Building new version of the PCB for 1130 MRAM

USING SOLDER PASTE AND HOT AIR GUN FOR ALL SOLDERING

Because so many of the chips are tiny footprints with very narrow pin spacing, I decided to use solder paste and the hot air station to solder them down. I mixed a bit of flux with the paste, which reduces some of the splatter after soldering. 

As the paste is turned into liquid solder, it forms balls. When the balls touch an exposed pad on the board, it flows onto the pad and other balls accrete to this structure. However, if a ball does not bump into a pad or other balls, it remains as a loose very small ball on the surface of the board. I had to clean these away otherwise they might move between pins to create intermittent shorts. 

I controlled the amount of splatter by applying the solder paste/flux mix carefully to reduce how much is away from the pads. I then dropped the chip into place with the pins aligned over the pad pattern. The hot air rework station as a control for the air volume, which I lowered because otherwise small parts like resistors could be blown out of place before the solder liquified. 

I used by PCBite with its small probes to test each pin for connectivity after a chip was soldered down. I touched up high on the pin with one probe and touched the far end of the trace with the other probe and listed for the beep of the continuity tester. I also touched adjacent pins to check for shorts. This was a slow process with all the chips and pins, but important to be certain that the board is correctly assembled. If a pin didn't have a good connection, I put on a dab of solder paste/flux and heated it until the connection was good while remaining free of shorts. 

First set of chips soldered down, circled in yellow

Another group of chips installed, circled in yellow

NEED TO ORDER A COUPLE OF CHIPS BEFORE FINISHING

I found that the main MRAM memory chip had developed a fault and one of the sense output NAND gates had developed a broken pin. I placed an order with Digikey and should have the parts before the week is out, when I can finish the board.

CONTINUING WITH NON CHIP COMPONENTS

I began to install all the resistors and capacitors onto the board. I used the solder paste and hot air station, even though these were easy enough to solder with an iron. In addition, there are a couple of connectors and all the pins that form the SLT connectors where the cables T1, T3 and T4 are attached.