Circuits Assembly - March 2008 - (Page 38)

Cover Story Beating the RoHS Heat Chris Munroe For RoHS PCBs, vapor phase offers flexibility and lower temperatures than convection. apor phase reflow is not a new process for electronics manufacturing. It is simply an alternative process for SMT reflow that has existed since the early 70s. Most engineers familiar with vapor phase recall the technology’s early shortcomings: environmental concerns over fluids used; throughput limitations; applicability only to single-sided PCBs, and an inherent problem with tombstoning. Technology advances – improved machines, chemical selections and process controls – have addressed many of these V Figure 1. In vapor phase, the assembly is immersed in vapor blanket until it reaches temperature equilibrium with the Teflon fluid’s boiling point. shortcomings. As a result, vapor phase is becoming a viable alternative to consider in volume manufacturing. As EMS providers have been challenged to increase schedule flexibility and implement Pbfree processes, engineers are beginning to recognize the benefits of a reflow technology that offers broader process windows, reduced changeover time and, in the case of RoHS-compliant assemblies, lower peak temperatures. Significant information is available on the vapor phase reflow process. The intent here is not to detail the process itself, but to present a general process overview and outline key advantages in relationship to Lean manufacturing principles and implementation of RoHS-compliant assembly. The vapor phase reflow process makes use of the heat produced by a boiling Teflon fluid (Figure 1). This boiling fluid produces a uniform temperature zone (vapor blanket) in which the assembly is exposed for solder purposes. Heat is transferred to the PCB as it is immersed into the vapor area until the assembly reaches temperature equilibrium with the fluid’s boiling point. The primary soldering benefits of vapor phase in comparison to infrared or convection include an oxygen-free (inert) environment without the need for nitrogen, fixed upper temperature exposure, and superior heat transfer on thermally challenged PCBs. Following are some of the most asked questions concerning vapor phase: What residues remain on the PCB following vapor phase reflow? Ion chromatography and surface 38 Circuits Assembly MARCH 2008 circuitsassembly.com http://circuitsassembly.com

Table of Contents Feed for the Digital Edition of Circuits Assembly - March 2008

Circuits Assembly - March 2008 Caveat Lector Letters Industry News Market Watch Talking Heads Focus on Business On the Forefront Screen Printing What Drives the Crowd? Mastering ESD Control in Automated Handling Systems Beating the RoHS Heat Trade Secrets True to Its Roots The Road Abroad - Strategic Alliance or Greenfield Facility? Tech Tips Wave Soldering Test and Inspection Process Doctor Pb-Free Lessons Learned Getting Lean Materials World Equipment Advances Apex Product Preview Ad Index Assembly Insider Technical Abstracts

Circuits Assembly - March 2008

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