Vapour Phase Reflow - Profiling For Lead Free AlloysAuthors: Allen Duck and Claus Zabel
Company: A-Tek LLC and Asscon Systemtechnik
Date Published: 5/17/2010 Conference: ICSR (Soldering and Reliability)
Gases as a conductive medium are very poor and while the Reflow process is often described as Convection it is in fact a conduction system. A heat source has a gas passed across it and energy is transferred from the source to the gaseous mass. The gas is directed via a Plenum and other flow control mechanisms to the PCB where the energy is transferred from the gas to the PCB [and everything else it comes into contact with.] The challenge is in maintaining control of the transfer given the overall inefficiencies in the system. If you assume a transfer ratio of energy in a gaseous system that is 1/10th of a hot plate then it is reasonable to assume you need to generate 10x the energy at the source that the PCB will require as the gas will not adequately convey the available energy. Added, the inefficiencies at the PCB end of the system are equally poor and further frustrate the control and energy efficiencies of the machines. Hence when we look at power consumed V power used the delta is significant. When you consider the issue of Delta Ts and the higher temperature demands of new alloys – it is obvious the response times in a gaseous system are simply incapable of sensing, providing, transmitting and receiving energy in a real time manner. These issues are exasperated with complex geometries and heavier mass PCBs, stacked parts and PCB to parts gaps that are so fine it is impossible to get gas flow under the parts to carry the heat into the central points of I/O.
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