silic0re wrote:Hi all,
Sorry to drag up this old thread, but I have a couple quick questions! I've made footprints for two leadless QFNs (TPS63001 [3x3mm, ~12 pins], and TPS65131 [4x4mm, 24pins]), and checked printouts of the footprints with real devices to make sure they're about right. I've also made the pins extra long to have a little more room for soldering -- they extend about 1.0mm from the edge of the devices.
I'm really not sure how I'm going to solder these things, but was thinking of dabbing on some solder paste to the footprint and going the toaster reflow route. Has anyone tried this?
While I've soldered plenty of small SMD's, I don't have much experience (nor success) with leadless devices. I once tried a SCP1000 barometric pressure sensor, whose pads are actually completely under the device and unexposed to to the sides. I designed a footprint that had much longer legs (about 3mm) so that one could put some paste on the footprint then reflow it after the part was on, but this didn't have very much success at all.
I think hot air would be ideal, but I don't have the equipment -- just a normal decent soldering iron, and patience. Still, I fear a repeat of my past leadless attempt, and think the toaster oven might be a viable alternative. Any thoughts or success stories?
thanks!
Hot air works fine - but you have to remember to pre-heat the PCB. Your goal should be to get the entire PCB to just below the melting point of your solder paste, and then use the hot air to bring it the extra 5 degrees.
One method I've used in the past without any fancy equipment is to put a bit of paste on the center pad and then squish it down as much as possible, then solder the outside leads. Then, heat up the center pad from behind using a via that is placed in the pad. It'll melt the paste and get the center pad soldered.
However - if you're going to have these boards professionally stuffed - you absolutely should not do this - as it'll make it nearly impossible for your board assemblers to get a good connection to that pad, as the via will wick away solder out of the pad and into the via. Of course - there are expensive fixes for this (having the vias capped or filled) - but those add significant cost.
You can also do something similar - but use a number of vias in some sort of grid or array, and make them as small as possible. Then tent the vias with as little soldermask material as possible on the component side. Your goal would be to just barely block the hole of the via. Then you can heat up the array of vias from the backside to melt the solder paste, while also leaving the board reflowable as the tenting will keep the vias from sucking up solder.