Implementing Selective Soldering -- Selective soldering significantly improves the conversion cost and quality of through-hole interconnections in complex PCB assemblies.

With the advent of each new PCB assembly, greater demands are placed on process capabilities by ever-increasing board complexity. The necessity for higher quality levels, combined with greater component densities and higher I/O devices, is challenging traditional soldering processes. Methods such as wave soldering and post-wave soldering are becoming increasingly challenged to ensure through-hole interconnections at acceptable quality levels for complex mixed-technology assemblies.

Given the added difficulties of high thermal mass through-hole components on dense, double-sided surface-mount boards, existing methods of mass soldering cannot keep pace with industry demands. Many components are already being processed at the limits of their heat ranges. This problem will be further compounded with the changeover to lead-free or high-temperature solder alloys.

Fundamental to the formation of through-hole interconnections in a complex assembly is the necessity to develop alternatives to traditional mass soldering. Methods that can selectively solder specific sites without disturbing adjacent surface-mount components are required. Of these methods, the most widely used one is aperture wave solder pallets. Although generally accepted, the use of aperture pallets can result in solder defects such as opens or solder bridges, limited vertical barrel fill and entrapped flux residues caused by the limited openings in the pallet.

By its very nature, traditional wave soldering requires compromises in process parameters, such as flux deposition, solder height and solder contact time. Post-wave soldering is often achieved by manual soldering, which can result in inconsistent quality and unpredictable throughput.

With traditional wave soldering, the compromises in process parameters can be incompatible with the excursion in thermal characteristics of the high-thermal mass components found in typical complex PCB assemblies. Equally important are component variations in thermal mass differential, lead-to-hole aspect ratio and component body temperature sensitivity. Match-ups can become impossible to achieve at acceptable quality levels.

Selective Soldering Technology

Programmable selective soldering systems offer the advantage of site-specific through-hole soldering without compromising process parameters. They offer controlled flux deposition and solder application while optimizing finite process parameters for variations in thermal mass differentials and aspect ratios for individual through-hole components. In these systems, a continuously recirculating supply of liquidus solder flows through a precision nozzle that moves to specified board locations by means of a closed-loop, servo-driven x-y-z positioning system.

Programmable selective soldering, also called mini-wave soldering, is a Windows-based process that uses CAD-derived information to determine specific solder locations, thereby eliminating hard tooling. The system can be used on a variety of circuit board assemblies without dedicated tooling for individual board layouts.

For optimum wetting and capillary action, an asymptotic preheating method is used to ensure an equilibrium heating environment that compensates for the thermal mass differential of large through-hole components or those components connected to ground planes. This heating equilibrium results in the formation of solder filleting on the destination side as well as complete vertical fill of complex through-hole components (Figure 1) without disturbing adjacent surface-mount components on the solder source side of the board. Typical high thermal mass applications include through-hole components such as DC power supplies and ceramic pin grid arrays (CPGAs). Because programmable selective soldering uses a software-driven menu, the solder parameters can be matched to the specific thermal characteristics of individual through-hole components.

Over the past several years, various selective soldering methods have been developed to meet different through-hole interconnect applications. Robotic point-to-point systems generally have slower cycle times because they solder only one interconnection in sequence and are effectively used in topside soldering applications. Fountain soldering, also called pin transfer or dip soldering, uses a tooling plate with dedicated nozzles to solder specific sites. A limitation of this method is that the solder parameters are identical for all locations and cannot be optimized for individual components. The necessity of expensive dedicated nozzle plates for each unique board layout adds to the overall cost.

Programmable selective soldering uses a solder nozzle that moves to specific sites, but it can also adapt to variations in lead projection and component geometries. Interconnections can be formed sequentially or simultaneously, as in the case of multiple row connectors.

Advantages of Selective Soldering

Unlike other selective soldering methods, programmable selective soldering can adapt to complex, mixed-technology assemblies because of its ability to optimize process parameters and its asymptotic preheating method that assures optimum wetting and capillary action. Programmable selective soldering can also accommodate lead-free and high-temperature solder alloys because it limits the exposure of temperature-sensitive components to the higher operating temperatures that would be incurred with lead-free wave soldering.

