It seems that everyone can do lead-free rework, so what is the big deal?
It’s all in the details.

Top-heat distribution across the component and constant heat transfer at the elevated temperatures needed for lead-free solder are critical factors to be taken into consideration when profiling a rework process. While heating the package connections, the ambient temperature of the die area must be minimized to protect the die from thermal-induced damage. Add to that the need to provide uniform under-heating to avoid board warpage, and the rework process becomes non-trivial.

It is a matter of common knowledge that top-heat methodologies can either be hot gas or infrared radiation (IR), and these technologies induce different heating results due to their diverse operating principle. Simply having a nozzle technology that relies on IR heating with an adjustable “aperture” that determines the heat flux seen by the package raises major concerns for all but the simplest of rework applications. If packages are remote from their neighbors and do not inhibit physical access, then divergence in the heating profile between these two disparate technologies is minimal. However, not many applications in the real world fall into this classification.

The challenge, therefore, becomes one of efficient heat transfer — into the areas where you need it, and away from areas heat spread could cause damage or disturb components. With a top-heat IR process, wider temperature variations across large components can be experienced, due to the way in which the incident radiation is absorbed or reflected by the surface of the component. At the elevated temperatures needed to reflow lead-free solder, these differences are exacerbated. Higher temperature leads to higher power, which leads to more beam spread — and more lateral disturbance.

In a hot-gas system, air is heated, controlled within a couple of degrees and by using an engineered nozzle design for each particular component shape and location, steered to where it is needed. It is an attractive notion, but an unrealistic expectation, to suppose that an IR source with moveable apertures can replace the need for an engineered nozzle solution (hot gas method) for today’s advanced rework applications.

If we add a secondary issue and allow the air to be replaced real-time by an inert gas such as nitrogen, a significant process advantage is realized. Reducing the prevailing oxygen in the air enhances wetting and improves the spreading behavior of the solder material — all of which go to producing a better solder joint control.

A rework engineer can choose from a variety of rework systems that handle these complex rework challenges to a greater or lesser degree of success. However, when it comes to the top-heating methodology, hot-gas seems to be the consensus.