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No-Clean Flux Residue and Underfill Compatibility
No-Clean Flux Residue and Underfill Compatibility
This paper covers an experiment designed to measure the electrical reliability of various combinations of underfill and no-clean flux residues.
Materials Tech

Authored By:
Eric Bastow
Indium Corporation
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Transcript
No-clean soldering processes dominate the commercial electronics manufacturing world. With the explosion of growth in handheld electronics devices, manufacturers have been forced to look for ways to reinforce their assemblies against the inevitable bumps and drops that their products experience in the field.

One method of reinforcement has been the utilization of underfills to glue certain surface mount devices to the PCB. This provides additional mechanical strength over and above the soldered connections.

Bumped SMDs attached to the PCB with a no-clean soldering process offer the unavoidable scenario of the underfill coming in contact with a flux residue. This may or may not create a reliability issue.

No-clean solder paste flux chemistries can vary. Some have halogens and others do not. Some have standard residues and others have residues optimized for pin probing.

There are also a number of underfill chemistries on the market. Furthermore, underfill curing conditions vary depending on whether the SMDs are exposed on the surface of the PCB or underneath an RF shield.

The paper discusses an experiment designed to measure the electrical reliability of various combinations of underfill and no-clean flux residues.

The results of the experiment indicate that solder paste/underfill combinations involving an underfill with good SIR performance and a solder paste with good SIR performance produced a combination with good SIR performance.

No chemical incompatibilities between the solder paste flux vehicles and the underfills were detected. The SIR results of the solder paste/underfill combinations logically follow the SIR results of the parent materials.

Keeping in mind the adage that electricity follows the path of least resistance; if one material performs poorly by itself, it will cause the combination to perform poorly regardless of the performance of the other materials in the combination.

If an underfill/paste combination is causing current leakage, both materials should be investigated separately to determine the culprit material. The combination should not be lightly dismissed as incompatible.

Furthermore, one should not immediately assume that one material is the problem and not the others without first investigating the materials separately.
Summary
No-clean soldering processes dominate the commercial electronics manufacturing world. With the explosion of growth in handheld electronics devices, manufacturers have been forced to look for ways to reinforce their assemblies against the inevitable bumps and drops that their products experience in the field. One method of reinforcement has been the utilization of underfills to "glue" certain surface mount devices (SMDs) to the PCB. This provides additional mechanical strength over and above the soldered connections. Bumped SMDs attached to the PCB with a no-clean soldering process offer the unavoidable scenario of the underfill coming in contact with a flux residue.

This may or may not create a reliability issue. No-clean solder paste flux chemistries can vary. Some have halogens and others do not. Some have standard residues and others have residues optimized for pin probing. There are also a number of underfill chemistries on the market. Furthermore, underfill curing conditions vary depending on whether the SMDs are exposed on the surface of the PCB or underneath an RF shield. This paper will discuss an experiment designed to measure the electrical reliability of various combinations of underfill and no-clean flux residues, as measured with J-STD-004B SIR (IPC-TM-650 2.6.3.7).
Conclusions
It was observed that Underfill B produced a unique visual anomaly as can be seen in Figure 16. This anomaly was especially pronounced on the SIR patterns that were covered with RF shields. It is not known whether the underfill looked this way after curing or not until after SIR testing. None of the other underfills exhibited this sort of phenomenon.

In a previous work, the author studied the impact of RF shields on no-clean flux residues. In that work, the results showed that entrapping a flux residue under an RF shield consistently produced lower SIR results than residues that were fully exposed (not covered). In that work, the RF shields were larger, produced a better seal (restricted outgassing), noticeably reduced the peak temperature of the SIR patterns below the shields, and were left on the patterns during SIR testing. In this study, certain materials and combinations, especially the underfills, seemed to benefit (produce higher SIR values) when covered with an RF shield. However, in this study, the RF shields were smaller, did not create a seal (see the corners of the RF shields in Figure 2), did not significantly reduce the peak temperature of the reflow and cure profile of the SIR patterns below the shield, and were removed prior to the SIR test. Therefore, it is unclear just what sort of impact these RF shield have on the results and how well they mimic a real world scenario. The author believes that the results in the previous work are more true to reality.

Underfill C produced poor SIR results. The poor SIR performance of Underfill C was transposed onto the SIR performance of the solder paste/underfill C combinations. Based upon the shape of the graphs (decreasing SIR performance with time), it would appear that Underfill C is somehow sensitive to the SIR chamber test conditions (temperature, humidity, and/or applied voltage).

Solder paste/underfill combinations involving an underfill with good SIR performance and a solder paste with good SIR performance produced a combination with good SIR performance. No chemical "incompatibilities" between the solder paste flux vehicles and the underfills were detected. The SIR results of the solder paste/underfill combinations logically follow the SIR results of the parent materials. Keeping in mind the adage that electricity follows the path of least resistance; if one material performs poorly by itself, it will cause the combination to perform poorly regardless of the performance of the other material(s) in the combination.

If an underfill/paste combination is causing current leakage, both materials should be investigated separately to determine the culprit material. The combination should not be lightly dismissed as "incompatible". Furthermore, one should not immediately assume that one material is the problem and not the other(s) without first investigating the materials separately
Initially Published in the IPC Proceedings
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