SMT: Desoldering And Remounting Of Components

4. Desoldering and remounting of components

Desoldering is an operation to remove defective or mismounted components from printed boards. In most cases, desoldering is the first step of a repair action. Removal of excess solder is always accompanied by the desoldering process.

4.1 Removal of excess solder:

Removal of solder may be performed at various stages of the repair operation. Sometimes it is useful to remove as much as possible of the solder before the component is taken out of or off its place; in other cases the component has already been removed and then excess solder must be taken off the solder lands or out of the plated holes before the same component or a new one can be replaced and resoldered.

For solder removal the following means are available:

1. solder suckers

2. suction soldering irons

3. solder wick

1. A solder sucker is a kind of suction pump with a PTFE orifice, which is placed in the molten solder to be removed. The pump works with a squeeze bulb or a plunger mechanism. Those with negligible recoil effects are the most suitable.

2 A suction iron is a soldering iron with a hollow tip to which a suction pump has been added. Exchangeable bits having different dimensions are available. An adjustable sucking capacity is preferred. Suction irons as well as solder suckers exist in many versions from various manufacturers.

3. Solder wick consists of woven copper wires impregnated with flux. The wick is pushed against the solder with the tip of a hot soldering iron and the molten solder is sucked up into the wick by capillary action. Solder wick is suitable for the removal of solder from joints on single sided boards and SMCs, but is less appropriate for plated holes in double-sided or multi-layer boards.

Desoldering with wicking braid is only accomplished with long periods of applied heat, usually 4 to 5 seconds, although occasionally this can be more than 10 sec. Most components do not suffer damage, but in the absence of heatsinks the rated body temperature of conventional components is approached. The suction soldering iron requires shorter operation times and lower temperatures than wicking braid.

Desoldering braid is available in various widths to match the size, heat, and space limitations of a circuit. Ultra-fine gauge, for use in confined areas, permits access to areas on surface mount and multilayer boards normally difficult for a desoldering gun to reach.

Braid also is inexpensive, facilitating its stocking in a variety of widths and lengths. It is packaged on bobbins in 5-10’ lengths, and on reels from 25’ to 500’ in length. This flexibility means that wick can easily be carried in a repair kit or kept handy at a workstation.

The proper size braid (see table below) will have a width equal to or slightly larger than the connection or pad. Wicks that are too narrow may not hold the volume of solder necessary to remove it all in one application, while wicks that are too broad may absorb too much heat from the iron, reducing heat transfer to the connection or pad. Care should also be taken in selecting the iron tip, which should be at least as wide as the braid to ensure uniform heating of the wick.

Common widths of desoldering braids and their applications

One good advantage of desoldering braid is its flexibility. In addition to cleaning pads and removing devices, wick can be used for eliminating shorts caused by solder bridges, correcting poor solder connections, removing components that must be remounted, cleaning flat surfaces, and removing excess solder. Desoldering wick is effective on flat surfaces and will clean residual solder.

In suction soldering irons, the vacuum is provided by a pump which in turn sucks the molten solder from the pad area. Here, the process is more controllable in terms of suction pressure, temperature of suction tip. Various tips are available to suit specific requirement.