How many times has a PCB fabricator heard, “Your boards don’t solder!” or a solder supplier been told, “Your flux doesn’t work!”?
For more than three decades, from the early days of roller tinning and reflowed tin-lead, through the emergence and evolution of surface-mount assembly technology, I have maintained a close interest in PCB solderable finishes, been involved in their development and optimisation, and given many technical presentations from the perspective of the PCB fabricator.
It was a privilege to be invited to attend SMART Group’s workshop on PCB solderable finishes, held May 2, 2012, where the emphasis was on their compatibility with solder paste and fluxes, and I enjoyed the opportunity to experience the practical realities of the soldering process, in the company of a group of assembly engineers, with two leading experts in soldering technology providing information, illustration and demonstration.
The workshop, led by SMART Group Technical Director Bob Willis and Henkel Global Technical Specialist Richard Boyle, was held at Henkel’s applications centre in Hemel Hempstead, UK and considered the benefits and disadvantages of ENIG, OSP, immersion silver, immersion tin, and solder levelling as current finish options, reviewed their performance during PCB fabrication and assembly, examined some of the common process problems experienced in industry, gave guidance on correct selection of paste and flux for different surface finishes, and included a practical session on solderability testing, printing and reflowing.
Willis set the scene for the workshop as he introduced his classroom session with the results of the 2012 IPC/NPL PCB Problems Survey, which indicated that of all the PCB problems reported by assembly engineers, by far the greatest percentage, 38.7%, were attributed to finish solderability. Against this background, his initial recommendation was for assemblers to ensure that they audited their PCB fabricators’ chemical processes, taking particular note of audit reports from the fabricators’ process suppliers, before, rather than after, any problems were encountered.
SMART Group Technical Director Bob Willis
Having emphasised the critical importance of effective process control at the fabrication stage, Willis explored a list of factors which could affect solderability after delivery--storage times and conditions, the effects of baking, handling, wash-offs and wipe-offs during paste-printing, curing of adhesives, use of temporary solder masks, hold-times between reflow operations--and used video recordings to demonstrate many examples of poor solderability. Further videos showed the workings of the various solderability test methods defined in J-STD-003, most of which were no longer appropriate in surface mount assembly technology--only the “Test E: Surface Mount Process Simulation Test” and “Test F: Wetting Balance Test” were truly relevant. “Know how your supplier is doing his tests and what materials he is using,” Willis recommended.
Acknowledging that not all PCB fabricators or assemblers could justify the investment cost of a wetting balance, Willis described a low-cost practical test developed in cooperation with NPL, using a test coupon with a series of parallel conductors on which were printed dots of solder paste at progressively decreasing spacing, and observing the extent of coalescence of the dots during reflow.
There was no universal solderable finish for PCBs. Bob recommended referring to the SMART Group’s Lead Free Experience 4 report for comprehensive reference information about the suitability of various finishes and the impact of solderable finish on yield in different production situations and conditions. He urged engineers to think about their total process when selecting an appropriate finish, to determine what worked best on their particular production line (balancing cost against functionality and reliability), and to make sure members of the purchasing department were aware of the real cost before they were tempted to take an apparently cheaper option.