Stenciling and First Bake

I’ve been a bit busy for the past few weeks in the professional sphere, developing firmware, evaluating hardware, and seeking out new projects. As a result, I have quite the backlog of topics I want to discuss. Today I’m going to follow up on the previous post about building a reflow oven, showing the first bake and results.

I had a remaining bare PCB from the Wireless RH/Temp Sensor Module project I’ve been working on, which turned out to be an excellent test for the oven. Knowing I would be doing this, I had already ordered a kapton stencil from OSH Stencils, as well as the acrylic jigs that they offer. They shipped with a nifty credit card blank for spreading the solder paste.

OSH Stencils spreader and jigs.

OSH Stencils spreader and jigs.

 

The pricing is very reasonable and for low volume projects, you can’t beat it. These should last for <100 boards, but if I’m doing anything >10 pcs., I would have them done somewhere else. The next step up would be a stainless steel stencil, which is still relatively affordable and could handle 1,000s of boards.

For solder paste, I got a 50g jar of lead-free from Sparkfun. It’s currently sitting in the fridge next to the butter.

The application process can be a bit tricky. The key is to make sure the stencil lines up with all of the pads in both the horizontal and vertical. It sounds simple enough but it can be a pain when there are portions of the stencil that are raised because they are curved a bit when you receive them. This can prevent you from seeing how well a cutout lines up with the pads. You can see how curved the stencil is in the photo.

Applying the stencil to the jig.

Applying the stencil to the jig.

Applying the stencil.

Applying the stencil.

I just used painters tape to hold the stencil in place, on all four sides. This also served to keep the whole thing together, in case I had to move it while I was working.

No photos of the application process, but it’s fairly simple. You have to make sure that you work the paste into all corners of each cutout. The easiest way to do this is to spread it from multiple directions. You will get some smudging underneath the stencil, due to it’s lifting – but it’s okay. You don’t actually need a lot of paste, considering the thickness of the stencil and the size of the cutouts. However, the result of this is that when you are placing parts they don’t feel like they are sitting in the paste very well. Rest assured, they are. Just make sure to cautiously place them, and make sure at the end you haven’t bumped anything out of place from earlier.

You have to also be fairly quick with the placement. I think the general rule is that you have a few hours with the paste exposed to the air before it starts becoming ineffective. I’ve never had a problem in the past letting it sit while I did something else – but it’s best to be quick about it.

Solder paste applied.

Solder paste applied and components placed.

Once the paste was on, I placed the parts methodically by component type, starting with the ICs. Precise placement isn’t super critical, but you will find that if you don’t center the passives you will get terminals separated from pads and tombstoning. On the ICs, you should try to get as close to centered as possible. I placed all of the parts except for plastic connectors, pushbuttons, and header pins. I would put these on later by hand to avoid melting them.

After placing the components, I carefully moved it to the tray that I would then place in the oven. I tried getting the thermistor as close to the board as possible but the wire was rigid and stubborn. I wanted to put it in one of the screw holes but it just wouldn’t do it and I ended up knocking some parts in the process.

Board ready to bake.

Board ready to bake.

 The bake process was incredibly simple – just making sure the shield and oven were powered and hitting the go button. I’ll admit I watched during the reflow phase, which is always incredible seeing the solder transition. It happened around 230C. The cool-down phase takes forever, and someone actually told me that on their oven they open the door at a specific time to better mimic the JEDEC solder profile during cool-down. I just let it go and worked on something else until it was done.

BAKING.

BAKING.

Soldered PCB.

Soldered PCB.

The end result looked pretty good. I had four passives that I had to touch up but nothing major.

I actually had someone contact me about a similar project using the same B&D toaster oven with a custom controller. The founder of the CountrolLeo2 Kickstarter, Peter, reached out to me to mention his project. I wish I would have known about it before I built this one – I would have at least gotten the controller. But as I told him, I needed something quick and didn’t really want to wait. Still, I think it’s a great build and he goes into some extensive detail about the process here. For what it’s worth, I still backed it with $25 because I like to help smart people get their products to market.

Building a Low-Cost Reflow Oven

Whenever I have a project that I’m working on that involves custom hardware, personal or professional, I have typically assembled PCBs by hand. It takes time, and can be tedious. In low volumes (<5 pcs.) it isn’t too bad. I could send them out to have them populated but this can get expensive, especially for low volumes. If you research reflow ovens or hot air baths, they can be quite costly and I couldn’t justify purchasing one without doing it all the time. But, there are other options.

I had heard about people converting toaster ovens into cheap reflow ovens, so I decided this would be the way to go. I did a bit of research and stumbled upon a few videos showing how it’s done. First from The Ben Heck Show:

That one was a little more complex than I really wanted to get, but the basic principle is the same. I also found the following video from R&TPreppers:

This one was a bit simpler, so I decided to follow it as a baseline for the build. I went ahead and got the RocketScream Reflow Oven Controller Shield, the solid state relay, and various other build materials like the fiberglass weave, pop riveter, hardware etc. (all from McMaster Carr.) I already had a thermocouple that I was planning on using in a temperature controller for my smoker. It turns out that the one I had was actually problematic, but more on that later. I eventually got one from Adafruit.

I also purchased the toaster oven from Amazon. I’m not sure why the price has gone up so much since, but I bought it at $28.00. It’s a fairly cheap one with quartz elements. Below are a few photos of its unboxing.

Black and Decker Toaster Oven

Black and Decker Toaster Oven

Unpacking the oven.

Unpacking the oven.

Unpacking the oven.

Unpacking the oven.

