REVERTING BACK TO CONVECTION REFLOW FROM VAPOR PHASE REFLOW FOR HIGH END SERVER PRINTED CIRCUIT BOARD ASSEMBLY

摘要

With the reflow of solder on large multilayered High End Servers printed circuit board assemblies (PCBA), the vapor phase (VP) reflow process was selected to heat the high thermal mass circuit board to the appropriate surface mount solder joining temperature. The vapor phase oven heats a vapor fluid to its boiling point temperature and its vapor condenses on the PCBA and transfers heat quickly and uniformly to the circuit board. The VP process minimizes the risk of damaging the PCBA and prevents degradation of product reliability by using a vapor fluid that boils at a temperature below the limitation of the raw board and components materials. However, the high cost of the vapor fluid and its potential environmental impact motivated the re-visiting of the convection reflow method for High End Servers PCBA. The convection oven chosen for this evaluation was a multi-zone conveyor reflow oven fitted with chilled liquid cooling zones. The major challenge associated with convection reflow technology is to prevent the over heating of lower thermal mass components since more thermal energy is needed to heat the larger components on the PCBA. This paper describes how the convection reflow process was developed and qualified for the high end server PCBA which was initially assembled using the VP process. A side by side comparison of VP and convection thermal profiles along with method of thermal mass compensation for the convection reflow fixtures, shielding of lower mass components, and reliability qualification will be presented.