Soldering the DirectFET without a paste stencil

I've gotten a number of hits in the past week from people searching for 'soldering directfet'. DirectFET is International Rectifier's brand name for a series of chip scale power MOSFETs. What this means is that it is a bare MOSFET with a little metal can that carries the drain terminal and covers the bare die when it's soldered down to the board. They're amazing devices -- a part not much bigger than my pinky nail can switch several kilowatts of electrical power with the most minimal heat-sinking. They are also, as you might imagine, impressively hard to solder successfully. There are 13 different package variations in three sizes. 

My site ended up in the search results for people trying to figure out how to solder these things because I designed breakout boards to enable several of these packages to be plugged directly in to standard 0.1" pitch breadboards. I've taken the page down, since I haven't gotten enough interest in them to justify taking them to production. However, I decided I ought to give people looking for information on soldering them something about how to do it. Unfortunately, I did not take photos of the process and don't have any parts on hand to replicate it with a camera in hand. Therefore, this will be text for now. 

The top of the package is a metal can, which serves as the drain terminal of the MOSFET. When you turn it over, you can see the bare MOSFET die. It has between two and nine metallized patches, depending on package variant, that form the gate and source terminals. One or two will be the gate and the rest will be the source.

The thing which should be immediately clear is that if you want to solder it down to the board, you need to do it in such a way that you are guaranteed to not form solder bridges; there's no way to fix them except to remove the chip and start over. As a result, you will need a board with solder resist that has the openings described in AN-1035: DirectFET® Board Mounting Guidlines. I put a number of them into an Eagle library, available for download. There's no way around that, unfortunately, but there is a way to make it easier. Think & Tinker (and others) sells laminatedable dry-film photoimageable solder mask. You put it on the etched & drilled PCB with a laminator and then use a UV exposure and mask to create the openings. 

This HOWTO assumes that you don't want to or can't get a solder paste stencil for your design or the chip. Printing solder paste and reflowing is of course the best way to solder down this chip; this is how to do without. 

In addition to the PCB and the DirectFET, you will need the following items:

  • A soldering iron with controllable temperature and a very fine needle tip -- 1/64" is good.
  • Fine flux-cored solder. 0.020" is a good size for this.
  • Liquid flux and a brush or a flux pen. The flux needs to be a no-clean type, because you cannot get it out from under the chip afterwards. Water-soluble fluxes are inappropriate, because they form corrosive residues that can't be removed afterwards.
  • A hot plate, hot air gun, or oven for reflowing the board.
  • A vise or hands-free set up to hold the DirectFET and PCB while you're working on them.
  • Lint-free wipes or swabs and clean isopropyl alcohol. 
  • A continuity meter.

While not strictly necessary, some sort of magnifier makes this process much easier. 

  1. Set your soldering iron to just a little bit hotter than required to melt the solder. 
  2. Secure the DirectFET upside down so you can see the die and its terminals. Coat it liberally with flux.
  3. Using your iron, apply a small dot of solder to each terminal and to the two ends of the can. You should form a small mound of solder fully wetting each terminal. It should not be enough to form a ball but enough to stand proud of the surface. This takes practice to get right.
  4. Remove excess flux from the DirectFET with a lint-free swab soaked in alcohol. Allow to dry. At this point, you can dead bug the device in the usual way with magnet wire, but that's beyond the scope of this HOWTO. 
  5. Coat the PCB where the DirectFET will go liberally with flux. Place the PCB in the place you are going to reflow it.
  6. Carefully place the DirectFET on the PCB over the footprint, making sure to line up the pads with the solder bumps such that no solder bump overlaps a neighboring pad.
  7. Reflow the DirectFET. It should pop right into alignment when the solder melts.
  8. Check for shorts with the continuity meter.
  9. If you did it right, you should have a securely mounted DirectFET.