Infra‐red Reflow for the Solder Attachment of Surface Mounted Devices

Infra‐red ovens and furnaces have been in use in the microelectronics industry for a long time, but recent design advancements and process developments have provided the precise furnace control dictated by surface mount technology. This paper addresses the process implemented by state‐of‐the‐art infra‐red equipment and the impact which that technology has on the surface mount industry. The temperature cycles that current infra‐red equipment utilises to reflow surface mount devices differ from the competing technologies in several distinct areas. Infra‐red equipment provides controlled preheat rates in the critical initial heating stages, followed by temperature equilibration and final reflow. The radiative and other processes involved in the preheat of product are discussed, with particular attention paid to the removal of volatile organics from the solder paste and the stress induced on all elements of the assembly. The control of temperature uniformity is discussed, and the effects of geometry, thermal complexity, and absorptivity are examined with reference to heat transfer theory. The ability of infra‐red equipment to provide close control of the ambient atmosphere during reflow cycles is outlined. Control of the atmosphere can benefit process control, reliability, and cosmetics. Inert atmospheres, such as pure nitrogen, can provide faster processing, more rapid and easier cleaning of fluxes, better solderability of secondary joints, and better cosmetics for the assembly. The inclusion of small amounts of hydrogen in the ambient can induce changes in the contact angle of the flux to polymers and ceramics, providing benefits for other process considerations, such as component shifting, solder balling and flux removal.