The part shortages are really causing me to consider new parts and new packages as I struggle to find parts to support my current and future products. One of that parts that has caught my eye are some new parts from Microchip (formerly Atmel), the ATTINY322x parts. On a new design, I really was looking for as many pins as possible, so I choose the ATTINY3227, which comes in a VQFN24 package. I have tried hand soldering VQFN parts in the past with no measure of luck, but I was compelled to give it another try. I have seen some good videos on how to do this with hot air, but presently I do not own a hot air solder system (I have used one in the past with some good results, but this was primarily done for high pin count, leaded parts, TQ144). As I rolling this idea around in my mind, I came up with a plan that seemed like it might work, so here is what I came up with.
My plan involved attempting to solder the exposed pad first, through the back of the board. To accomplish this I designed a VQFN-24 footprint in eagle, using a through-hole pad (0.045" hole with a 0.085" diameter exposed pad) for the exposed pad. Here is the footprint for the device:
In order to hold the chip in place on the board while I solder the back side of the board, I decided to build a jig to hold the chip and the board in place. So I opened up Fusion360 and using measurements from the PCB layout I came up with a design for the jig. Here are the 3D design and the completed jig:
Here is the jig loaded with first the chip and then the board:
In the above image, the exposed pan of the chip is visible through the PCB through-hole, giving me a warm and fuzzy feeling that I have the PCB inserted into the fixture correctly. Prior to placing the PCB into the jig, I place flux on the exposed pad on the chip. After placing the PCB in the jig I applied some additional flux into the through-hole on the PCB. I then used a fine tip on my soldering iron to apply copious amounts of solder into the through-hole, repeatedly inserting and removing the tip, until I was sure that solder made it down to the exposed pad on the chip. I then removed the PCB, with the chip attached and flipped the PCB over and began solder the pin contacts. I tilted the PCB in the board holder and zoomed in tightly to view the edges of the chip and the exposed pin pads. I then applied solder to the chip and dragged the tip across the pins a coupe of times until there was visually a fillet at each pin with no visible shorts and/or solder blobs. I repeated this for each face of the chip, until all of the pins were attached. Here is an image (through the microscope eyepiece) of the chip, soldered into place:
There was an indication of a slight misalignment of some of the pins, but no obvious shorts (also the corners of the chip did not align well with the silkscreen markings). I will need to do a quick shorts test (adjacent pins and to ground) with an ohm-meter (while under the microscope) and then add a few components so that I can power up the board. I might then write a quick pin wiggle test, raising a single pin at a time high and checking that the pins are connected to the PCB, while also potentially exposing other pin short faults. The good news, is that there are plenty of vias on the board, such that all but 2 of the used pins are attached to a via, allowing for some easier probing. More on that later .....
Overall the approach was a huge success! If I go much further with this product, I might alter the jig to align the board to the fixture by a couple of the through-holes on the board edges (connectors and guide pins) as the routed edges of the board are off a bit (leading the the slight misalignments mentioned above. (I am working on two versions of the PCB right now, one with the ATTINY3227 in a VQFN-28 and the other with a ATTINY3226 in a 20-SOIC 7.5mm wide package). Always good to have a backup plan given the challenge to my soldering skills and also as a hedge to potential part shortages.
More later....
