Successfully implementing high-volume, high-performance selective soldering

Dealing with mixed through-hole and surface-mount PCBAs

The selective soldering process is an essential part of the electronic assembly manufacturing operations throughout the world. Complex printed circuit board assemblies (PCBAs) place greater demand on the overall accuracy and repeatability of the selective soldering process. This requires the selective soldering process to be more precise.

Florian Strohmayer, Applications Development Manager, Nordson Select, Hagenbach, Germany

Any company who deals with soldering of through-hole and surface mount mixed-technology printed circuit assemblies quickly discovers the selective soldering process is not only more efficient than wave soldering, but also provides the opportunity to meet customer requirements with significantly more flexibility. One such company is the second largest EMS provider in the north of Israel. With headquarters in Nazareth, RH Technologies Ltd., employs more than 1,000 people worldwide and builds electronic systems and assemblies for the advanced technology field. With worldwide facilities in China, Israel, Romania and the USA, this company serves end-customer markets throughout the aerospace, communications, industrial, medical and defense sectors.

In general, most selective soldering applications are unique and therefore require application engineering to determine the most appropriate soldering solution. To meet these requirements, the users need to have a complete understanding of the soldering process, and have a good relationship with the equipment supplier. This partnership should be based on prompt communications with the supplier having a worldwide logistical support system that can quickly respond to any need or potential problem.

Implementing selective soldering for a high-volume, high-performance application involves choosing a selective soldering platform designed for demanding through-hole and SMT soldering applications. This was a deciding factor when this EMS provider chose a new system for their third selective soldering line. According to Yacov Rozenberg, Chairman of the company, this was a critical factor since they wanted one system that had 6 solder pots within one machine to provide high-volume production, as well as system flexibility.

To solve their productivity challenge, they partnered with Nordson Select to implement an Integra 508.5PD 3S selective soldering system for their latest high-volume application. This system provides five-zone in-line operation with independent areas for concurrent fluxing and preheating, as well as dual solder pots in three soldering stations to ensure high output while producing excellent solder joint quality.

Selective soldering can be divided into five basic sub-processes: conveying, fluxing, preheating, board handling and soldering. In high-volume selective soldering applications, a SMEMA chain conveyor with positive PCB locations and automatic conveyor width adjustment is essential for transporting the board between process stations. Dual maintenance-free drop-jet fluxers provide fast and accurate flux application of flux to precise locations on the PCBA eliminating the need for post-soldering cleaning of the board assembly.

Preheating in selective soldering is not intended to reduce thermal shock as it is with wave soldering, but rather to pre-dry the flux and remove solvent and bring the flux to the correct viscosity before soldering. In this case, preheating is accomplished with full surface topside and bottom-side infrared preheating with closed-loop pyrometer control to achieve optimal thermal distribution. Accurate board handling is necessary to confirm that adjacent SMT components will not be affected by the selective soldering nozzles. Often selective soldering systems use either a single nozzle for flexibility or a multi-nozzle arrangement for greater output. The EMS provider‘s high-volume application required having both soldering technologies available in a single selective soldering system to provide throughput and flexibility.

Increasing throughput

The Integra 508.5 is a multi-station selective soldering system designed for high-volume applications with maximum throughput and due to its modular design, the machine can be matched to a wide variety of high-performance requirements. When configured with dual drop-jet fluxers, the system can be operated in two different modes and is capable of processing up to 10 boards at one time.

The parallel processing mode enables fluxing and soldering of two boards in each fluxing and soldering station at the same time, which doubles machine productivity. The Z-axis control raises both dual solder pots and solder nozzles in each of the three soldering stations simultaneously, doubling the output. Using in the parallel processing mode, the distance between the dual solder pots and dual solder nozzles is automatically adjustable between 80–240 mm. This allows each solder station to process two boards at the same time, at a variable distance which is beneficial for soldering of multi-up panels. A panel of any multiple of two boards aligned in the direction of the dual nozzles will derive the full benefit of fluxing and soldering two boards at the same time, including 2-up, 4-up, 6-up or 8-up panels. With an odd number of boards within a multi-up panel, a partial benefit in productivity is still obtained. For example, a 3-up panel has the full benefit of fluxing and soldering the first two boards at the same time while single fluxing and soldering the third board. A 5-up panel has the full benefit of parallel fluxing and soldering four boards and single fluxing and soldering the fifth within the panel. Alternatively, the double processing mode allows soldering with multiple size solder nozzles, within the same program, with independent Z-axis control. This operates each of the small and large solder nozzles individually, which enhances flexibility and increases productivity of the system.

