Hot Air Rework

When using hot air units, the proper nozzles must be selected, and the circuit assembly must be positioned properly.

Figure 1: Positioning the circuit board during rework so that it is perpendicular to the heating nozzle will help ensure even, uniform heating.

In the beginning, when surface-mount components started to take over, edging out through-hole components, a number of hot air-based rework systems appeared on the scene. Many were large, some small, but all had various means of delivering hot air to the component with varying degrees of process control. Despite their diversity, they all had one thing in common -- they were fine for removing components from a circuit board, but none were particularly good at the component replacement process.

As a result, replacing components was a highly manual task, dependent upon operator skill, often relying on such techniques as speed soldering or soldering one lead at a time. Many began to realize that, for most applications, these large, complex machines were unnecessary since replacement was going to be accomplished by hand anyway. Thus, small, efficient, bench top hot air units for removal have become the norm.

When using these machines, two factors are important for successful rework, in addition to the obvious heating/process settings: 1) the proper selection and design of interchangeable nozzles that direct the heated air flow to the component and 2) proper positioning of the circuit assembly during the removal process.

Figure 2: Despite the availability of machines for component rework, re-soldering components is almost always a manual task, dependent upon operator skill.

Nozzle Selection
Nozzles have evolved in design. The thinnest walled nozzles work best since they can reach the component on assemblies that are densely populated. A thick-walled nozzle might not fit, necessitating time-consuming and tricky manual removal of adjacent components. Nozzles have evolved to use a more open heating design, rather than a fluted design that directs heat onto the component leads and away from the component body. In particular, ball-grid array (BGA) component removal requires a uniform heating of the entire package to simultaneously reflow all of the solder connections beneath.

A difference of opinion exists on whether outer edge heating is most effective. Some will argue that double-walled nozzles, which direct heated air at the periphery of the component, avoid overheating the die in the package itself and protect the die from thermal damage. The argument against outer edge heating maintains that the component is designed to see reflow temperatures and that any argument that one can effectively heat the leads of a part, which is designed to dissipate heat and not the body of the package, is nonsense. The entire package reaches reflow temperature during the reflow cycle no matter where the air is directed. Besides, if the part is defective and is being replaced, the point is moot.

Circuit Board Positioning
Positioning of the circuit board during rework is also critical. Circuit boards should be properly positioned in a fixture prior to removal. Positioning means placing the circuit board so that it is absolutely perpendicular to the heating nozzle and the correct distance below the nozzle. If positioned too high, hot air will heat too wide an area and extend the dwell times. If positioned too low, the nozzle efficiency is reduced-leading to possible overheating. Follow the hot air system manufacturer's instructions and experiment on your own with practice boards to find the best parameters for you.

Board Preparation
Often overlooked is board preparation -- a task as important as the rework process itself. Tape off or mask areas or components adjacent to the target component with Kapton or high-temperature tape, metal shields, deflectors or heat sinks to minimize the spread of heat to solder joints or plastic parts that should not see reflow temperatures. Board preparation is not performed at the expense of heating uniformity, nor is it conducive to creating thermal stress. Prior to applying these masks, which can be done quickly, pre-bake the PCB to remove moisture from the assembly, minimizing the potential for delamination, measling or popcorning of components.

Preheating also shortens the ramp to reflow temperature, minimizing thermal stress in the board. The baking time may vary, but a minimum time would be 20 to 30 minutes, longer for larger boards or boards that have seen extended service in a high-humidity environment. Baking times will be recommended by the customer or by a process engineer.

Component Removal
What do you do once the part is hot enough for removal? Vacuum wands or pens, integral to the machine or attached as a hand-held accessory, are generally used to remove components from the circuit board assembly once the solder has become molten. The gentle nature of a vacuum lift will usually prevent damage to surface mount pads. While the vacuum grip is just strong enough to overcome the surface tension of molten solder and lift the component away, the grip will not hold if solid solder connections remain, except in cases of very fine pitch leads and tiny pads. However, vacuum pens will certainly not pull away a device that has been bonded with adhesive. Removal then is more difficult, risking damage to the circuit board, and requires the use of sturdier tools such as tweezer tools or dental type probes.

Some of these parts may be actually bonded to the board's surface, with epoxy or other adhesive, and this material may be on the underside of the component. Often, the parts are not readily visible or apparent, and the operator might not actually be aware of them. One indication would be the presence of adhesive or epoxy on other parts on the same side of the board. Also, visual inspection technology is now available that can look under components, providing there is enough space between the leads to see.

The only way to remove these components, without destroying the board, is to heat the board and part sufficiently to soften the adhesive, then gently and slowly pry the part off. The peripheral leads may need to be lifted first to remove them from the pads and bend them upwards, so as not to destroy the pads when the actual body of the component is pried or twisted loose. Unfortunately, it is unlikely that the component will be salvageable afterward.

Several members of the Circuit Technology Center team contributed to this feature story.