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.

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