STEP 1 - Apply Flux Pen to PCM DAC pads
Hopefully, if you read through the Preparation section, you should be at a similar position as that in the photo at right. You can actually see in this photo where the liquid flux has been applied in the photo. Position the board so that you can easily "wipe" sideways while holding the iron tip parallel to the length of the PCM2702 DAC chip (see the SMD Soldering page). What follows is a continuation, using both graphics and photographs, of a step-by-step process in how to populate the board.
STEP 2 - Apply Solder for an Anchor Pt on the PCM pads
You should pick one of the two pads near the "Q1" crystal. The idea is to anchor one corner, then solder the pins on the other side. Once you solder the anchor pin at the corner, you will still be able to rotate the chip slightly to make certain that the pins line up with the pads.
The pads on the end closest to the USB jack are connected to the ground plane. These are more difficult to solder starting out and will lead to frustration if you use one of those as an anchor point. So, stick to the pad marked in the image at top left.
The technique is pretty simple. Pick up the PCM chip with your tweezers - make sure you have it oriented the same as the pads on the board (note the dot goes up at the crystal). While holding the chip in your left hand with the tweezers, use your right hand to hold your soldering iron to melt the solder you just applied to the anchor pad. While the iron has the solder melted, use your other hand and the pair of tweezers to move the chip into position and the corner pin into the melted solder. Do this while lining the pins up as best you can - release the soldering iron. When the solder cools, release the tweezers. Re-melt the pad and use your tweezers to move the chip slightly if the alignment is off. The silkscreen doesn't matter as much as the pin pads - make sure the pads of the chip are lined up with the pads on the board.
STEP 3 - Solder the PCM Pins Opposite the Anchor Pt.
The BantamDAC team recommends the "Tin and Wipe" method to solder the PCM chip pins to the board. There are many other techniques, to be sure. However, you will find that the pads on the BantamDAC board are fairly saturated with tinning. It takes very little solder - very little. Place your soldering iron horizontally along the edges of the pins (the solder tip is at right angles to the ends of the pins).
While applying a small amount of solder to the tip of the iron, make a down-and-to-the-side wiping motion with the soldering iron tip across the pins. Your wiping motion will be in line with the pins and travel outward sideways - as if you're trying to stretch the ends of the PCM pins with your soldering tip. Move down gradually as you continue the wiping motion - never allowing the soldering iron tip to "rest" on the pins. Add solder if you see you don't see the tips of the pins shine with melted solder. If you get too much (a bridge between pins), repeatedly wipe sideways while alternately wiping your iron tip in the brass wool. The solder will flow away from the pins and probably onto adjacent pads. In fact, if you do this correctly, the adjacent pads will probably look like they've had solder applied to them.
Step 4 - Solder the Other Side of the PCM Pins
Flip the board around - if you're right-handed you want to always wipe to the right, so you'll need to flip the board around to solder the other side of the PCM chip. Proceed as described above.
Step 5 - Inspect the PCM Chip and the Pins
At this point, your board and chip should look like the photo at left. It looks pretty rough, but there's little you can judge without cleaning it up. So, do so - use a toothbrush and 90% isopropyl alcohol ($1/qt at Walmart) - or similar - to clean the pins and the board.
Here we see the chip and board cleaned up.
Check the pins with a magnifying glass and a good light. You can also try taking a digital photo of the board and enlarging it on your computer to inspect the joints. A great way to inspect the joints is to hold the board up to a bright light (such as the desk lamp on the table in the photos). The light will shine translucently through the board and allow you to see any bridges or non-connections.
Finally - with extreme caution - you can also use your DMM to measure zero resistance between the pins and the pads on the board that are connected to them. You do this by placing on probe on a pin at the top of the chip where it exits the plastic. Trace the pad connected to the pin and follow it to its next connection with a part on the board. Place the other probe at the contact point on the part (not next to the PCM pins) and see if you read zero resistance. Be very careful in using this method. A DMM injects a small amount of voltage into a circuit to measure the resistance. It's possible that this voltage is enough to short out the PCM chip. So, if you use this method, be forewarned of the risk!
If you find suspect pins - bridged or unsoldered joints - then rack the board back in the helping hands and repeat the soldering procedure.
As mentioned earlier, note the solder on the adjacent pads - a result of the wiping motion in soldering the PCM chip. Not to worry - we need to tin all of those pads, anyway.
file last changed:Monday, October 13, 2008 3:41:09 PM
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