WORKSHOP VIDEO #59: 946A+ solder reflow hot plate

This week we take a look at the 946A+ solder reflow hot plate from our last Shenzhen equipment haul. There’s a ton of no-name 946A+ out there, ours happens to be D-GOLD. They’re all pretty similar, and run about $50 in Shenzhen.

Using a kitchen hot plate is a common hack for DIY reflow soldering, even SparkFun has used them. This is the industrial version, actually intended for reflow soldering and board preheating. The difference is the big block of metal surrounding the heating element, it has more thermal mass and and spreads the heat more evenly than a lightweight kitchen hot plate can.

Many people prefer these to a reflow soldering oven because the PCB is exposed and you can tweak the component positioning on the fly. The downside is that the solder paste heat profiles can’t really be adhered to, at least without constant attention. Our strategy is to preheat the board to a lower temperature, then ramp it up to let the solder melt.

To demonstrate how it works, and compare to the reflow oven demo, we’ll use Sjaak’s RGB controller PCB as a test. We applied the solder paste with the solder paste dispenser, and manually placed the hand-full of parts on the board.

For more than one board you might speed things up with a solderpaste stencil. Generally the board and stencil require some kind of frame and unwieldy rig, but the Stencil8 board is a great alternative. Thanks to Arachnid Labs for arranging it.

We’re using ancient Mechanic Paste solder paste, a standard lead/tin mix. We’ll borrow from the reflow oven profile and start off at 180C. After the plate warms up the PCB goes on. After a few seconds it warms and the flux starts to activate. Now we crank the temperature to 240C and watch for the grey solder paste to melt into shiny solder.

When the plate reaches 240C we turn it off and let the board cool naturally for 10 minutes, like the natural cool phase of the reflow oven. After 10 minutes the board comes off, slightly charred. We probably cooked it a little too hot and long, but the result is perfectly usable.

This is way more involved than the reflow oven, and there’s always the looming hazard of an exposed hot metal plate to fall into. Overall we still prefer the reflow oven for one-offs, but definitely see the appeal of the hot plate in a small workshop.

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    1. I imagine oven… I have seen a video of someone doing a double sided board in an oven, but it takes two reflow sessions. One for each side.

      You might want to make sure you use an adhesive of some sort for the components on the bottom.

  1. I devised a simple jig to lift the board off the hotplate once done. if you look at the graphs of the temperature profiles with and without liftoff, you’ll see the thermal inertia of the hotplate is a problem if the board is left on the hotplate. see link for details

    1. Wouldn’t a hot plate dedicated to soldering rework be designed to cool of according to the right profile?

    2. Erich, I think I saw that on Hackaday a little while back. Cool project :)

      I just picked up a dual hot plate – because it was $20 at a local Thrift store and seemed barely used. I need to pick up an IR thermometer or use my meter, thermo-couple and Kapton tape to get some data before I try another test. My first test worked, but I was free wheeling it and it reflowed faster than I was expecting.

      One of my original ideas was to use a small elevator type setup that would raise and lower a tray with the pcb on it to help control the temp… that seems a little much, though. Your idea is more practical at this point.

      1. I too thought about automated lifting of the plate, or a platform lowering the hotplate, perhaps pneumatically with an inner tube and aquarium pump to ensure even contact with the hotplate… in the end, I bodged the liftoff jig up in about 10 minutes with spare bits and pieces lying around and haven’t seen the need to make it any more complicated. As someone said on hackaday for someone’s PID controlled setup, once you now the timings for your hotplate, you don’t really need anything fancy controlling it. I have had good results even with dsPIC reflow soldering.

  2. The stencil is a huge plus, even if you are only doing 3 or so boards, manually applying solder paste in the right volume is a really time consuming process and if done wrong, easily leads to bridging after reflow. $25 for a kapton stencil isn’t a bad price to avoid all that.

    Course, if you are doing a medium to large board with a number of BGAs, a steel stencil is really a must have, even for one offs.

  3. How much did the hot plate cost and where can we get one?

    We need to heat up mobile phone LCD assemblies to separate the digitizer from the LCD screen.

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