With the use of a single drop-jet fluxer and dual solder pots in each of the three soldering stations, the system can be used in the double processing mode with two different solder alloys in each of the solder pots without requiring to physically exchange the solder pots. Additionally, with this unique parallel and double soldering mode capability, a production floor must no longer sacrifice between throughput versus flexibility, or flexibility versus throughput, since both capabilities are available via a simple click of a mouse.

Selective soldering is widely used in the automotive electronics industry, which uses quality assurance programs such as ISO, QS9000 and Six Sigma. To implement these types of programs, failure mode and effects analysis (FMEA) studies are required. Vendors who supply production equipment to this industry must frequently provide assurance that their assembly processes, including selective soldering, are fully under control.

The selective soldering system implemented by the company is equipped with a data logging system that monitors all aspects of the selective soldering process. This information is continually gathered and stored in an SQL database and can be instantly exported in XML format for detailed analysis. Parameters such as the temperature of the solder alloy, PCB temperature, accurate flux application, and error messaging are readily available. Any additional process parameters can be easily added to adapt to specialized traceability requirements.

While drop-jet dispensing is a proven technology for no-clean processing, flux solvents can evaporate over time or impurities within the flux can cause a drop-jet orifice to partially clog and slightly dispense flux off center. The amount of flux dispensed by a drop-jet dispenser is so minuscule and is extremely critical to the quality of the selective soldering process, that automatic flux verification is essential especially in a high-volume lights-out operation.

The selective soldering system in use at this EMS provider is equipped with an in-process, closed-loop flux verification system that confirms the presence and accuracy of flux application by the drop-jet fluxers. This system ensures micro-deposition of flux to extremely small soldering locations with unparalleled accuracy and guarantees minimal flux consumption. Monitoring of this process has proven to greatly increase the consistency and overall solder quality during high-volume production.

It is widely known that only clean and oxidation free solder nozzles can be properly wetted to guarantee consistent solder quality. The automatic solder nozzle tinning system on this selective soldering machine does not spray an adipic acid, or a liquid or powdered flux. The automatic solder nozzle tinning system keeps the surfaces of the solder nozzles meticulously clean by automatically removing oxidation residues and re-tinning the surface of the solder nozzles, without resulting overspray or contamination of the board or the machine.

Conclusion

Based on numerous case histories, including the positive experience of the EMS provider, high-volume, high-performance selective soldering is establishing itself as a cost-effective alternative to wave soldering. It provides faster time to market with increased flexibility to meet unanticipated changes in customer requirements.

SMT Hybrid Packaging, Booth 4–101

www.nordsonselect.com


Zusammenfassung Résumé Резюме

Selektives Löten kommt oft dann ins Spiel, wenn oberflächenmontierten und bedrahtete Bauteile auf einer komplexen Baugruppe bestückt werden. Die Kriterien sind neben der hohen Genauigkeit und Reproduzierbarkeit auch die Prozessgeschwindigkeit. Wellenlöten wurde hier bisher oft praktiziert, doch mit einer präzisen selektiven Lötanlage, die parallel mehrere Boards mit Flussmittel versorgen und löten kann, gibt es wirtschaftliche Alternativen mit hoher Flexibilität und Leistung.

Le brasage sélectif est souvent utilisé lorsque les composants pour montage en surface et contenant du plomb sont montés sur un assemblage complexe. Les critères ne sont pas seulement la haute précision et la reproductibilité, mais aussi la vitesse du processus. Jusqu‘à présent la soudure à la vague a souvent été utilisé, mais avec un système de brasage sélectif précis qui peut alimenter et souder plusieurs cartes en parallèle avec du flux, il existe des alternatives économiques avec plus de flexibilité et de puissance.

Селективная пайка зачастую применяется в тех случаях, когда смонтированные с использованием поверхностного монтажа и соединенные вместе детали устанавливаются на комплексные узлы. Основными критериями при этом являются высокая точность, повторяемость и скорость. Прежде для этого часто использовался метод пайки волной припоя, однако высокоточные установки для селективной пайки, способные подавать флюс и паять одновременно несколько плат, являются более экономичной, универсальной и эффективной альтернативой.

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