Reflow for leaded and lead-free assemblies

How critical is the requirement to use separate reflow ovens for my leaded and lead-free assemblies?

Y. P.

Expert Panel Responses

It's highly unlikely that dedicated Reflow ovens are required to process lead base and lead free alloys for traditional SMT applications. Intimate contact within the solder interconnects is prohibited by design, limiting lead migration to the Reflow oven's atmosphere. Modern convection dominant Reflow ovens exchange atmospheres at relatively high rates, continuously introducing fresh gas (air or N2) and exhausting as required. If desired, atmosphere samples can be collected and analyzed from strategic locations within the Reflow oven's process chamber to confirm. Additionally, oven components or subsystems that inherently collect process by-products such as exhaust manifolds, traps, collectors, condensers and exhaust stacks can be examined as well. Consultation with specific oven manufacturers is recommended, they should be able to provide oven related details.

Al Cabral
Regional Sales Manager
Finetech

Al Cabral is Regional Sales Manager for Finetech and Martin rework products. His expertise includes through-hole, surface mount and semiconductor packaging with an emphasis on soldering and heat transfer. Al has been a significant contributor to the development and optimization of reflow and rework processes and systems, particularly lead-free transitions and microelectronic applications.

This question is an important one for Wave solder as pot contamination can be a big issue. But on the Reflow solder side, there really is no issue. Solder paste stays on the pcb and does not "contaminate" the Reflow oven. And even if Tin Lead or Lead Free solder fell on to the bottom of the oven (as part of a pin-in-paste application for instance) it would not affect any performance at board level. The big difference between running Tin Lead and Lead Free is profile. You have to be sure to run the appropriate profile for the material being processed. The Lead Free profile can be 20-40 degrees hotter than the Tin Lead profile so using the Tin Lead profile for Lead Free parts will certainly not provide acceptable results. So the real trick is to segregate the profiles �not the machines. Otherwise, full speed ahead.

Marc Peo
President
Heller Industries Inc.

Mr. Peo has been with Heller Industries for over 20 years and has been President for the past 8 years. Marc has authored several industry articles on Soldering, Flux collection, nitrogen use and Lead Free conversion.

Developing a unique profile for tin-lead and lead-free assemblies is critical. Tin-lead solder (e.g. Sn63/Pb37) melts at 183C whereas the common tin-silver-copper (SAC) Pb-free alloys melt at 217C. For Sn63 assemblies it is common to have a peak temperature of approximately 220C, for Pb-free assemblies it is common to have the coolest portion of the board reach a temperature of at least 230C in order to achieve good wetting and solder spread. To make sure that your assemblies have good reliable solder joints you want to insure that all the solder joints on the PCB fit the minimum required solder profile provided by your solder paste vendor. In order to do this a new profile will be required moving from tin-lead to Pb-free solders.

Dr. Brian Toleno
Application Engineering
Henkel Electronics

Dr. Brian Toleno is the Application Engineering Team leader for Henkel Technologies. He is responsible for the technical service and application engineering for Henkel's electronics assembly materials, including solder paste, underfills, PCB protection materials, and underfills.

It is not critical at all. Unlike Wave soldering which is an additive process where solder is added to processed assembly thus requiring separate pots for different alloys, reflow process melts alloys thermal energy melts alloys (in form of solder paste) that is already applied onto PCBs. So the only difference are temperature settings. As long as an oven is able to achieve required temperature there are no concerns for lead contamination.

Dr. Rita Mohanty
Director Advanced Development
Speedline Technologies

Dr. Mohanty, PhD in Chemical Engineering, has over fifteen years experience in both industry and academic relating to engineering and electronic polymers, electronic packaging and assembly. She has experience in SMT process optimization and Lead-free transition.

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