I purchased the Kada 862D++ IR welder from the China Group to solder the MPC5200 to the board. See my previous post about my experiments with this machine. Well, today I decided to get serious and actually solder an MPC5200 to our board. Below are some pictures.

StencilQuik Package BGAs are difficult because you can't inspect the solder joints after the chip has been soldered to the board. Assembly houses use an x-ray machine to look at the solder joints after they have soldered the board. However, I don't have an x-ray machine so I looked around for some way of ensuring that the solder joints would not cross-connect or touch a nearby via. Fortunately, I found StencilQuick. These little gems are a little expensive ($70 for a pack of 10), but they are worth it for prototype runs. Basically, they are a laser cut template that you permanently stick to the board to mask everything under a chip except the connections. Here is a shot of what they look like.
Squeegee The stencils don't work very well if you can't completely fill the holes and ensure that there is no solder paste residue left on the surface of the stencil. To make this a little easier, I purchased a squeegee kit from the StencilQuik guys. I guess I could have found a stiff (yet flexible) edge to scrape off the excess solder paste, but these just seemed to be easy and cheap (about $25, if I recall). Here is a picture of the package.
Solder Paste Name I purchased this Kester solder paste from DigiKey. It is a no-clean 60/40 (roughly) solder paste for about $30. You have to have it shipped overnight or 2-day to ensure quality. You have to keep it in a refrigerator. Apparently, if this stuff heats up then it does not work. The reason for that is pretty simple: solder paste is solder balls suspended in a flux. If it heats up, then the solder balls drop to one side or the bottom of the bottle and you are left with only flux in part of the tube. If you expect to use the solution in that part of the tube to solder something, you will be disappointed because there won't be enough solder in it to make a connection. By keeping it cool, you keep the little solder balls suspended in the flux evenly for a longer period of time.
Solder Paste Specs Here is a shot of the solder paste specs. It is Sn63Pb37 no clean solder paste. You have to warm it up to room temperature to use it (I just left mine out overnight). This tube is meant to be put in a soldering machine, fixed with a needle on one end and a plunger on the other. However, I just needed a 'glob' on my BGA so I poked a hole in the needle end and used the handle of a screwdriver to push out some solder paste. It worked... I got a glob of solder paste on my board.
No BGA Here is a picture of the BGA area on the board before I applied the stencil. I has already been throughly cleaned with alcohol rags.
StencilQuik Applied Here is a picture of the BGA area after I applied the StencilQuik. Notice that the stencil is slightly miss-registered. This is about as good as I could place it my first time. I hope to do a little better in the future. Close inspection of the actual connection holes seems to be fine, however.
Applying Solder Paste Here is a picture I took while applying the solder paste. Warming the solder paste to room temperature was a good idea. The paste was very malleable. However, here is a little bit of wisdom -> tape down the sides. I thought that the stencil was securely in place so I decided to skip taping down the sides. As it turns out, I was correct. The stencil did not move or crease. However, as you can see, the solder pase does 'smear' out onto the board. That is not a big deal. It can be cleaned. However, it does collect on the edge of the stencil (between the stencil and the board). If you don't clean this out, then you run the risk of shorting out lines that have vias along the edge of the stencil. Lesson learned.
Solder Paste Cleaned Here is a picture of the stencil all cleaned up. I first scraped off as much of the excess solder paste as I could with the squeegee. I then used alcohol wipes and q-tips to clean up the remaining solder paste (be gentle at this point so as not to disturb the solder paste in the actual connection holes). Here is another piece of wisdom -> use lint free cloth. The cloth I used had lint. Of course, little pieces of lint fell off the cloth and got stuck in and around the holes. On close examination, the little lint pieces were covered in solder paste and they were making shorts to other connection holes. Arg! So, I carefully cleaned each hole with the best cloth I had and then slowly inspected and fixed every hole. In the future, I am going to use lint free cloth and skip this work.
BGA Upside-down This is just a picture of the underside of the MPC5200 I used on this board. Of course, You have to align the BGA in the connection holes and make sure that pin one is where it is supposed to be.
BGA Mounted Here is a picture of the BGA placed on top of the stencil and ready for soldering. When you place the BGA, you can feel is 'sink' slightly into the stencil holes. I didn't push down on the BGA at all because I could imagine 'squishing' solder out of the holes around each hole and potentially making a solder bridge to another pin. I just gently placed the BGA and verified that it was sitting in the holes.
BGA Soldered Here is a picture of the BGA soldered to the board. I set the pre-heating plate on the KADA 862D++ to 100C and then used the IR beam at 300C for 30 seconds. Sorry I didn't get a picture of this. I was so worried about the process at the time that I forgot to snap a picture. The board 'bowed' slightly and there was a little smoke. I couldn't actually see the BGA move at all. However, after it cooled, the BGA was definitly stuck to the board.