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May, 2020
Inline Robotic Soldering Iron Eliminates Copper Dissolution in High-Mass Joints
By William M. Hunter, Managing Director — Americas, mta automation, Inc.
some solder joints are impossible to solder using any form of wave solder. Until now, manufacturers have skipped these problematic joints dur- ing the inline process and solder them offline using handheld or robot- ic soldering irons. In general, solder- ing offline is inefficient and con- strains throughput, while hand sol- dering is inconsistent. The MPS700 is an inline, bot-
T
tom-side system that uses a robotic soldering iron ideally suited to solder problematic joints. This allows man- ufacturers to process the entire board inline. Overall throughput improves, because problematic joints do not need to be processed offline. Quality and consistency improve, because hand soldering is eliminated and wave technology is not forced to sol- der joints beyond its technical limits.
Problematic Joints There are several reasons joints
can be problematic, and the MPS700 is designed to address them all. However, the most serious problem seems to be copper dissolution in high-mass joints. All solder joints must be preheated to reflow temper- atures before solder will wet to the
TM
he most common through-hole soldering technology is the selective mini-wave. However,
joint. The selective mini-wave circu- lates molten metal over the joint until it reaches reflow temperature, then the solder will wet to the joint. As molten solder washes over
joints to preheat them, some of the copper from the board dissolves into the molten solder and is removed from the board. With small joints, exposure to the wave is limited, so dissolution is minimal and accept-
able. However, high mass joints require much more exposure to the molten solder to reach reflow temper- ature. As a result, the copper layers can become dangerously thin before they reach reflow temperature. In contrast, soldering irons pre-
heat each joint to reflow temperature using a heating element (the solder tip). Once the joint is heated, solder wire is fed to the joint and melts on
contact. Solder is only added — not removed — so there is no dissolution of the copper at all. The MPS700 eliminates copper dissolution in high-mass joints.
Wave Workarounds There are a few workarounds.
The most common workaround for wave soldering machines is to preheat the boards as much as possible using an oven before the wave. While pre- heating heavy copper boards can take a long time, this solution works fairly well if the entire board can be preheat- ed to near reflow temperature. At this point, the joints do not
require much exposure to the wave to reach reflow temperature, so copper dissolution is minimized. However, preheating to such high tempera- tures is not always feasible. Some components cannot be heated above 176°F (80°C). In other cases, the boards have plastic assemblies that limit preheat temperatures. If there are components or assemblies with temperature limits, preheating the entire board is not a viable option to mitigate copper dissolution. Another workaround is to delib-
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erately make the copper thicker than needed, such that, as the copper dis- solves, the final thickness is still
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