A FPGA Controlled Reflow Oven

FPGA Reflow Oven

For Christmas, [Hamster]’s wife gave him a mini-oven. Later that day, he tore it apart and built this FPGA controlled reflow oven.

We’ve seen plenty of reflow oven builds in the past. Most of those projects use a microcontroller to do closed loop control, sensing the temperature and toggling the heating element to hit a set point. This build uses the Papilo One FPGA development board as a controller. It implements a state machine that meets the reflow profile of the solder paste, ensuring SMD components are soldered properly.

The oven uses a MAX31855 to read temperature from a thermocouple. This device provides amplification, cold junction compensation, and analog to digital conversion which spits out the temperature over SPI. To control the heater, a 40A solid state relay is used.

The VHDL code that drives this oven is linked in the writeup, and has some interesting bits for those looking to experiment with FPGAs. It includes an SPI interface, display driver, and the temperature state machine logic.

Adding a SIM card to the Photon Q 4G LTE

[Charles] is a big fan of phones that have physical keyboards. He thinks they are better suited for writing lengthy emails, but unfortunately his HTC Desire Z was getting old so he had to replace it. [Charles] therefore decided to import the Motorola Photon Q from the USA which exposed one major problem. The Verizon phone uses CDMA so there is nowhere to put a GSM SIM. But a bit of hacking allowed him to add a SIM card slot to it. Even though he’s not the one who originally found this hack (XDA thread here), his write-up is definitely an interesting read. To perform this modification, he needed a hot air reflow station, a soldering iron, a Dremel with the appropriate cutting wheel and several SIM card slot assemblies from the Galaxy S3 (as the first ones usually get burned during the disassembly process).

Obviously the first steps involved opening the phone, which may have taken a while. Using hot air, [Charles] removed the EMI shield covering the SIM card IC . He then extracted the latter using the same technique. Finally, he removed another EMI shield covering the contacts to which the SIM card slot should be connected. A few minutes/hours of delicate soldering and case modding later, [Charles] could use his SIM card on his brand new phone.

Electric skillet reflow soldering guide

skillet-reflow-tutorial

It’s no secret that we’re bizarrely drawn to macro videos showing solder paste during the reflow process. This electric skillet reflow guide provides the fix we’ve been jonesin’ for while including some helpful tips for first-timers and veterans alike. Not sure what we’re talking about? Look at the grey paste at the top of this image. As it heats up it’s drawn under each component as seen in the lower half of the image.

This particular guide is aimed at one-off assembly so a solder paste stencil is not used (we learned a lot about those earlier in the month). It instead uses the painstaking toothpick application technique. It takes time but the upside is that once you get the hang of it you’ll apply the perfect amount of solder each time. After placing all of the components [Count Spicy] carefully transfers the board to an electric skillet, covers it with the glass lid (so he can see what’s going on), and sets the temperature just above the solder’s specified melting point.

Since the skillet is cheap and easy to find you really just have to order the solder paste to get into this type of assembly. Our only gripe is that you can’t really follow a temperature profile with this rig. For that you need to move up to some PID controlled hardware.

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Reflow soldering improved with carbon dioxide

co2_reflow-oven

This is exactly what it looks like. [Oleg] calls it soldering in inert atmosphere, but it’s just a toaster oven reflow hack dropped into a container full of carbon dioxide.

Why go to this trouble? It’s all about solder wetting. This is the ability of the molten solder paste to flow into all of the tinned areas of a board. [Oleg] talks about the shelf life of hot air leveled PCB tinning, which is about six months. After this the tin has oxidized. It will certainly not be as bad as bare copper would have, but it can lead to bad solder joints if your PCBs are more than about six months off the production line. This is one of the reasons to use solder flux. The acid eats away at the oxidized layer, exposing tin that will have better wetting.

But there is another way. Soldering in the absence of oxygen will also help the wetting process. CO2 is heavier than air, so placing the reflow oven in a plastic container will allow you to purge air from the space. CO2 canisters are cheap and easy to acquire. If you keg your own homebrew beer you already own one!

If you’ve got everything but the reflow oven just look around for a few examples of how to build your own.

Quick fixes for SMD population problems

quick-fixes-for-poor-PCB-work

Here’s a collection of tricks to get over some surface mount prototyping issues the next time you find yourself in a bind. But first we have to address the soldering atrocity seen on most of the components above. [Rxdtxd] admits he’s using a firestick for soldering his SMD parts. The non-brand 40W iron is just about the worst thing he could be using (well, we guess a candle would be worse). Try to overlook those joints and enjoy his solutions to a couple of other problems.

First up is what to do when you lift a fine-pitch trace like would be found on a TQFP footprint. The fix for this is to grab a junked transformer and use a bit of the enameled wire from the wrappings as a jumper. The wire is quite fine, and the insulation will burn off when soldered which means you don’t need to strip it first.

The second and third tricks both deal with resistors. As you can see above he placed two 1K resistors on a single resistor footprint to make his 2k resistor. The 0603 packages were both soldered standing on end, then connected with a lead from a through-hole component. The other resistor hack piles five components on top of each other to build resistance in parallel. This is not a great idea as it will fail over the long-term, but it will get you though the prototyping stage as long it doesn’t require precise tolerance.

Populate SMD boads using a toothpick and tweezers

cnlohr-hand-applied-solder-paste

Here’s a demonstration which proves you don’t really need special tools to populate a surface mount PCB. We’ve seen this board before, it’s the glass PCB server which [Cnlohr] developed and demonstrated by connecting the real world to Minecraft. It’s a tiny board and we were happy to have the chance to see his method for populating the parts before reflow soldering.

In the video after the break [Cnlohr] starts by dispensing a glob of solder pasted from its storage container. He mentions that as long as you store the stuff in the refrigerator it’s rather easy to work with. Because most of his projects are single boards it’s not worth it to have a solder stencil produced. Instead he picks up a bit of the solder glob on the end of a toothpick and applies it to each pad.

This isn’t really as bad as it sounds. The fine pitch TQFP footprints can just be dragged with a bit of the paste. After this application — which took around seven minutes — he grabs some tweezers (not the vacuum type) and begins placing each component. If he missed some paste he’ll discover it in this step and add where necessary. The last step is a trip through his toaster oven.

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Hands free hot air station

In an effort to ease the process of soldering Ball Grid Array (BGA) chips at home [Roger] rigged up a hands-free solution for his hot air equipment.

The main component in the build is an Aoyue hot air rework station that he already had in his workshop. He wanted an adjustable mount that would hold it steady when reflowing parts so he hit Amazon and bought a $14 articulated lamp. After ditching the funnel-shaped shade he bolted a cable clamp to the socket housing. This can be tightened on the hot air wand, with the spring tension of the lamp making it easy and quick to reposition the nozzle. [Roger] sent this project directly to our tips line and we’ve embedded the rest of the project images after the break.

If you’re looking for a more DIY rework solution you should checkout this hot air pencil hack. It uses a desoldering iron, a fish pump, and some metal mesh as a heat sink to put out a stream of very hot air.

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