Considering Temperature Control as a Cleaning Method

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How can accurately controlling the thermal profile of your solder reflow process improve the cleanliness of your electronics?

A proper thermal profile more effectively complexes no-clean flux residues, mitigating a potential product reliability issue. The residues become benign, non-corrosive and non-conductive; ion chromatography (IC) cleanliness test results improve (as do SIR test results). Additionally, solder joints benefit from improved intermetallic formation.

What to consider

Many variables impact solder joint quality, including several directly related to the reflow profile:

• The actual temperature(s) of the surfaces to be joined (accurate profiling is critical)
• The dwell time above liquidus and maximum temperature
• The uniformity/distribution of heat
• The cooling rate

Component density in various areas of an electronic assembly can significantly affect the actual temperature attained in an area and the actual time at that temperature. Best practices for thermal profile control and monitoring should be followed.

Foresite investigated the effects of thermal profile variation on product reliability, and lead investigator Eric Camden presented the findings at the SMTA International 2014 Conference. Sample Foresite UMPIRE 2 board assemblies were reflowed at the recommended peak temperature and at temperatures below and above the recommendation, then evaluated with IC, SIR and cross-sectioning. A lead-free, no-clean paste with a recommended peak processing temperature of 250°C was used.

Findings

The acceptable range of peak temperature to produce quality solder joints is narrow – samples produced 20°C below the recommendation consistently produced unusable samples. Those processed only 10°C below the recommendation had poor intermetallic formation and failed IC and SIR tests. Processing at the recommended peak temperature resulted in improvements in all areas.

Exceeding the recommendation by 10°C resulted in the best intermetallic formation and acceptable IC/SIR results. However, watch for unintended consequences such as thermal damage to components, reinforcing the need to maintain good, tight, process control.