Controlling Copper Dissolution
One of the greatest dangers of any method of soldering is copper dissolution. This occurs when the copper knee that the component is inserted into comes into contact with excessive heat and melts.
It has become a much more common phenomenon in recent years, with the reason being that lead-free solders have a much higher flow point than leaden solders. Copper dissolution is not, however, limited to only lead-free solders; it can occur with lead-free and lead-tin all the same.
When it occurs, it often results in a poor connection if not a damaged board. Therefore, it’s important to take steps to prevent it.
Lead-Free Solder Raises Risk Of Copper Dissolution
Copper dissolution occurs due to excessive heat. This can occur when soldering a component and the solder is at too high a temperature for the copper knee of the hole in the board. Another possible cause is too high a temperature in the soldering machine or oven itself, such as if using lead-free solder in a reflow oven. This is why thermal management is so important when soldering components.
In either case, part (or all, in some cases) of the copper knee melts away. Since the copper knee is essential to forming a proper connection, the result is poor joints and thus more defective product having to be reworked or scrapped.
Copper dissolution was previously far less common, becoming a more widely observed phenomenon in the 1990s. What changed?
Lead free solders entered the marketplace. Since lead is toxic, there are many reasons to consider a lead-free solder over a leaden one. However, since lead-free solders also have a higher melting point, dissolution starting occuring at a greater frequency.
Additionally, selective soldering is itself a risk factor. Selective soldering largely replaced wave soldering in much of the industry as it allowed for greater precision. However, the solder fountain does subject copper joints to more heat whether using lead-free solder or not.
In other words, selective soldering and lead-free solders come with an increased risk of copper dissolution. What can be done if you intend to use lead-free solder and your product requires the use of a selective soldering machine?
Selective Soldering Parameters Cut Down On Copper Dissolution
What can be done to minimize these risks? After all, not every manufacturer can switch to reflow ovens for their product. Plenty of industries require through-hole components and have to rely on selective soldering to stay in business.
Additionally, the international regulatory environment makes it so that some jurisdictions almost preclude the use of leaden solders entirely. Therefore, it is also the case that many operations must-needs continue to use lead free.
The solution happens to be fairly simple, and any programmable selective soldering machine – including those made by RPS Automation – is capable of taking the appropriate steps to minimize risks of copper dissolution.
Whether copper will dissolve is a question of heat and time. In other words, longer a copper knee is subjected to heat, the greater the chances of dissolution. What you must do, therefore, is select the appropriate length of Time On Joint, or the amount of time devoted to a solder joint.
It was noticed over the years that the shorter the dwell time on any joint, the lesser the frequency of boards with dissolved copper. Optimal TOJ should be set between 1 to 6 seconds, as copper will dissolve after being subjected to solder for 7 seconds or more.
TOJ, however, does need to be balanced with connection quality. You may need to calibrate your selective soldering machine for a TOJ that creates better joint with your PCBs and components.