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Selective Soldering with Dispensable Solder Paste
December 31, 1969 |Estimated reading time: 3 minutes
Paste print and mass reflow are the standard processes for soldering PCBs. The exceptional wetting characteristics of solder paste, along with the precise temperature control of reflow ovens, combine to produce billions of circuit boards each year. Sometimes, however, printing is neither possible nor practical. In these situations, states John Vivari, Nordson EFD, dispensable solder paste can be a reliable, cost-effective, and easily implemented solution for rework, prototyping, high-volume repair, and other selective soldering applications.Dispensed Solder Paste Vs Solder Wire
Dispensable solder paste offers a number of advantages over the wire solder often used for these applications, including:
- More consistent, predictable deposit size, for greater process control;
- Higher flux content for better wetting and joint reliability;
- Independent of operator skill level, so that any employee is able to consistently produce high-quality solder joints;
- Visual confirmation of reflow completion helps operator minimize part heating;
- Greater process flexibility, in that paste permits the use of different heating methods like ovens, focused hot air and lasers, reducing the risk of thermal damage;
- Paste can be dispensed in locations that are inaccessible to solder wire;
- Paste is available in a wider range of formulations than wire. Flux type, paste viscosity, rheology, metal content and alloy are all customizable to a wide degree, allowing paste to accommodate non-standard processes, parts, and materials.
Dispensable solder paste comes in many formulations designed to succeed where wire solder and preforms fail. Formulations are available to tackle the most difficult to solder surfaces like stainless steel, and survive the most stressful heating methods, such as lasers and soldering irons.
Depending on the process and production volumes, dispensable solder pastes are usually applied with precision fluid dispensers or auger valves. Manual dispensing can also be an option when only a few joints are being made on a sporadic basis. To facilitate travel through the dispensing needles used with these methods, dispensable pastes contain a higher percentage of flux than comparable print pastes.
For operator-controlled benchtop processes that involve a relatively small number of solder deposits, a precision dispenser will usually be the most appropriate tool for applying solder paste.
On projects that require a series of identical solder deposits, dispensers have a digital timer, which is a reliable, convenient way to keep all the deposits the same size. For projects where joint sizes vary, the timer can be turned off so that the operator can manually control the size of each deposit, starting and stopping the dispense cycle. In either instance, using paste instead of wire will allow the operator to visually confirm the size of each solder deposit before reflow, then verify complete reflow and wetting afterwards.
For automated and semi-automated operations, auger valves mounted on tabletop robots are a good dispensing option. In applications like prototyping and BGA rework, this approach can dramatically improve productivity by combining consistent deposit size with fast, precise positioning. In addition, robotic dispense systems can often be coupled with automated heating systems to deliver even greater quality and consistency.
Conclusion
Dispensable solder pastes offer a highly reliable, cost-effective and easily implemented solution that delivers numerous advantages over solder wire in many rework, prototyping, high-volume repair, and other selective soldering opportunities.
Paste formulations are available to tackle difficult-to-solder surfaces, such as stainless steel. In addition, paste can be reflowed with any type of heating method to meet the requirements of a specific application.
John Vivari, senior application engineer, Nordson EFD, may be contacted at john.vivari@nordsonefd.com
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