Sunday, July 10, 2011

Reflow Soldering

I’ve been working with SMDs for quite some time now. Upto now I’ve been using a magnifying glass, a point tip, fine solder, and copious amounts of flux, but when you’ve got a bunch of chips with tiny little pins (or maybe even NO pins!) reflow soldering is the way to go.

Very tiny balls of solder suspended in flux form a sort of paste. This solder paste is applied to the pads over which the chip is placed. When heat is applied the solder balls melt, and helped by the flux the solder wets the pads/leads. On cooling, the solder forms a (usually) good joint.

Wanting to do a bit of reflow myself, I bought myself some solder paste (known as ‘PPD’ in Lajpat Rai Market). The paste seemed a bit dry, and in hindsight I should have added some more flux to it. But I’m getting ahead of myself. Let me tell you a bit about how I got my SMD soldered.

Normally, a stencil is used to apply the solder paste. Stencils cost a bomb to make, and spending so much just to solder a few boards is not worth it. I used Sparkfun’s tutorial as a reference, and also had a look at Fly Electric and Bill Shaw’s attempts.

The chips with flux and paste – I didn’t really care much about neatness. The surface tension of the solder ‘fixes’ itself.

I used one of my PIC’882 dev boards with an MCP2200 20pin SOIC, and an SOT23 LM1117. The solder paste came in a small plastic container. I used a flathead screwdriver to apply the paste. I fluxed the pins of the chips, and then used a pair of tweezers to place the chips on the board.

Left: The aluminium foil on the toaster tray ensures I don’t die of lead poisoning when I toast bread later.
Right: my new multimeter has got a cool thermocouple temperature sensor

I had a regular toaster that I placed the board in. To measure temperature I used my multimeter with a thermocouple. I wasn’t to particular about time and temperature (though I suppose I should have been!). Ramp-up to 210°C took about 3 minutes.

Not wanting the temperature to go over 220°C, I turned off toaster switch when the temperature reached 218°C. The temperature went to 220°C, dragged by the thermal inertia, and then started to drop. Once the solder melted - I relied on simple visual inspection to check when the solder melted - I turned the switch on and off a couple of times over the next two minutes to keep the holding temperature roughly constant.

Making sure the temperature doesn’t exceed 220°C

After it seemed like the the solder had properly reflowed, I opened the toaster door to let the board cool in still air. Once the temperature reduced to 80°C, I picked the board off the toaster tray.

Left: Maybe I shook the board
Right: there’s nothing that a little rework can’t fix.

The alignment of the LM1117 was a bit messed up – I don’t really know how that happened. One the MCP2200 there was one solder bridge, but all the other pins seemed adequately reflowed. I think the dryish solder paste combined with the uneven application and this being my first attempt necessitated a bit a touching up on the MCP2200.

Guess I’ll have to reflow a few more boards to get perfect!

Sunday, July 3, 2011

Electronics makes the summers a happy time!

Two orders arriving in a span of less than three days! It’s almost as good as Christmas!

I bought some stuff from iTead Studio. It came decently packed in a small cardboard box. The components were sealed in static resistant bags, and then wrapped in bubble wrap. So everything came safe.

My haul from iTead

I bought a waterproof ultrasonic sensor, two nRF24L01+ 2.4GHz radios, a regular ultrasonic module, a PIR sensor and some Arduino style long pin headers.

The iTead order was placed on the 16th of June, and it arrived on the 28th.

My second package came today. This was a PCB order I had placed with Seeed Studio on the 15th of June, but it came only today (2nd July). In all fairness Seeed had shipped it by the 22nd, but China Post was the bottleneck. Still, I’m not complaining too much.

I bought some other stuff too from Seeed – a Bus Pirate probe cable to use with the BP I made. The cable has some pretty decent probes. Not the best quality, but reasonable for the price. You may ask why I didn’t build one myself – I was too lazy :-P.

The probe attached to my Bus Pirate

I picked up a current transformer to fool about with (at some later date), and a small solderless breadboard. They look cute and I simply can’t find them in India.

Left: The breadboard and CT Right: the CPLD dev board

FPGAs fascinate me. But since I’m a Mechanical Engineer I’ve never got the chance to formally learn about these. I don’t know whether that’s a good or bad thing (since I feel that the easiest way to lose interest in something is to conform yourself to structured study), but that’s another story. Buying an FPGA dev board would not only be expensive, it may have been too big a jump for me, so I bought a CPLD instead. They’re cheap and I’ll get a good, albeit simple, introduction.

 SD card
Left: The micro SD cards Right: the vague data on one of them

When I’m buying electronics I get carried away (much like my sister or mom when they’re out buying shoes or handbags) – I bought two 2GB micro SD cards; one to fool around with, and the other to use. Both the SD cards were detected by my computer. One of them was blank, but the other seems to have been used before. There were some files, and one of the folders was called “HTC Sync”. It beats me how they got on there.

My boards – back and front

Its quite interesting after you pay by credit card to see how much stuff you bought without intending to. This order was placed because I wanted to get PCBs made. :-P The PCBs are the second revision on my Arduino-ish PIC dev boards. These boards are not Arduino compatible, nor do they use Wiring/Processing. But they are inspired by the modular approach Arduino uses of being able to socket-in shields. These are raw Rev2 PCBs; I’ll post populated photos soon.