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

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.

Did you check the compatibility of the customer-recommended saponifier with your mass soldering and touchup fluxes, soldermask and water temperatures when making the change? Does your customer use the same soldermask, solder paste and flux that you do? If not, perhaps the new saponifier is not compatible with your materials.

Did you talk to the technical representatives from the new saponifier and cleaning equipment suppliers to be sure the process and equipment setup you are using is fine? Have you tried your original saponifier to see if it works with the new inline cleaner? (If it does, go back to using it!) are you sure that your incoming boards are not contaminated? Do you check incoming board cleanliness and maintain a clean storage area?

Keep in mind the many process variables that come with new equipment, such as spray pressures and spray configuration. Even more pertinent is the possible incompatibility of the new saponifier with your soldermask, flux and perhaps water temperatures.

You have changed two major variables in your cleaning process: chemistry and equipment. Since you did not have the problems before the changes, the most probable explanation is that the new saponifier is incompatible with your flux and soldermask or water temperatures. I assume you have had technical personnel from the equipment and the saponifier supplier in your plant working the problem. Keep me posted on the outcome.

Send your process, technology or training question to les.hymes@worldnet.att.net. Please type "ASK LES" in the subject line and indicate your name and company or institute affiliation. All questions june not be answered.

Les Hymes is the owner of Les Hymes Associates, Surprise, AZ; (623) 544-4646; e-mail: les.hymes@ worldnet.att.net.