In the last ten years solder paste has become a major part of the soldering process.
Solder paste has improved dramatically in performance and consistency over the past ten years. That does not mean that all the problems of applying and reflowing paste are eliminated, but that with care consistent results are not difficult to obtain. The following suggestions can help in producing sound results.
Consistency. Develop a sound process and stick to it rigidly. Although the performance of the paste has been improved this is no excuse to allow variations to creep into the process. For example do not permit stenciled boards to sit around for varying lengths of time before placing the components. Our pastes do not vary much in tack during the first few hours after stenciling, but there is inevitably a small variation. Try to eliminate all the variables from the process, no matter how small. Look very carefully for the not so obvious variations. In one case opening the cover to the stenciling machine changed the process characteristics.
Stenciling. In a similar manner you can leave today’s paste on the stencil without the properties changing very much. But why make life more difficult? The variations are small, but inevitably there will be small changes, and by setting up the process carefully they can be avoided. It is so often the small things that collectively cause problems and are individually extremely difficult to find. Eliminate the variables in every part of the process. Develop good processing parameters for the stencil machine and then make sure they are followed at all times. Check the machine routinely for accuracy, and also remember that the boards must also comply with the processing accuracy required.
Stencil Cleaning. If the stencil is correctly designed, and the boards are flat, there should be little paste on the bottom of the stencil. But with time some paste will find it’s way there and transfer to the board surface. The result, —- solder balls. Whenever they are found after reflow, look at the bottom of the stencil for paste. If it has to be cleaned every few boards then check the stencil design. It should seal tightly onto the pad during stenciling.
Reflow. The reflow parameters are given only as a guide. The design of the assembly, and the sizes of the various joints all affect the thermal profile. The reflow oven has to be set to reflow the most thermally massive joints. A good rule of thumb is to keep the solder molten for the shortest time that will form a sound joint, and cool as quickly as possible without thermally shocking the assembly. There is no way that the optimum parameters can be calculated they must be developed from practical testing. However they must also be developed in a careful and controlled manner. They will not be found by haphazardly changing the machine parameters.
Hirsch Metals sells a variety of solder pastes including No Clean” Contact Paste and “Water Soluble” Contact Paste