ELEC 332

Assembly

Soldering Commercial PC Boards

This process is described in the SMD Soldering Video.

Soldering Milled Boards

Soldering a board which has been milled on the T-Tech is a bit more complicated than soldering a commercially fabricated board.

The vias are not plated through and must be manually connected from top to bottom.
The pads are not pre-tinned so the bare copper must be cleaned before soldering, and a more liberal use of flux is required.
The bulk of the unused copper is left on the board, rather than being etched away. This makes it easier to cause a short when too much solder is used on a pad.

	1.		Inspect for copper slivers.
			Because copper is a very ductile material, it is possible to leave thin slivers which are still attached to the remaining metal. This is especially common at acute angled corners, as shown in the photo. Examine the board carefully before beginning work and remove any such slivers with the knife or tweezers. It is important to remove these before they can cause trouble, because they can be very difficult to find once they have been covered with flux.
	2.		Use the glass brush to clean the pads.
			A copper surface will oxidize when exposed to air, slowly if clean, and much faster if assisted by fingerprints. Some of the blank board material we use is covered with a thin, clear film to prevent oxidation. In any event, there is likely to be a barrier between the copper and the solder which will prevent the formation of a good joint. Often heat and flux will remove this barrier, but sometimes it can't. The best way to be sure of a good joint on a bare copper board is to mechanically clean the pad before soldering. The most convenient way to clean the pad is with the glass fiber brush. Scrub the pad with a few firm strokes and, voila, bright, shiny copper. For best results the fibers should protrude about 2-3 mm from the tip of the handle. When you run out of fibers, there are refills in the drawer. Since the glass is brittle, little bits will break off and be left behind on the otherwise clean joint. These should be removed with the blower bulb. Resist the temptation to wipe them off with your finger tip, as this will leave fingerprints on the board and glass fibers in your fingertip. Since they are transparent and tiny, they will be difficult to remove.
	3.		Use additional flux and pre-tin where appropriate.
			Attempting to solder directly to a bare copper pad can be difficult because the copper will oxidize significantly before it is hot enough to melt the solder. To avoid this, place a small amount of gel flux (from the syringe) on each pad to be soldered before placing the component. In some cases it may be helpful to pre-tin the pad, removing any excess solder with solder wick.
	4.		Connect vias with a piece of bare wire.
			All connections between the top and bottom of the board must be made manually. In a few cases these can be made by the leads of thru hole components (be sure that it is possible to solder the top side in this case), but most will be standalone vias. These should be connected before installing other components on the board. For each such via, clean and flux the area adjacent to the hole on both sides of the board as described above. Place the board in the vise with the ground plane side up. Using the spool of 28 ga. bare wire, insert the end of the wire so that about 5 to 10 mm protrudes from the opposite side of the board. Solder the wire and clip it flush with the board. Clip about 2 mm from the end of the wire on the spool (to remove the bump of solder) and repeat until all vias are filled. Turn the board over and solder and clip all of the protruding wires on the other side.
	5.		Install remaining comonents as with a commercial board.
			Installing components will proceed essentially the same for a commercial board, except that any thru hole components which connect to the top of the board must be soldered on the top of the board.
	6.		Check for solder bridges.
			On a commercial board, all excess copper is removed. On a T-Tech board, only a narrow strip of copper is removed to isolate the trace. This means that there is a much greater opportunity to form solder bridges. Check that no solder has flowed beyond the boundaries of its pad to connect to either an adjacent pad or to unremoved copper. If it has, simply remove it with solder wick.