Reflow Soldering Under Vacuum: A New Method

Estimated reading time: 5 minutes

With PCBs becoming increasingly complex, unexpected assembly issues are inhibiting quality. Adding vacuum and nitrogen to reflow — whether in a new reflow oven or integrated onto an existing SMT line — offers benefits like higher quality with an economical cost and continuous operation. Raimund Faber, SMT Maschinen- und Vertriebs GmbH discusses the causes of voids and how these defects affect quality, and one method to eliminate them during reflow without sacrificing oven flexibility or throughput.

Engineers are always coming up with new products or enhancing and improving existing ones. This is particularly true in the electronics industry, especially with the ever-increasing complexity of printed circuit boards (PCBs). More complexity is most often accompanied by unexpected issues that hamper product quality and performance. Assemblers need high-quality, high-performance products, manufactured with trouble-free operations and safety, but without neglecting economic considerations. Meeting these criteria, a vacuum and nitrogen module* has the advantage of being an integrated part of a new SMT reflow soldering system or retrofitted into an existing SMT line at any time. The ability to continuously operate this line is a distinctive advantage.

Soldering Defects

Void formation in reflow and wave soldering processes can be caused by various factors. The causes are not always easy to define, and they affect lead-free and leaded solder joints alike. Some causes include crystalline inclusions of flux, bubbles from out-gassing solvents and humidity from components and their materials, out-gassing of non-polymerized photoresists of solder masks, and inclusions of air below components as a result of an imperfect print and/or pad design. There are a variety of other reasons contributing to the creation of voids. The size of the voids, micro or macro, is of importance for the finished assembly quality.

Figure 1A. Normal soldering process.Figure 1B. Soldering with vacuum. Occurrence of voids in the soldering process is a manufacturing reality. Voids can impair the reliability of electronic board assemblies and influence electric integrity, as well as mechanical and thermal performance. Effect and consequences on the reliability of the solder joint depend to a large degree on the nature and/or type of void.

Negative effects on product quality include limited heat dissipation of components or solder joint structures, reduced stability of solder joint, performance limitation of components in high-frequency applications, reduced stability  against vibration, reduced capacitance of components (i.e. power modules), and increased solder problems (bridging, solder splashes, etc).

Three standards (IPC-A-610D, IPC-7095B, and J-STD-001D) deal with electronic assemblies affected by voids. They are applicable for BGAs only and provide guidance for accepting voids within certain limits. For all other solder joints, a rule-of-thumb value of ~ 25% voiding is commonly used in the industry.

Vacuum-based Reflow

A quality-conscious PCB manufacturer has to be aware of and control voids if his main business is to supply board assemblies to the automotive, medical, defence, and aerospace electronics industries. A close-to-perfect solder joint is of utmost importance for reasons of reliability and safety. To support these high-reliability manufacturers, a continuous vacuum-based process is the most promising equipment development.

The ideal vacuum module will eliminate voids using a continuous reflow soldering process. A vacuum-technology-based process is a promising path to follow, but it must be a continuous process in order for assemblers to meet economic return on investment (ROI) criteria. The conventional vacuum processes are stop and go, lack certain beneficial features, are less environmentally benign, and cannot match SMT assemblers’ cost of ownership (COO) demands.

Direct vacuum integration provides the manufacturer with a high degree of flexibility, and is significantly more economical than a separately operated system.

A vacuum unit positioned immediately behind the peak zone can be called up when required, as the vacuum process is centrally controlled with the option of being switched on or off. During the continuous reflow pass, the module can be automatically switched to vacuum operation. The PCB product is exposed to vacuum on preset parameters, kept under sub-pressure for a short period of time, and then vented. The cycle time for the vacuum reflow soldering process is 60–90 seconds.

Using this vacuum method, voids in the solder joint are reduced to a minimum, improving quality significantly.

Figure 2. An integrated vacuum and nitrogen module is an independent unit that can be fully integrated into a SMT reflow soldering system.

Using this vacuum module technology does not impose any restrictions on the usual requirements and/or parameters of the SMT reflow soldering system. Dual-sided PCBs can be processed; board width, height, and length settings are not affected; smaller boards can be processed with a carrier; etc. The use of nitrogen and cooling modules is also possible. With economics in mind, the vacuum reflow module is designed with a small foot print and only one sealing surface. For ease of use and maintenance, integrated software controls operating mode, including automatic setting of all parameters.

Environmental friendliness is increasingly important in SMT assembly. At reflow, this means energy efficiency. The vacuum reflow module adds only 1.5 kW/hr to reflow’s power requirement. Other environmentally friendly features include a low consumption nitrogen system and ABS process gas cleaning system. Unlike typical liquid gas cleaning systems — which need to be manufactured, transported, stored and disposed of in compliance with strict environmental hazardous material standards — this module uses a dry cleaning medium that has lower disposal requirements and easier compliance standards. Using a modern nitrogen/vacuum module like this avoids the environmental burden and protects resources from start to finish in the process chain.

Figure 3. Reflow oven with the independent module.Conclusion

Adding or retrofitting a modern vacuum and N2 module permits assemblers to operate an SMT reflow soldering system as a fully integrated vacuum soldering system for high-end, quality products in mission-critical assemblies, turning off the module when it is not required. The process technology will result in a significant reduction of voids in solder joints and improve product reliability and safety. It also increases versatility; with one system, electronics manufacturers can solder under vacuum, reflow solder, and cure, also in an inert atmosphere.

* SMT Maschinen- und Vertriebs GmbH, Wertheim, Germany, introduced the SMT Vacuum Plus N2 module at Productronica 2009 in Munich.

Raimund Faber, GM, sales & marketing, SMT Maschinen- und Vertriebs GmbH & Co. KG may be contacted at Roter Sand 5, D-97877 Wertheim, Germany; +49-(0)9342-970-0; info@smt-wertheim.de; www.smt-wertheim.de.

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