To compare the effectiveness in conversion cost and process quality between selective soldering and traditional soldering methods, three sample assemblies were studied (Table 1). These assemblies are all complex telecommunications multi-layer boards consisting of double-sided surface-mount and double-sided through-hole technology. The through-hole interconnections for each board were formerly soldered by a combination of mass soldering and post-wave soldering plus a variety of secondary operations, such as solder masking.

The cost to convert a circuit board and components into a functioning assembly is a critical measure of operational efficiency. The National Electronics Manufacturing Initiative (NEMI) has published a roadmap of conversion cost benchmarks to ensure competitiveness within the North American electronics manufacturing industry.1

The measurement of conversion cost must consider all cost of ownership factors. Included must be all cost variables such as direct and indirect labor, consumables such as solder, dross and flux, equipment depreciation, inspection, rework and repair costs, tooling, and floor space and utility costs. The cost of consumables alone can be significant for a wave soldering operation when the volume of solder, dross, nitrogen, flux and electrical energy used by a typical wave soldering machine are all considered.2

When all cost factors in each assembly process are considered, conversion costs between programmable selective soldering and the former soldering methods can be compared (Figure 2). The conversion cost for the sample boards with the former soldering methods ranged between $0.0155 and $0.0167 per solder interconnect (I/O). After programmable selective soldering was implemented, these conversion costs were dramatically reduced to between $0.0052 and $0.0079 per I/O, which is well below future competitive benchmarks.

Maintaining a robust process capable of achieving high yields for complex PCB assemblies is the goal of every assembly and process engineering staff. Design constraints such as high component density, limited clearance between components, and 50-mil through-hole microconnectors can add to that challenge. Each sample board studied had historic defects per million opportunities (DPMO) levels between 88 and 167 DPMO. These defect levels were attributed to the design constraints of each unique board and the former soldering methods used, including secondary labor-intensive operations. After implementing programmable selective soldering, the defect levels for each sample board improved to between 24 and 32 DPMO (Figure 3).

Another benefit of programmable selective soldering is its ability to solder through-hole components without exposing adjacent components to excessive thermal excursions. An example is fine-line micro ball grid arrays (microBGAs) that can be incompatible with the wave soldering process when through-hole components must be wave soldered in the board after the microBGAs have been reflowed. This process incompatibility can result in a thermally induced fracture of the solder ball joint.3 The fracture is caused by the internal stress of the coefficient of thermal expansion (CTE) mismatch as a result of thermal shocking during the through-hole wave soldering process. Programmable selective soldering can solder these same boards without thermal shocking, which eliminates fractured solder ball joints as well as bridged vias underneath fine-line microBGAs.

Finally, programmable selective soldering, as an end-of-line process step, minimizes the accumulation of work-in-process inventory that can result from post-wave hand soldering. This non-uniform demand condition is often a bottleneck in a production process that involves hand soldering as a post-wave operation. Programmable selective soldering can also eliminate the environmental issues associated with a typical wave soldering operation.

Conclusion

Compared with historic cost and defect data, programmable selective soldering can produce through-hole interconnections in complex, mixed-technology boards that far exceed future conversion cost and process quality goals. Because programmable selective soldering is a data-driven technology, it can be easily and cost-effectively adapted to a variety of assemblies. The results are reduced conversion costs, increased factory utilization and significantly improved process quality.

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References

1. Davidson, T., and Barthel, B. (1999, May). Roadmapping the PCB assembly future. Circuits Assembly, pp. 70-75.

2. Klenke, B. (2000, September). Implementing selective soldering: A cost effective alternative to traditional mass soldering. SMTA International Conference Proceedings, pp. 111-116.

3. Wenger, G. Internal document. Lucent Bell Labs, Engineering Research Center.

Bob Klenke is vice president of sales and marketing of ERSA Inc., Soldering Machine Div., Plymouth, WI, e-mail: [e-mail].

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Copyright [copyright] 2001 Miller Freeman LLC