Reflow Soldering and Accelerometers
Here’s my random ramblings for my first surface-mount soldering escapade:
I received a solder paste kit from Celeritous, and it came exactly as pictured on their site. Unfortunately, notice there are no instructions in this kit.Fortunately, they gave several hints, and there is so much information available online that I wouldn’t let it stop me from trying to use their solder paste kit. In fact, it is by far the easiest all-in-one solution I have yet found, and they are very kind and ship promptly. I will be doing business with them in the future, and I wouldn’t hesitate to recommend them to anyone. While they didn’t give step-by-step instructions, they did make several references. For example, they mentioned the internal red cap should stay in place while using the plunger, which is helpful as there are two red caps. This implied to me that I should remove the plunger from the manual syringe and place it in the solder paste syringe, which I should use to directly place solder paste. Making that explicit would have been helpful, but I was able to tease this all out successfully.
The first thing I did was take an old board I had lying around from another project and practice dispensing paste, then reflowing it to see what happens. The boards were designed for through-hole components, and I didn’t place any components on the board: I just wanted to see what dispensing the solder paste was like, and what happened during reflow.
It was hard to pick a needle, so I tried a really small one (it was light blue, if the colors are standard). It did its job, and would be great for fine-pitch work, but I was looking to get more aggressive with solder and wanted to try a bigger needle. Lesson 1: remove plunger first, then needle. It seems obvious, but I was lost in the moment. Solder kept oozing out of the syringe until I did so. I later plugged in the green needle, and got what I was looking for.
Here’s the sacrificial board. I set the toaster oven to convection, 425 F (roughly 220C), and used the Stay On setting. The paste seemed to turn a duller, more fluid gray than when it was dispensed as the oven warmed up. After about two and a half minutes, something magical happened: Each blob of paste, in order of size, turned silvery in an instant and within a few seconds the messy paste gathered up in to tiny, professional-looking solder balls. Even intentional shorts I made cleaned themselves up. After the final patch had balled up nicely, I shut off the oven and opened the door to allow the board to start cooling. It wasn’t long before I was able to take it out and get a real good look.
This is too easy…Let’s go straight to the project! I printed out a layout from Eagle with all of the component values, and organized everything on what little workbench space I had. Then, I put solder paste down on all of the pads. I was surprised how simple it was after I figured out that you can use both hands to apply the solder paste (one on the business end of the syringe, one on the plunger).
After removing the plunger from the syringe, I started placing the largest part, the SD slot, and worked around that. Then, I moved generally from left to right, top to bottom, larger to smaller, to minimize the risk of knocking around parts I had already placed. I placed all of the parts except the card slot with my tweezers, and then pushed them in to the paste to assure they would lie flat against the board. Most of the parts of the same values were clustered together in rows, so in general it was easy to place them all at once. The last parts I placed were the accelerometer chip and the decoupling cap on the far right. The paste itself was tacky enough to hold parts in place, but the longer it sat around the more it turned into a pool of soldery mess. I was beginning to get concerned that I had used too much paste and the board would end up one giant jumper.
Time to cook! I let everything warm up to 350 F (roughly 180C) for 2 mins, then pumped it up to 425F until I was happy with the silvery appearance of the solder. Good thing I did a test run! The solder got dull gray and runny, just like before, and all my capacitors (and some of my resistors) were left floating in a pool of solder paste. If I weren’t expecting that, I’d be really concerned and may have pulled the plug on the whole thing. Well, probably not, but I was nervous…there’s ten dollars worth of parts in there!
As before, once the solder hit its magical reflow point it turned a wonderful silver and pulled back, balling nicely around the appropriate joints. One of my resistors was off of one pad, and when the solder balled up it pushed one end askew. When I turned off the oven and opened the door, I nudged it back in to place with my tweezers while the solder was still molten. 425F is hot, so I was very careful and wouldn’t recommend sticking your hand in one of these EZ-Bake ovens as a rule of thumb.
I gave the board a quick visual inspection, and found that there was only one solder jumper, but it was between the XOut and an unconnected pin on the accelerometer (15, 16). Since one of the pins wasn’t connected, I wasn’t overly concerned. The rest of the joints looked pretty snazzy in general. It didn’t look like it came off of a professional assmebly line, but it would take more than a casual glance to realize that. Most of the joints flowed perfectly, and most of the parts aligned properly. The resistor I had to nudge was askew, but I tested everything before powering it up and it all reported as expected.
Cleanup time…No advice from Celeritous. They seem to indicate the needles go bad, and that’s just life. I thought I could do better, so I got a disposable plastic tub (one that formerly held lunch meat) and put some isopropyl alcohol in the bottom. I let the needles soak for a while, and capped both ends of the solder paste syringe again. Turning back to the needles, I noticed that they were still filled with solder paste. I stripped some 24AWG stranded wire, pulled a length of one single wire from the bundle, and ran it through the needle. Success! I soaked the needles for a bit more time in the alcohol, and now that there was plenty of surface area the remaining solder paste dissolved right off the needles. I eventually put the needles on the manual plunger/syringe combination and ran alcohol through the needles several times each. That seemed to be the key, and soaking probably isn’t necessary at all. Simply running alcohol through them with the plunger may work, and if not, running a wire through it first clears most of the remnant paste. They’ll definitely live to dispense solder again.
I hand soldered the connectors and, since the iron was warm, tried to grab the jumper on the IC with some solder wick. It worked like a charm, and I highly recommend having some wick around if you try surface-mount stuff. Just put it over the jumper, add an iron, wait for the solder to turn molten, then remove the wick with the iron. If you remove the iron first, the wick will weld to the part and you’ll have to re-heat it with the iron to make the solder molten once again. It does all the hard work, soaking up just the jumper and leaving solid electrical contacts. I’d like to point out that the accelerometer is a BGA part, so there are no legs. The solder balls peek out from the bottom edge, and the solder wick was still able to grab it. Handy!
Once the board cooled, I plugged it in to my Arduino and let it rip. Success! The error light was blinking! While that sounds bad, I had left out the SD card in case an undocumented feature/bad joint reared its ugly head on power-up. I powered it down, added an SD Card, and powered it up twice. Both times it recorded acceleration measurements, and it appeared at a glance to have a bit less noise on the readings as compared to my breadboard and foot-long wire setup. Awesome.