Getting everything together, I would try to emulate the R&T build as much as I could. I knew I didn’t want the electronics inside the enclosure because I didn’t want to deal with cutting display holes in the front panel. I decided to mount the electronics on the outside of the unit, with the understanding that I would need to protect the live terminals on the SSR. I have yet to do that, but I could easily make a plastic guard or something similar. For now I just leave it unplugged when I’m not using it.

Solid State Relay

Solid State Relay

Arduino Uno + RocketScream Reflow Oven Shield + SSR

Arduino Uno + RocketScream Reflow Oven Shield + SSR

The RocketScream Shield required adding headers, as a lot of shields do. I ordered the headers from Mouser in the correct pin count. I always hated cutting the breakaway headers – they end up not cutting very well.

Assembling the RocketScream Shield

Assembling the RocketScream Shield

Next I gutted the oven. B&D used anti-tamper torx screws on the bottom of the case. I have torx, just not the ones with the pin hole in the middle. Nothing a pair of pliers can’t remedy. I’ve already voided this warranty so I don’t particularly care at this point.

Disassembling the oven.

Disassembling the oven.

Disassembling the oven.

Disassembling the oven.

Taking out the guts.

Taking out the guts.

Stripping it down.

Stripping it down.

Once I had the oven dismantled, the next step was to test the shield. I snagged the MAX31855 library, the PID library, and once I loaded up the example sketch from the repo provided by RocketScream, the thermocouple registered the correct temp on the display. I first had it hooked up in reverse and heating it up caused the reading to decrease. This would have been very bad in the oven, causing it to just continue heating.

Testing on the bench.

Testing on the bench.

Then I made patterns on the outside of the case for the Arduino and SSR hole patterns and drilled some holes in the enclosure. I also added a hole for a grommet, to safely guide the wiring in and out of the unit.

Drilling holes

Drilling holes

I made an internal hole for mounting the original thermocouple, which was threaded with a nut. I wasn’t sure how it would perform compared to the example projects I’ve seen from others. What I found was that because the shell of the thermocouple is electrically connected to the case, when I first tested the unit – the SSR switching on and off caused a lot of noise in the thermocouple measurement. The conclusion was that one side of the thermocouple must be electrically connected to the shell. Testing with a DMM confirmed this. Rather than trying to make this one work, I just got the glass-bead type and shoved it through the hole I already made.

Installing the thermocouple.

Installing the thermocouple.

Inside thermocouple.

Inside thermocouple.

Next I installed a fiberglass blanket on the inside around the perimeter of the heating case. I bought a riveter to do this, and I don’t know why I never had one before – it’s an awesome tool. I’ve used them in the past but forgot how useful they are. Rivets are cheap. The tool was a little pricey but I feel like I’ll use it a lot. I had to cut holes in the fiberglass to accommodate the components and wiring, which was really easy because of the nature of the fiberglass weave. I riveted it in four spots on each side, and it stayed pretty snug.

Pop riveter.

Pop riveter.

Fiberglass insulation.

Fiberglass insulation.

Installing the fiberglass.

Installing the fiberglass.

Riveting the fiberglass.

Riveting the fiberglass.

Next came the wiring. The elements are in parallel, and I had to make a couple patch wires, but I ended up using just wire nuts for the connections (with the exception of one which I soldered.) I included the original light from the front panel in the wiring because – why not? The thermocouple leads and the broken hot side were fished through the grommet on the outside, where I connected them to the shield and SSR, respectively. I made a short jumper from the shield to the SSR and the wiring was done.

I only had an external DC wall-wart for the Arduino power, which I just plugged in separately.

Rewiring the heating elements.

Rewiring the heating elements.

Installing the SSR and Arduino + RocketScream.

Installing the SSR and Arduino + RocketScream.

Wired up.

Wired up.

Ready for first test.

Ready for first test.

Finally, I tested it. After replacing the thermocouple it ran great! The Arduino will output data points as it’s running, but I haven’t looked at the curve data yet. Watching it, however, the temp ramp and values seemed to be correct. The cool-down period does take a while, and I have heard of others opening the door at a specific point in the cycle to better mimic the JEDEC curve.

Testing the oven.

Testing the oven.

I have already done a single board for the Wireless RH/Temp Sensors in this oven that I will post about later. For now there are some changes that I will probably make to the system including:

  • Putting aluminum foil on the inside of the door with a viewing slit, to retain the heat better.
  • Making a guard for the SSR to prevent anyone from getting shocked.
  • Installing a dedicated DC supply in the unit for the Arduino.

Seriously, Arduino?

I’m in the process of converting a toaster oven into a reflow oven, and I opted to use the Arduino Uno + Rocket Scream Reflow Oven Controller Shield as the PID controller.

So I’ve disassembled the oven and went to mount the Uno on the outside using the four mounting holes which are meant for 4-40 screws. I noticed that when I installed the standoffs on the enclosure of the oven that one of the standoffs would likely short to the AREF connector underside pins.

What’s worse however, is that on the same hole, the head of a standard 4-40 screw INTERFERES with the AREF and 10-pin socket header.

4-40 screw hole where the head won't fit.

4-40 screw hole where the head won’t fit.

Standard 4-40 screw in screw hole - head doesn't fit.

Standard 4-40 screw in screw hole – head doesn’t fit.

Screw doesn't fit here. PCB fail.

Screw doesn’t fit here. PCB fail.

Really? That’s a pretty big oversight to include screw holes for 4-40 screws and not check if the screw head would interfere with the placement of other components. PCB layout fail, Arduino.