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Trends in Wavesoldering Technology
December 31, 1969 |Estimated reading time: 7 minutes
Trends in Wavesoldering Technology
The fundamental mass-joining technology has evolved to meet industry demands for better, faster and cheaper assembly.
By Rolf Diehm and Johannes W. Neupärtl
Virtually unlimited manufacturing versatility provided at an affordable price is one of the more persistent requests customers make of equipment vendors. To satisfy this demand, proactive manufacturers are offering new equipment designs that provide maximum flexibility. For example, modular wavesolder machine designs encompass more than easy change-out of preheater sections and fluxer units. Rather, the new equipment enables an expansion of capabilities by adding or replacing preheat and fluxer sections in existing equipment (Figure 1). Specifically, a soldering system with a foam fluxer may, via an additional module, incorporate spray fluxing.
Figure 1. A modular wavesoldering system can enable an expansion of capability, e.g., by adding preheat and fluxer sections.
Such a strategy might be used when a manufacturing line is in transition to an alternate flux chemistry or to support a contract manufacturing line. Similarly, as volume increases, it may be feasible to add a preheater module to support new required manufacturing speeds. Also, a modular wavesoldering system opens the opportunity to field-upgrade to localized nitrogen inerting at the solder waves to capture this technique's solderability benefits. Finally, a modular system possesses a versatile control unit capable of incorporating the added units into its overall machine control.
Interchangeable ElementsIn addition to expanded functions provided by a modular design, flexibility should include an ability to exchange, or rearrange, key processing elements such as the fluxer unit, individual preheater sections and easily interchangeable solder nozzles. For example, rotating fluxer "magazines" facilitate performance of required maintenance at minimal machine downtime. Interchangeable elements also permit exchanging IR preheat sections for forced-convection modules, or exchanging standard solder nozzles with specialty units suited for specific soldering tasks.
Conveyors and Board Transfer TechnologyAt least one manufacturer of wavesoldering systems offers segmented conveyor systems that permit fluxing, preheating and soldering of assemblies at different conveyor speeds. This can be useful for tailoring parameters for challenging designs. With segmented systems, a separate conveyor in the fluxer section helps to contain flux in that area, thus reducing maintenance requirements. Meanwhile, the "downside" of segmented conveyor systems (i.e., a minimal board length is necessary for effective board transfer) has been addressed in the most recent equipment designs. Now, conveyors permit effective transfer of boards as small as 3.5", accommodating most assemblies processed today.1
Figure 2. Segmented conveyor with pin-and-chain-to-finger transition. Recent advances include rapid finger change-out of bent and residue-coated units.
Finger conveyors, the prevalent method of board conveyance in North America, offer reliable processing of assemblies through the machine (Figure 2). For best results, it is necessary to perform periodic maintenance to replace bent fingers and remove accumulated solder and flux residues. Recent advances in finger conveyor technology have featured innovations such as rapid finger change-out to help machine operators keep equipment performing with minimal effort.
Figure 3. A local inerting system for solder waves. Its chief benefit is reduction of solder dross.
Processing AtmosphereIndustry experience and research have demonstrated the beneficial effects of inerting the wavesoldering atmosphere via the infusion of nitrogen gas (Figure 3).2,3,4 There are several methods by which nitrogen can be introduced, each having a definite goal and specific impact on the overall process. One, used chiefly as a dross-reduction strategy, is local inerting of the solder waves. Some recently introduced wavesoldering systems accept the manufacturer's local inerting systems, which can be incorporated at the time of purchase or as a field upgrade. Another strategy, called full-process inerting, addresses dross by virtually eliminating its formation. Here, depending on the specific technology used, gas consumption may not be much greater than that required to inert the solder waves alone. This is because full-process inerting uses nitrogen that would otherwise be exhausted after treating the waves, and directs it through the remainder of the process. In the case of a full-tunnel inerting machine, the entire process benefits from the continuous nitrogen purge. Other advantages of full-process nitrogen-inerting include:
- Virtual elimination of dross
- Protection of OSP-treated printed circuit boards (PCB) via low-oxygen preheating5
- Support of volatile organic compound (VOC)-free fluxes through a continuous purge of the preheater with heated, dry nitrogen gas, thereby preventing condensation of moisture in equipment6
- Support of lead-free alternative solder alloys that require effective inerting to counter oxidization.7
Soldering Parameters for Varied PCB SectionsAt one time or another, almost all users of in-line soldering equipment are deterred by assemblies that must be soldered from end to end via only one wave setting and conveyor angle. Although acceptable soldering can be accomplished with this limitation, soldering with only one set of parameters can cause difficulty in certain regions of the assembly. For example, a high wave needed for effective topside soldering in one section of the board may not be desirable in the vicinity of large tooling holes. Some sections of an assembly may also have different conveyor angle requirements for achieving optimum results.
