Intro: Soldering SOP or Micro Surface Mount Chips
I use this for soldering small surface mount chips on DIP sized brake out boards.
Solder paste in a syringe style dispenser. Low melting point paste is a plus.
Tweezers, the sharp point style
A chip holder made from a block of wood and a springy wire and a conductive surface.
Sewing pin or needle tool
Step 1: Prepare the Chip Holding Device
Drill a small hole in a block of wood and bend a wire with some spring in it in a U shape. Make it so when stuck in the hole it applies pressure downward onto the surface of the block. Attach a metal surface for static control. I used a copper adhesive peace. With adhesive copper eventually the heat it will effect the adhesive and some staples will help to keep it in place. Sometimes the board your working on wants to move around so staples can also be used to create an edge preventing lateral movement.
Step 2: Placement of the Chip and Board.
Wipe the circuit board lands and chip legs with a rosin pen. This not only helps with soldering but is sticky and helps the chip stick to the circuit board preventing movement after being properly placed on the board. Place the chip on the circuit board with the tweeters. Find the location on your homemade chip holding board where the wire presses down without wanting to slide laterally. You probably will want to adjust the bend in the wire to get the downward pressure you want. The wire is then placed on top of the chip to hold it in place. You can use the tweezers to place the holding wire in the center of the chip. A magnifying glass will probably soon be needed. If the chip has leads on only two sides there is a technique to make very small accurate adjustments to the chip placement. To do this hold the chip on the non-lead sides with the tweezers length resting flat across the circuit board. See the picture. Now the long edge of the length of the tweezers are parallel with the board and pressing both downward on the board and pinching the sides of the chip. While maintaining the downward pressure on the board and pinching the chip rotate the tweezers axially in a clockwise or counterclockwise manner keeping both tips on the circuit board. The tweezers tips will flex and roll slightly to the right or left taking the chip ever so slightly with it. You will do this a few times each time inspecting on side of the chips pins and then the other until perfectly placed. Since you will go back and forth adjusting each side of the ship the holding wire may get in the way. Placing the board at a 45 degree angle to the wire minimizes this interference.
Step 3: Apply Solder Paste
Apply solder paste. If the chip has the really small 0.5mm pitch you want to apply it by putting some on the tip of a pin. How much? A drop on the head of the pin is to much, more like a micro dab. Use the magnifying glass. You will see after putting the first dab down you will probably want to remove part of the dab with the pin. Smear the dab across all the land traces. The paste will short all the traces together, that's fine. When the paste melts it will wick its way off the spaces between the lands, but NOT IF THERE IS TO MUCH PASTE ON THE BOARD. For this reason it is better to have to little paste than to much. You can always add more paste and melt it a second time. Getting solder shorted pins cleaned up is more difficult than adding more solder paste later but you fix it if you have to. With 0.5mm pitch chips apply the past to the board and not the chip pins. Doing so can create solder shorts. The solder will wick it's self between the chip legs when it melts. See the picture for proper paste application. You can also use a small piece of tissue paper folded over a few times and held with tweezers to wipe off excess paste. It is easier to clean up excess paste now rather than after you melt it.
Step 4: Melting the Solder Paste.
Use a heat gun to melt the paste. As it starts to melt pull the gun further from the board since the excessive air flow can push the melted solder across the board.You can use this to your advantage to force solder onto the chip pins or put solder where you don't want it so pay attention. You can also use this air flow to blow solder off shorts between the board lands. If you end up with shorted pins apply more rosin from the rosin pen before heating again. It helps with the surface tension of the solder. You can use a pin to scrape between the lands to remove the shorts. Correctly placing the pin before heating the board helps since you don't want the pin pushing the chip off the lands as the solder melts. In some cases putting rosin on the sewing pin then heating the pin first then applying heat to the board and you can use the pin as a solder wick. Note there are two pins are shorted in this picture.
Step 5: The Finished Product
The best test is to put the chip in a breadboard circuit and test it's functions. If it fails inspect for shorts.