Reading time ( words)
“What is your most common PCB problem?”
A survey conducted by Bob Willis had revealed finish solderability to be the predominant contender, and it was clear that the choice of solderable finish applied to surface mount boards could have a significant effect on the assembly yield and cost of the final circuit. A further survey question revealed the relative popularities of surface finish as: gold 43%, tin 24%, solder levelled 22%, copper OSP 8% and silver 3%.
SMTA Europe organised an informative and enlightening webinar this month entitled “Guide to PCB Solder Finishes—Process Defects Causes and Cures,” with soldering specialist Bob Willis as presenter.
Willis began with detailed descriptions of PCB surface finish options and their typical attributes and applications, together with their relevant IPC specifications: IPC-4552 for electroless nickel/immersion gold (ENIG), IPC-4553 for immersion silver, IPC-4554 for immersion tin, IPC-4555 for organic solderability preservative (OSP), and IPC-4556 for electroless nickel/electroless palladium/immersion gold (ENEPIG). He recommended visual aids for the benefit of the Goods Inwards department and showed photographs of “good” examples of each finish at a range of magnifications.
As well as giving a comprehensive indication of material specifications, builds, thicknesses, tolerances and test requirements, his illustration of the “Manufacturing Notes” summary from a customer’s engineering documentation included clear designation of solder resists and solderable finishes, and the areas and features to which they corresponded. Willis considered it not sufficient to specify finishes in general terms, but to be precise and unambiguous, so that engineers knew the exact description of the finish and its compatibility with their process. It was also useful to identify the chemistry supplier, whether that supplier could be changed without notice, and whether the finishing operation was being contracted out.
Packaging was also an important consideration for protection of boards in shipment and storage. Traditionally, boards were interleaved with tissue paper to prevent damage to solder mask, legend ink and pad surfaces. Vacuum sealed packages in anti-static material were now common, some packages being provided with zip-lock seals. Whatever method was chosen should be agreed between supplier and customer and included in the purchase specification.
Many factors had to be taken into account if the as-received solderability of PCBs was to be maintained throughout the assembly process, and Willis listed several—some obvious, others less so. Baking was one example. Was baking necessary if the boards had been packed and stored correctly? Sensible handling was essential. But what about wash-offs to remove mis-printed solder paste? Similarly, what about removing adhesives or temporary solder masking? If ROSE testing was carried out, did the test conditions affect solderability? For double-sided work, what was the hold time between the first and second side, and was the soldering process nitrogen reflow or vapour phase? Was there a hold time before completion of the assembly sequence? What about upgrades, retrofits and repairs? All these circumstances required investigation and characterisation with respect to their potential effect on solderability. It was also important to know what the end-customer’s perception of quality and the inspection standards he was working toward.
Willis explored solderability test methods, with reference to IPC-A-600, IPC-A-610, J-STD-002 and J-STD-003, and explained the differences between edge dip, rotary dip, solder float, wave solder and surface mount process simulation. And he stressed that the solderability of components was also an important issue, not to be overlooked.
With plenty of photographic and video illustrations, he demonstrated wetting, non-wetting and de-wetting of surfaces in reflow and manual soldering, and explained some of the causes and mechanisms. “Always look a little closer!” was his advice when examining soldering defects.
The preferred method for solderability measurement on surface-mount boards was the wetting balance, used to determine the wetting force between molten solder and the test surface as a function of time. Willis showed many examples of the graphs output by the equipment, demonstrating the behaviour of several finishes after various thermal cycles.
But a wetting balance was an expensive laboratory facility, and Willis has always been an advocate of low-cost practical tests. His solder-spot test pattern could be standalone, or incorporated as a simple coupon in the border of a production panel. It consisted of a series of 0.020” parallel tracks on 0.040” pitch, printed with dots of solder paste with progressively smaller spacing. If it was included in the production panel, then it was printed in the same operation as the solder paste print, with the dot pattern incorporated in the stencil. Once the assembly was reflowed, the test pattern witnessed the wettability of the surface finish by that paste under those reflow conditions by observing how many dots had fused together. This method had proved to offer a convenient and cost-effective process control procedure.
Willis moved on to a practical discussion of assembly process defects, illustrated as usual with clear photographs and microsections. He described phenomena such as solder wicking, inconsistent OSP coverage, reduced solder wetting, tin whiskering, champagne voiding and silver corrosion, stressing all the time not to throw defective boards away before they had been systematically and comprehensively inspected and tested to determine the root cause of the fault. He invited questions, gave detailed and meaningful answers to queries and resolved a few mysteries. He had published a series of process defects guides which were downloadable from his website.
It was an hour well spent! Many thanks to SMTA Europe, and especially to Bob Willis, for providing an invaluable insight into some practical aspects of solderable finishes for surface mount assembly, and investigation of causes and cures of soldering defects.