No-Clean Doesn’t Always Mean No-Residue
No-clean flux technology is widely and successfully used in electronic assembly processes for many industries such as automotive, medical, and consumer products. But if several critical parameters are not effectively controlled, failures related to the no-clean process can occur.
We see many cases where no-trouble-found (NTF), returned assemblies are actually failures due to some aspect of the no-clean process. In these cases, one (or more) of these critical parameters was not properly addressed. In other cases, the fact that the no-clean assembly had failed was obvious.
The Critical No-Clean Flux Process Parameters
The goal of a no-clean process is to render harmless any residue that does reside on the finished assembly: non-conductive, non-corrosive, or electrically benign. These are the process parameters that become especially critical in a no-clean flux process:
The cleanliness level of incoming components, including the bare board, especially in specific, critical areas (overall cleanliness of a board can be misleading).
Soldermask quality. For example, insufficient cross-linking that results in absorbent porosity.
Solder paste flux that is entrapped under low standoff components when the exposed flux edges harden and the venting of volatiles is obstructed.
Liquid flux that flows to areas where it is not exposed to sufficient heat, for a sufficient time, for the volume present, to fully activate.
Hand solder and rework methods that do not sufficiently control flux volume, location or thermal activation.
Control of any cleaning operations that are utilized. For example, brush cleaning after rework.
The Challenge
No-clean soldering has shifted the critical process parameters from effectively cleaning any corrosive residues from the assembly, to controlling the cleanliness of incoming material (boards and components) and ensuring the complete thermal complexing of any residual flux.
We know that no-clean flux technology is certainly capable of producing reliable electronics. We also know that product issues related to no-clean flux processing have gone overlooked or misdiagnosed.