Hot Air Rework Profiles For SMT Assemblies

Rework is the removal and replacement of a single component on an assembled board, and a hot air rework station is the tool used for most surface mount parts. The station applies heated air to the component and its joints, melts the solder and allows the part to be lifted. The difficulty is that the heat has to be concentrated enough to melt the joints under the part without melting or overheating the joints and the parts around it.

This article covers the preheat, the nozzle choice and the profile, and the checks that confirm the surrounding assembly survived.

Why Preheat Comes First

A hot air station heats a small area, and it has to raise the joint from room temperature to the melting point of the alloy. If the board starts cold, the station has to supply all of that energy through a small nozzle, which means a high air temperature and a long dwell. Both of those stresses the component and the laminate, and neither is necessary, because the board underneath can be warmed first.

The preheat brings the whole assembly, or the region around the work, to a temperature below the melting point of the alloy. It reduces the temperature the nozzle has to reach, shortens the dwell, and reduces the thermal gradient across the board, which is what causes delamination and pad lift. The preheat also reduces the risk to the joints outside the nozzle area, because they are already close to their own melting point and are less shocked when the heat spreads. The temperatures involved are the same ones that govern the assembly profile, described under lead free versus leaded solder.

Nozzle Choice And Heat Transfer

The nozzle sets the shape of the air stream that reaches the joints. A nozzle sized to the component body delivers heat to the whole component at once, which is what a ball grid array or a quad flat pack needs, because the joints are underneath the body and cannot be reached from the side. A nozzle that is too large heats the neighbouring components; one that is too small heats the centre of the part without reaching the joints at the periphery.

For a component with leads on two sides, a nozzle that delivers air across the row is often better than a nozzle that surrounds the body, because the leads are directly exposed. For a shielded component or one under a heat sink, the heat has to travel through the shield, and the profile then needs a longer dwell at a lower temperature. Where a component cannot be removed with hot air at all, a different technique such as infrared or a conduction tool may be needed, and the choice is part of the rework plan rather than a decision made at the bench.

Setting The Profile

The rework thermal profile has the same four stages as an assembly profile: a ramp, a soak, a spike and a cool down, but compressed in time and applied to a small area. The soak brings the joint and the component to a uniform temperature; the spike melts the solder; and the cool down lets the joint solidify before the part is moved. Moving the part while the joint is still liquid tears the joint and leaves a partial solder deposit on the pad.

The profile is verified with a thermocouple attached to a joint under the component, or to a thermocouple on a thermocouple board that duplicates the assembly. The measurement is what matters, because the air temperature at the nozzle is not the temperature at the joint. The cooling stage is often the least controlled and the most damaging: a joint that is cooled by a blast of cold air can crack, and a board that is cooled too slowly allows the intermetallic layer to grow. The measurement of the pad temperature and the way it relates to the paste deposit are covered under placement order and pad positioning.

Protecting The Neighbours

The neighbouring joints and the components near the work are the main risk in rework. Adhesives and plastic bodies soften, small chip components can be disturbed by the air stream, and a nearby joint that is already marginal can be pushed over the edge. The measures that help are to shield the surroundings with a metal mask or a heat shield, to direct the air so that it flows away from the sensitive areas, and to keep the nozzle as close to the component as it can be without blocking the flow.

Components that have been through reflow more than once accumulate thermal exposure, and the specification for the number of reflow cycles the assembly may see applies to rework as well as to production. A board that is reworked several times is not the same board as one that has been through the line once, and the reliability data collected at the production stage may not describe it. Treating rework as a controlled process, with a recorded profile and a limit on the number of cycles, is what keeps the difference manageable.

Removing And Replacing The Component

The component is lifted with a vacuum tool or tweezers once the solder is fully liquid, and the lift has to be vertical, without twisting, because a twist shears the joints and can tear pads. Residual solder is then removed from the pads with a wick or a vacuum tool, and the pads are cleaned and levelled with a flat tip so that the replacement part sits on a uniform surface. Uneven solder left on the pads is the most common reason a replacement component does not sit flat.

The replacement is placed with the same accuracy as the original, using a placement tool where the pitch demands it, and the joint is made with the same profile that removed the part. Where the replacement is a fine pitch device, the paste or the solder is replenished rather than relying only on the residue, because the volume on the pad after removal is not controlled. The design decisions that make this possible, including the keep-out around a part likely to be reworked, are part of design guidelines for manufacturability.

Verification After Rework

The verification of a reworked joint is visual for the accessible joints, X-ray for the hidden ones and electrical for the function. A visual check confirms that the fillet has formed and that no solder bridge remains. X-ray confirms that a ball grid array joint has reflowed and that the void content is inside the limit. An electrical check confirms that the component works, but it does not confirm that the joint is mechanically sound.

Two additional checks are worth doing on a critical rework. The first is a check of the surrounding joints for disturbance, since heat spreads and the air stream may have moved a nearby part. The second is a check on the board itself for delamination and for pad lift, which shows as a discoloured or blistered area around the pads. Both are quick, and both catch the damage that appears in the area around the work rather than under the component that was replaced.

FAQ

Is a rework profile the same as a production profile? It has the same stages but is compressed and applied locally. It is verified with a thermocouple at the joint, because the nozzle temperature is not the joint temperature.

How many times may a board be reworked? The limit is set by the product specification and by the thermal exposure each cycle adds. Repeated rework accumulates intermetallic growth and stresses the pads, so a board reworked several times is not equivalent to a new one.

Can every component be removed with hot air? No. A heavy shielded part, a connector with a large thermal mass or a component under a heat sink may need a different technique, and the rework plan should identify those cases before the board is built.