Dry solder joints and assembly cleanliness: Les defines solderability and troubleshoots cleaning process problems.

Q: What is a "dry joint?" Why is it important that a circuit board have no dry joints?

A: To achieve a solder joint between two mating surfaces, liquid solder must wet and flow over those surfaces to form a metallurgical bond. The IPC-T-50 Terms and Definitions document defines solder wetting as "the formation of a relatively uniform, smooth, unbroken and adherent film of solder to a basis metal."

In the industry, the term dry joint refers to a condition where the solder does not wet and flow on the pads of the printed wiring board and the component terminations. Instead, the solder beads up, similar to the way water does on a freshly waxed surface. Dry joints do not form robust metallic connections that will reliably carry electric current. Dry joints generally result in no connection or an intermittent connection. In soldering technology, the term used to describe the dry joint condition is "non-wetting" or "poor solderability."

Solderability refers to the characteristic of the surfaces being soldered and the ability of a metal surface to be wetted by molten solder. To aid in assuring adequate solderability, flux is used to clean and activate the surfaces to be joined. The flux removes oxides and other contaminants from the surfaces to be joined, raises the surface energy of the areas to be soldered and helps the solder to wet, flow and bond to form the connections. If the solder does not wet and flow on the surfaces to be soldered, no metallurgical bond will be achieved, resulting in dry joints.

Q: We manufacture IPC Class 3 assemblies using mildly activated rosin (RMA)flux. Until recently, we successfully cleaned all of our products after soldering and touchup with water and a saponifier in a batch cleaner. We recently purchased an inline cleaner and set up our process to match our customer's process. The customer also asked us to use a different saponifier. No matter how I adjust the process time, temperature and concentration, we cannot seem to get the same good results we did with the batch cleaner! Ionic contamination results are fine, but the board is not as cosmetically clean as it was with the original equipment and saponifier.

A: You apparently had a satisfactory process but lost the recipe when you changed your cleaning equipment and chemistry! My answer will consist of some questions that may help you to troubleshoot your new system.