Toaster-Oven Reflow with Normal Solder (no Solder Paste)

I recently successfully soldered a 4mm-by-4mm QFN chip using my toaster oven but without solder paste. There were two reasons to avoid solder paste. The first is that somebody threw away the solder paste syringes that I stored in a fridge (the solder paste was also beyond the expiration date, but still worked okay). The other reason, which is more important, is that reflowing QFN chips without a stencil using solder paste proved difficult. The problem is to put an appropriate amount of solder paste on the pads (that is, to put little enough). When I put too much, which happens very easily, solder bridges form and are hard to remove.

I needed to solder a QFN chip to an evaluation board I designed and I decided to try to do that with normal solder. I tinned each pad with normal solder, including the center pad. I made sure that the amount of solder on the pads was uniform; every pad had a little hill of solver (probably 0.25mm or so high). I then added flux, placed the chip on the board, the reflowed it.

An inspection of the joints under the microscope showed that there was too little solder on the pads; the solder was completely flat on the pads. I added more solder to the pads with a soldering iron with a needle-point tip (Hakko 888 with a T18-I tip). I also added solder to the center pad through a via to the back side of the board. Following this fix, the chip was soldered correctly (this was verified by correct operation of the board). The rest of the components were soldered using an iron, but perhaps I should have reflowed them too.

The is the picture of the board; it was designed to evaluate an AX5031 transmitter using a Texas Instruments Launchpad. I forgot to order a 16MHz crystal with a footprint that matches the boards, so I improvised with a larger crystal.

2017-11-15-12-07-57.jpg

In a discussion in Facebook, Ohad Miller commented that he solders similar QFN chips using a soldering iron. He solders the pads pretty much like I added solder to the pads. He solders center pad using an large via (he makes these vias large specifically so that it is possible to solder through them). I didn’t try this but it seems like a good way to go.

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