Beating the RoHS Heat Print E-mail
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Written by Chris Munroe   
Friday, 29 February 2008 19:00

For RoHS PCBs, vapor phase offers flexibility and lower temperatures than convection.

Vapor 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 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 Pb-free 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.

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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 insulation resistance tests have been completed to validate that the VP process and fluid used do not leave residual contaminates on the PCB that are detrimental to the assembly’s long-term performance.1

What has been done to determine that VP solder joint quality is as good as convection or IR? There are many independent studies on vapor phase reflow. In conjunction with the available studies from independent laboratories, EPIC has completed a number of validation protocols to show the capability of the vapor phase process to solder both lead and Pb-free PCBs. Test cycles included thermal cycle, thermal shock, shear, vibration and cross-sectional analysis of the solder fillets. In all cases, the solder joint quality was as good or better than those produced using convection reflow processes (Figures 2 and 3). Also, vapor phase reflow has been used in high reliability military and aerospace programs since the early 70s.

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How safe is the Teflon material for the environment and workforce health? The material used for vapor phase is an FDA-approved Teflon. There is no risk of harmful exposure to the material, and no fluorocarbons are produced at the operating temperatures used for reflow soldering of PCBs.

EPIC has conduced two independent laboratory studies to validate there are no health and safety risks to its employees when exposed to the Teflon materials. Employees are monitored on an ongoing basis, and air quality samples are taken yearly.

Why recommend vapor phase reflow when the industry is predominately convection reflow?

Solder joint quality is improved with vapor phase, with an inert soldering environment, and superior heat transfer. As the industry changes to Pb-free, manufacturers have encountered suppliers that are not informing the industry on component plating changes. Vapor phase is much more forgiving to these changes and improves the solder process window.

There should be no concern on the use of vapor phase reflow in manufacturing today, with few exceptions that also pertain to convection and IR. Components such as non-sealed switches and SMT sensor devices should be reviewed for compatibility. Also, with the heat transfer characteristics of vapor phase, soldering SMT electrolytic capacitors needs to be monitored closely. It must be understood that components on the PCB during reflow will see the peak temperature set by the boiling point of the fluid used.

Benefits to Lean

Vapor phase reflow also benefits the Lean Manufacturing environment. A key concern with vapor phase reflow in volume manufacturing is throughput; many manufacturing engineers question how they could accept a reflow process that is going to bottleneck the SMT placement process.

The reason for this concern is the inherent design of VP systems exposes the PCB to the vapor process longer than the tact time of the placement processes. To deal with this, vapor phase systems are now designed with throughput in mind. Inline machines are available that can produce double-sided SMT PCB at rates comparable to inline convection and IR reflow processes. These designs include inline batch carrier and inline edge chain systems, designed with sub 30-sec. tact times. Just like any other process machine design, each of these has pros and cons. Batch carrier machines tend to be more mechanical and prone to more maintenance, while inline systems are more gated by the size of the PCB being processed, and less flexible when it comes to changeovers.

Lean manufacturing principles in low-to-mid volume manufacturing emphasize minimal changeover time. Changing from product to product needs to be seamless and completed faster than the time taken by upstream processes. It is standard process for a reflow oven to be processing the last PCB of one production lot, while changes for the next lot are being completed on the placement machines. Changeover time can be an issue in convection reflow if the reflow zone temperature changes are significant between the two products and the edge carriers need to be resized.

Inline vapor phase systems designed with edge chain conveyors will have the same constraints as inline convection systems, and it is necessary to wait for preheat changes to materialize before the new process starts. Conversely, inline batch carrier vapor phase systems are capable of processing one product lot’s board size changes while another lot is still being soldered. PCB size adjustments can be completed; with IR preheat followed by vapor phase soldering, there is no delay to changeover once the sizing is completed.

Lean principles also apply to new product development. It is imperative to maximize capital expenses. To do this, manufacturers must find ways to launch new products on production lines used in sustaining production operations. Standalone prototype lines are becoming harder to justify. It is difficult to dedicate multi-million dollar, state-of-the-art placement lines to NPI activities, yet using obsolete equipment offers no benefit to a true product validation on the production line. How do engineers find time on production lines to properly profile the PCB through the reflow process? By running sample assemblies with thermocouples through reflow numerous times to get the right profile, with manual inspection of the solder joints and flux residue to determine the correct target profile. Engineers develop matrix charts on board size, layer count and complexity to approximate the profile so process development time is minimized.

How does vapor phase improve this process? With its heat transfer characteristics, and the uniformity at which it accomplishes this, it is easier to understand the profiling relationship between PCBs. Vapor phase reflow profiling can be classified by process type to the point where there are fewer profiles to develop. For instance, standard multilayer 0.062"-thick boards can follow the same Pb-based vapor phase profile regardless of component complexity. Heat load monitoring during soldering permits the systems to profile almost automatically. Ramp rates and soak times at peak temperature can be engineer-defined and controlled by the systems, regardless of product mix. In a true one-piece flow on a prototype, it is much easier to get it right the first time using vapor phase processing. Inadequate reflow temperature or over-temperature on the first article is virtually eliminated.

VP and Pb-Free Components

Pb-free components are going to be introduced in Pb-based processes. This challenge to engineering and quality is a huge concern and one that needs scrutiny. EMS providers rely on component suppliers to ensure the Pb-free transition on component terminations is seamless to their soldering processes, but that rarely happens. Termination changes require additional modification to solder profiles and flux chemistries to ensure proper wetting of the solder to the Pb-free termination. Nitrogen in convection reflow is becoming more a requirement than an option, and nitrogen is expensive.

Vapor phase reflow offers an advantage over convection in this Pb-free transition phase. The inert environment and consistency of heat transfer permit vapor phase to be more forgiving with Pb-free component terminations. Less active no-clean flux chemistries have proved adequate in soldering Pb-free terminations that demand high-activity fluxes in convection reflow.

Another advantage of vapor phase reflow is its lower processing temperature. Convection or IR process temperatures can reach 245° to 265°C at the component level. VP temperatures stay at 230°C. The lower temperatures make it possible to use standard FR substrate material rather than (higher cost) FR-406. Higher temperature substrate material can add 10 to 15% to PCB cost. Vapor phase also offers processing advantages with large mass connectors because the thermal equilibrium is superior. In convection reflow, particularly in higher-temperature Pb-free processes, correctly soldering large mass connectors may overheat the rest of the PCB.

Also, cost savings is attributable to lower energy consumption. In addition to the elimination of nitrogen, electricity use with vapor phase is much lower.

Vapor phase systems under development will automate solder reflow. These systems will have the ability to monitor and control the thermal process during soldering. This would further reduce the number of hours spent profiling production assemblies, and would ensure the proper profile is sustained, no matter what the production load is on the system or process. Vapor phase systems can also incorporate vacuum environments that will produce zero-void solder joints, key to improving solder joint strength.

While vapor phase reflow still has throughput constraints, overall the technology has advanced significantly the past few years. Rising energy costs, continued pressure to do more with each production line, and the migration to Pb-free components make vapor phase a viable alternative to convention and IR reflow processes. As such, vapor phase will see a comeback in the Pb-free, Lean manufacturing environment.

References


  1. Steve Fraser and Chris Munroe, “Using Vapor Phase Reflow in Lead-free Processing,” SMT, April 2005.

Chris Munroe is director of engineering at EPIC Technologies (epictech.com); This e-mail address is being protected from spambots. You need JavaScript enabled to view it .

 

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