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The definition of failure is “the lack of success in doing or achieving something, especially in relation to a particular activity.” If the activity is concerning a soldering process, such a failure can have a downstream impact far beyond the actual solder joint. In this regard, it is first necessary to understand what constitutes a good solder joint because appearance is too often deemed a success.
These challenges to solder joint reliability were exemplified when, in July 2006, the RoHS directive came into effect, and the higher thermal demands of lead-free solders forced all manufacturers of soldering irons to focus on improved heat transfer. This requirement was further complicated by the ongoing decrease in component sizes and the fact that many PCBs are becoming more like heat sinks due to multiple layers and other factors. The importance, therefore, is for soldering irons to provide:
- A fast response (speed)
- No overshoot (control)
Certainly, most systems today offer good or even excellent performance in thermal energy capability, but difficulties emerge in those systems using conventional ceramic heater technology, especially concerning:
- Tip-to-ground resistance (difficult to maintain)
- Tip-to-ground voltage leakage (difficult to maintain)
- Thermal transfer efficiency
- The potential for solder splatter (due to temperature overshoot)
- A requirement for calibration of the thermocouple
In this article, we will explore the considerations necessary to achieve good solder joints and offer some practical rules for good solder joints and how to achieve them reliably. We will also discuss other thermal energy factors to keep in mind.
Figure 1: Components of a good solder joint and their relative placement during the creation of a solder joint.
The considerations necessary to achieve good solder joints are (Figure 1):
- The formation of an intermetallic layer
- Solder joint structure
- Joint temperature (military standard)
- Tip temperature vs. joint temperature
- Maintenance of the soldering profile (similar to that found in a reflow oven)
When copper comes in contact with molten solder, it forms two distinct intermetallics between the copper and the tin contained in the solder (Figure 2):
- Layer of “e-phase” (Cu3Sn) next to copper
- Layer of “h-phase” (Cu6Sn5) a thicker layer above
Tin is depleted by the formation of intermetallics, so in tin-lead solders, there will be a resultant lead-rich region.