Surface-mount Technology - Rework

Rework

Defective surface-mount components can be repaired by using soldering irons (for some connections), or using a non-contact rework system. In most cases a rework system is the better choice because SMD work with a soldering iron requires considerable skill, and in many cases is not feasible at all. There are essentially two non-contact soldering/desoldering methods: infrared soldering and soldering with hot gas.

With infrared soldering, the energy for heating up the solder joint is transmitted by long- or short-wave infrared electromagnetic radiation.

Benefits:

• Easy setup
• No compressed air required
• No requirement for different nozzles for many component shapes and sizes, reducing cost and the need to change nozzles
• Fast reaction of infrared source (depends on system used)

Disadvantages:

• Central areas will be heated more than peripheral areas
• Temperature control is less precise, and there may be peaks
• Nearby components must be shielded from heat to prevent damage, which requires additional time for every board
• Surface temperature depends on the component's albedo: dark surfaces will be heated more than lighter surfaces
• The temperature additionally depends on the surface shape. Convective loss of energy will reduce the temperature of the component
• No reflow atmosphere possible

During hot gas soldering, the energy for heating up the solder joint is transmitted by a hot gas. This can be air or inert gas (nitrogen).

Benefits:

• Simulating reflow oven atmosphere
• Some systems allow switching between hot air and nitrogen
• Standard and component-specific nozzles allow high reliability and faster processing
• Allow reproducible soldering profiles
• Efficient heating, large amounts of heat can be transferred
• Even heating of the affected board area
• Temperature of the component will never exceed the adjusted gas temperature
• Rapid cooling after reflow, resulting in small-grained solder joints (depends on system used)

Disadvantages:

• Thermal capacity of the heat generator results in slow reaction whereby thermal profiles can be distorted (depends on system used)

Reworking usually corrects some type of error, either human- or machine-generated, and includes the following steps:

• Melt solder and remove component (s)
• Remove residual solder
• Print solder paste on PCB, directly or by dispensing
• Place new component and reflow.

Sometimes hundreds or thousands of the same part need to be repaired. Such errors, if due to assembly, are often caught during the process. However, a whole new level of rework arises when component failure is discovered too late, and perhaps unnoticed until the end user of the device being manufactured experiences it. Rework can also be used if products of sufficient value to justify it require revision or re-engineering, perhaps to change a single firmware-based component. Reworking in large volume requires an operation designed for that purpose.