In response, one-machine design enables the user to program the soldering "recipe" with various wave heights, conveyor angles and speed values every 1.2" up to a total of 23.6".8 In high mix production environments, or when soldering complex assemblies, this feature's flexibility can help to increase yield.
Reducing Solder Bridges and Improving WettingThe diversity of PCB designs has increased over the last few years. At the same time, the designs are becoming more complex. As a result, defect reduction on the portion of an assembly that requires wave soldering is continuously placing increased demands on the wavesoldering process. In practice, there are two universal problems that have motivated manufacturers of soldering systems to seek solutions bridging and open solder joints. In general terms, these defects can be traced to specific causes. Many times bridges result from connectors that are placed perpendicular to the conveyor. Opens are often attributed to lower cost, single-sided PCBs.
Debridging methods. Manufacturers of soldering systems offer varying technical solutions to address this problem. In some production environments, bridges are allowed to form but are subsequently removed by the use of hot, directed process gas. Others believe that innovative nozzle design, which prevents bridge formation through special wave and solder peel dynamics, is the solution.
Improving wetting. In order to increase wetting, and thus prevent opens, new wavesoldering technologies have been developed. Inserts that can be placed in existing nozzles to produce resonate frequencies within the wave are an example. These inserts favorably affect flow dynamics and wetting, helping to ensure good fillet formation, even for single-sided PCBs.
Mini-wave Selective SolderingOf growing interest is the use of selective-soldering technology for applications not addressed effectively by traditional mass-soldering methods. On the other hand, state-of-the-art "mini-wave" soldering equipment offers useful features to facilitate selective soldering, including nitrogen-inerted mini-waves with dross-free material that permits flow into "tight" areas for fine-detail soldering. The latest systems offer conveyor transfer and robotic board-handling through the processing sequence. These systems also offer quick and easy changeover of their board grippers and application-specific nozzle assemblies.
Ergonomic Design and User-friendlinessIncreasingly, machines are offering good visibility and easier access to key process and control sections. Operator interface monitors are providing 3-D displays of the machine and its most critical sections. Such interfaces, with their easy-to-interpret graphical toolbars, provide an intuitive means of accessing the control software to check actual parameter values, change parameters, etc. As an additional means of simplifying operation, some manufacturers provide on-line maintenance support for timely diagnosis of machine problems from a remote location.
Other InnovationsReduced-oxide soldering application (ROSA) is an electrochemical process for removing oxides from metal surfaces before soldering. This technology, the subject of a recently completed research project involving Rockwell-Collins, NORTEL, Allen-Bradley, the U.S. Army and SEHO, has demonstrated, under certain conditions, the ability to restore solderability to oxidized PCBs.9 Its electrochemical aspect makes it a reusable solution, i.e., once the solution has discharged to reduce surface oxides, it is recirculated to a cell for reduction and reuse. In certain applications, this process may eventually be used to restore PCB solderability before component insertion and subsequent soldering.
REFERENCES1 MWS 8200 technical data sheet, SEHO GmbH, Kreuzwertheim, Germany, 1997.
2 "SEHO Inert-Gas Soldering System Evaluation," Ford Manufacturing Technology Development, March 1990.
3 H. Hsiao, J.R. Lin, E.K. Chang, S.M. Adams, "Reducing Solder Defects Under Nitrogen With Varying Concentrations," SMI 1997 Proceedings, p. 371-76.
4 J.F. Koon, "Environmentally Friendly Processing Supports Cheaper/Faster-Better in Circuit Card Assembly Manufacture," Future Circuits International, No. 1, Vol. 2, p. 195-230.
5 G.M. Wenger, D.A. Machusak, "Evaluations of Organic Solderability Preservatives," Proceeding NEPCON '93 West, p. 436-42.
6 Product test data, SEHO USA Inc., Ashland, Va., 1996.
7 M.A. Whitmore, "Progress Toward Development of Lead-Free Solders," International Tin Research Institute, 1994, p. 16 (Table 6.1).
8 NGW software manual, SEHO GmbH, Kreuzwertheim, Germany.
9 D. Hillman, R. Wagner, J. Reinig and D. Pauls, "Reduced Oxide Soldering Activation (ROSA): Enabling Technology for Solderability Restoration," DARPA Contract DAAL01-96-C-0109, June 8, 1998.
ROLF DIEHM, technical director, and JOHANNES NEUPÄRTL, marketing director, may be contacted at SEHO (Seitz & Hohnerlein) GmbH, Frankenstraße 7, 97892 Kreuzwertheim, Germany; ++49-9342-889-204; Fax: ++49-9342-889-200.