In an EMS environment, overall quality rose almost 80% over a conventional wave.

SMC has always considered wave soldering a reliable process for most through-hole applications and a conventional process used in manufacturing. Recently, however, we were faced with producing a high-volume assembly that required through-hole and SMT technologies with extremely tight lead clearances near tall SMT components. We started to use our traditional form of wave soldering, but found it fell short of our quality standards.

Initially, we noticed typical problems such as solder voids, solder bridges and excessive solder, all related to using selective solder pallets contributing to daily rework. Solder dross was also detected and a significant factor in the high defect rate. This method required constant monitoring and maintenance to produce good quality product at a high-volume rate.

Our second requirement with this assembly was the need to add a water-soluble flux to the solder wave process. Maintaining a repeatable amount of flux in some of the narrow and deep apertures of the selective solder pallets became a challenge. A drop jet spray head would have been ideal for controlling flux quantities in target locations and would have contributed less over-spray than an atomizing spray fluxer, but having to use an organic flux with a 33% solids content would have caused the drop jet to corrode and clog.

We then experimented with foam fluxing with no success, adopting an atomizing spray fluxer to flood the pallet. Although this approach improved quality in the areas of no solder and solder voids, it also resulted in much more downtime because of the excessive flux. Flux found its way into every crack and crevice, and the excessive flux contributed to extreme amounts of dross that required constant monitoring and frequent cleaning. We also had to wash the solder pallets after flux was applied. Any flux or moisture not removed when the pallets were reused in the solder wave caused significant solder splashing and contributed to the defect and rework problem.

We realized quickly that our standard process was not going to provide the quality required and we were faced with the challenge of finding an alternative method that would improve quality, meet short cycle times, reduce downtime, eliminate the need for costly selective solder pallets, and of course, it had to be competitively priced.

Selective soldering seemed a likely candidate as a solder wave replacement, and SMC is experienced with its common point-to-point method. Selective soldering is known for producing acceptable quality levels and reducing labor, rework and masking costs. The selective method could solder leads in some of the tightest locations, apply flux repeatedly and accurately, and doesn’t require the use of selective pallets. The problem with selective point-to-point soldering for our application was its lack of speed. We needed a method that would provide high-quality yields, while producing a complex assembly at high production rates.

Our search led us to an alternative form of selective soldering, a method known as multiwave soldering. It has the benefits we require, with regard to the point-to-point method, and meets cycle time requirements. Fluxing, preheating and soldering boards in parallel in an inline transfer system enable the multiwave to attain most of its speed, and in most cases can outperform speeds of traditional solder waves. The system solders via a technique similar to a solder dip and has an arrangement of selective nozzles designed to correspond with the PCB lead pattern. Nozzles attach to a plate and can be designed to fit in some of the tightest areas. For leads close to tall SMT components and clearances as tight as 0.10", the system’s positioning capabilities are repeatable, and have shown to be superior to soldering with selective pallets. The plate is situated in a solder bath, and at the time of soldering, solder is pumped to fill each nozzle in an inert nitrogen environment. Soldering all leads simultaneously is performed at high speeds, and the concept is simple. Nozzle plate changeovers are quick, although fabrication lead time and cost means the multiwave plate is not ideal for high-mix production, as changeovers would require dedicated plates for each new assembly. The multiwave is a low-mix/high-volume soldering system with a lot of flexibility when it comes to PCB design.

The multiwave concept of using a selective nozzle pattern eliminated the masking process and the need for excessive flux and washing pallets. Solder splashes were eliminated, and overall quality improved drastically. Initial studies comparing our previous wave process to the multiwave process showed improvements in overall quality by almost 80%, with some defects totally eliminated.

The top two contributors to reducing manufacturing costs are reduced rework and increased uptime. Applying minimal amounts of flux and nitrogen over the pot reduces hazardous wastes and minimizes de-drossing. Preventive maintenance is minimal and downtime for de-drossing has been cut to fifteen minutes a week from two hours a day for a two-shift operation.

Downtime and rework due to through-hole soldering are at all-time lows; the system meets internal throughput requirements, and costs associated with solder pallets and solder wave management have been eliminated. Wave soldering is not something of the past, but for about the same price, we are able to take a selective approach to high-volume soldering.

Frank Grimard is manufacturing engineer at Creation Technologies – Lexington (formerly SMC); fgrimard@smcems.com.