Selective Soldering: Design, Process Challenges and Practical Solutions

Estimated reading time: 8 minutes

Not just the formulation of the flux, but the surface energy of the PCB became significant in the control of spreading of the flux and it was recommended to use solder masks with a lower surface energy for selective soldering applications. Schouten showed design-of-experiment results illustrating remarkable differences in flux spreading for different solder masks.

For precise flux application, dropjet dispensing techniques had been developed. Dropjet could suffer from satellite effects analogous to those experienced in inkjet technology, resulting in extraneous flux droplets. New high-frequency dropjets had recently been introduced, with internal pressure compensation to eliminate satellites, and these had increased the robustness of the fluxing process and increased yields.

Moving from conference room to demonstration shop, Wim Schouten’s team gave a comprehensive description and practical demonstration of selective soldering using the Vitronics Soltec ZEVAm machine and its off-line SmartTeach programming software as an example of a flexible entry-level system. Workshop delegates had plenty of opportunity to learn hands-on how to programme, set-up and operate the machine, which featured high frequency fluxing, inline pre-heating, a movable electromagnetic solder pot and automatic wave height and solder height measurement.

Returning to the conference room, Charles Cawthorne, electronics manufacturing technologist with MBDA Missile Systems, and SMART Technical Committee member gave a user-view of selective soldering implementation and process optimisation in a defence-manufacturing environment, sharing his experiences gained during the progressive transition from wave-soldered through-hole assembly technology in the early 1980s, where typical layer count was six, the finest component lead pitch was 2.5 mm and the maximum number of leads on a package was 16, through the mid-2000s, with a typical layer count of 24, surface-mounted components with lead pitch down to 0.5 mm and lead count per package as high as 728.

In the early days of surface mount technology, hand-soldering had been a practicable option for those components still in through-hole format. But as assemblies became increasingly dense and complex, hand-soldering no longer gave reliable or repeatable results, particularly in meeting the IPC-A-610 Class 3 barrel-fill requirement of 75%. Consequently, MBDA Missile Systems had installed a selective soldering system in the late 2000s. Cawthorne described the factors that had influenced the choice of selective over conventional wave soldering and commented on considerations of process set-up and optimisation, process yield and board design, illustrated with case histories.

In choosing their system, MBDA needed the flexibility to assemble PTH connectors on both sides of the PCB, as well as bottom terminated components and BGAs on both sides. They required the ability to apply topside heating during PTH soldering and to enable different solder dwell times on individual PTH joints to offset thermal sinking effects of ground and power planes. Obvious limitations of selective soldering were the impact on process time and the clearance requirements to accommodate selective nozzles.

Cawthorne discussed process set-up details, including fluxing, thermal profiling, nozzle drag rates, point soldering dwell times, nozzle size selection and the number of passes over particular components. Regarding space constraints and thermal relief on ground and power planes, he emphasised the importance of design-for-manufacture and involving the PCB fabricator and assembler as early as possible during the design phase. Flux choice and application technique were crucial to yield, as was accurate thermal characterisation of the PCB. For lead-free assembly, copper dissolution effects were a significant consideration and he recommended reading NPL Report MAT 26, as previously referred to by Nigel Burtt.

Returning to the subject of fluxes for selective soldering, Ross Bennett, global key account manager for Kester GmbH, described the development of a no-clean flux designed specifically for the needs of the selective soldering process. Echoing the earlier comments of Wim Schouten, he re-emphasised that the characteristics of a normal wave soldering flux were not generally suitable because of reliability issues with unspent flux and corrosive residues, poor connector barrel-fill, and the challenges of sustaining flux activity on thick boards during long periods in the soldering process and avoiding flux spread to adjacent components.

Kester’s development goals were to pass IPC SIR testing in the raw or partially activated state and to control spread and sustain activity over long dwell times to enable good barrel filling in challenging applications, together with a wide process window and no clogging of dropjet heads.

The main factors governing hole filling had been observed to be the quantity of flux deposited, the top side board temperature and the combination of solder dwell time and solder pot temperature. It was recommended that design-of-experiment be carried out to optimise the settings for a given assembly, taking into account variables such as board design, board thickness, components, and soldering equipment. Extensive development and testing had resulted in a zero-halogen, no-clean formulation that gave excellent soldering performance whilst minimising jet clogging and leaving no tacky residues.

Just as important as the attainment of acceptable hole filling was the avoidance of solder bridging between adjacent pins, particularly on fine-pitch components. In his second presentation, Wim Schouten covered this topic in detail, beginning by defining “fine pitch” by reference to the IPC road-map. In 2011, 2 mm was considered fine pitch. Currently it was 1.27 mm and trending to 1 mm by 2021.Page 2 of 3

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