Pictures from the inside of a flammable gas sensor from 1980’s England. Elton John says that “when you’re made in England you are built to last”. Well, this device is made with a Lead Acid battery potted to the case, so this one is built to last… Until the battery fails.
Note the conformal coating over the main board (though not all boards). The device uses 74 logic, and quite a few different daughter boards connected to the main PCB. The battery feeds 8V into the main PCB. It is charged by an external PSU via the barrel jack on the side. There is minor visible corrosion from the battery leaking. Two 5V regulators on the back of the case. Well built, except for the battery. Fundamentally flawed – it was not meant to live beyond the life of the Lead Battery.
In this video I describe some tips on fixing bridging from a small pitch FPGA, here a Xilinx Spartan 3, PG208 package. The largest non-BGA package available for this family.
I started with a toaster oven, and some solder paste, dispensed with a small syringe. From there I placed the IC carefully on top of the PCB, and put in the toaster until the solder reflowed. Once it was cooled, I noticed some bridges from using a little too much solder paste. This video explains how I cleaned up the solder bridges using some solder wick. It comes down to needing to tin the tip before each application of the wick. When it comes to tinning the tip, it’s not something that is required in all soldering, but here, in this scenario, it is required. It also helps to use fresh wick each time.
The kind of problems that you can run into when soldering a device like this may be the following:
Don’t bother trying to solder this by hand with an iron. Just don’t. Use a toaster, and solder paste. You don’t need a fancy reflow controller, just stick the board in a toaster until the solder reflows. Set it to max temperature (I use broil). Don’t use the toaster for food. Next.
Don’t put the solder paste on top of the IC pins. Put the solder paste down on the pads, and then put the IC on top of them. Doing the opposite will not work as well.
Do make sure that you have soldermask between all the pins on your gerbers before sending out. That is a rookie mistake, to have a footprint made that somehow omitted or doesn’t include the solder mask. Without it, the bridges will happen more easily.
Do use a good quality soldering iron. Consensus these days is that the entry level iron you should use is either the Hakko 888 or the Weller I have. Each runs about $100-150.
Do double check your footprint pads are the right size. Don’t make the leads too long on high speed designs, or the capacitors / collectors won’t be able to dispense electricity to the IC fast enough.
I purchased a surplus Rayovac PS133 Nicad AA/AAA battery charger, but had trouble getting it to work. The LEDs which indicate that it is actively charging would not light up. After looking around a bit inside, I made the following changes: 1) Add a resistor to the LED for the first battery, so you know when 1 is connected (glows at half brightness). 2) Add some solder to the positive connectors, so that the batteries can make contact. Without the solder or some type of material on the positive contacts, the batteries would not make contact with the case on. My particular batteries are Panasonic N-700AAC Nicad batteries.. See the following video.