’t cause any reflow issues at all.  The boards are high quality and as noted on their website, shipped the next day.  I had them within 72 hours of ordering.

Solder paste application

The first step of the process involved applying solder paste to the circuit board.  You can use stencils, or just do it by hand.  Just by nature of the board layout and surface tension, paste application is very forgiving.  As long as you generally get the paste amounts right, parts will suck themselves into place and be perfectly aligned during reflow.

The most useful tools in this process, as with hand soldering board components, are: a Panavise , tweezers, and a microscope or loupe.  The Panavise is a small desktop vise that securely holds your circuit board while you work on it.

A Panavise securely holds your board while you work on it.

When you order your solder paste, you’ll probably be surprised to find it’s just a tube of paste.  There is no tip or even a plunger to remove the paste.  Generally, the smaller tubes are 10cc and the larger ones 30cc.  I order my syringes and tips from CML Supply.  I find the 22 gauge tapered plastic tips to be perfect for most applications.

Application of the paste is pretty straight forward.  For passive components with two pads, a small dot on each side is adequate.  For multi-pin parts such as ICs and connectors, you can simply apply a ribbon of paste down the row of pads.  If you get any bridges, they’re easy to clean up later.  Using tweezers, simply place each part on top of the applied paste in the appropriate locations.

A circuit board with solder paste and all components applied ready to go into the reflow oven.

Reflowing the board

The reflow process is pretty straight forward.  You simply board the board in the oven, start it, and come back about 10 minutes later to a completed board.  I use profile 3 on the T-962, which I’ve found to work best for my particular application.  One annoying caveat of the T-962 is the timer.  It can be off by nearly as much as several minutes.  Fortunately, it doesn’t seem to serve any real purpose other than user feedback.

Cleaning up mistakes and through hole parts

After reflow is complete, it’s not abnormal to have a few solder bridges.  Here, solder wick is your best friend.  Make sure to get a good quality one, and make sure it’s fresh and not oxidized or it will be very hard to work with.  I like a small braid which heats up quick and works well with fine pitch parts.

Make sure to use a chisel tip on your iron, or you run the risk of overheating the area prior to the point at which the wicking occurs.  I turn my iron up slightly for this, setting it around 380-400° C.  The photo below shows a before and after of a solder bridge on a TQFP32 IC.

Finished board

While it does take a little bit of preparation, once you have the process in place I find reflowing even a single board quicker than hand soldering.  It really shines in batches of 3-4 boards that are identical.

Below you can see a finished mPrinter revision 3 PCB, with all the through hole parts mounted.  There are some additional headers that have been soldered on that will simply be test points on production boards, but in general it will be identical to this board.

9 thoughts on “Small-scale surface mount reflow

  1. badger

    Hi Andy,

    as for the bridges.. it most likely means that you applied too much of the solder paste. If you plan to assemle more of the same PCBs you might consider to create a stencil for applying the solder paste. It will
    help you to apply the paste way much faster an in adequate amount. ;)

    Badger

    1. admin Post author

      Badger,

      You’re absolutely right. Just a time and costs savings really. When prototyping these in batches of 2-3 boards, it’s not worth $100 on a stencil for a single revision that might change. For production, we’re using an assembly house.

  2. Pingback: How to: Small-Scale Surface Mount Reflow « adafruit industries blog

  3. Mark Moran

    Thanks for the post. Very helpful.

    Which CNC mill do you use? I’ve been successfully etching boards, mostly the Datak positive photo-sensitive ones, but I’m very interested in getting a desktop CNC and hopefully using it to make double-sided PCBs. I’ve heard mixed reviews of a number of them, including Zen Toolworks 7×7/12, Roland MDX-15/20. Can’t you solve the via problem by inserting tiny pieces of wire or even just filling them up with solder?

    I’m also considering using a laminating machine with transfer paper, but that still requires etching. Thanks for any advice.

    1. admin Post author

      Mark,

      I have a Harbor Freight Micro Mill that is retrofitted with steppers. I made my own mounts on a lathe and manual mill. I use CamBam and LinuxCNC. I machined a holder that bolts onto the head and use my Dremel as a spindle for high-speed cutting when necessary.

      If you want something off the shelf, definitely don’t go that route. It took a few years to get it “tuned”. I added a 20lb gas piston on the Z-axis to act as a counter weight. That helped dramatically.

      I’ve found the most important thing to be a good driver. I tried a HobbyCNC, then onto some eBay options, before finally settling on a Gecko 540. I should have just gone with that from the beginning – it’s night and day better than the others.

  4. Felix Rusu

    I use beverage can aluminum and toner transfer with transparency film. Then I etch in muriatic acid. Makes pretty good stencils. I live can aluminum because it’s very flexible and won’t bend easily (versus the copper that will bend easily and ruin the stencil, there’s plenty of opportunity for that while making it). Other toner transfer materials work very well for close to 100% toner transfer and peel right off (no soaking needed)..

    See details here: http://lowpowerlab.com/blog/2013/01/29/diy-metal-stencils-video-update/
    And steps here: http://lowpowerlab.com/blog/2013/01/23/diy-home-made-metal-stencils-step-through/

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