An introduction to surface mount adhesives

In the electronics industry, surface mount technology plays a critical role in the evolution of high speed, automated printed circuit board assembly. Surface mount adhesives hold the component in place during pick and place, soldering and board handling. The product characteristics of surface mount adhesives are continuously improved to meet the rapidly changing needs of our customers. These improvements are accomplished by listening to, and working closely with our customers, and the dispense equipment manufacturers. The relationship between the adhesive and the dispense equipment is a symbiosis with a clear and measurable impact of the dispense equipment process on adhesive performance. This paper explores the general requirements for applying surface mount adhesives and how these can be influenced by changing the dispense equipment settings. Today, surface mount adhesives can be applied on the board by means of three different techniques: dispensing; screen printing; or pin transfer. For pin transfer, screen, or stencil printing where the adhesive is exposed for long periods of time to the open environment the critical application requirements are water absorption and curing characteristics. For high speed pneumatic and positive displacement dispensers, where the adhesive is in a closed environment, the critical application requirements generally include dot profile, excellent dispensing performance at high speed, good bond strength and fast cure. Understanding the simple relationship between chemistry and physics and the different mechanisms that are able to improve this relationship will automatically lead to increased board assembly speed while improving the finished board yield. This paper shows in detail how the adhesives are subject to external influences such as temperature, pressure and humidity and how intrinsic product characteristics are effected. It also offers trouble shooting information about the different approaches that can be taken in order to avoid (humidity absorption), or take advantage (temperature increase) of these intrinsic changes.