SMART Group - Surface Mount and Related Technologies

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Donal McDonald – Obituary

Donal McDonald

I am the bearer of very sad news, a great friend of SMART and a real industry character Donal McDonald passed away suddenly in San Diego, just before the APEX show. I have known Donal for about 27 years and was proud to call him my friend, he was the life and soul of any party and a guy who would do anything to help his friends

Like many of us who travel globally to the shows, we didn’t meet as often as we would have liked, but we had many many good times in many wonderful places and he will be missed by everyone in our travelling village. The Craic has gone out of these events and everyone knows that they will not be the same again, we will keep many good memories, but Donal will make no more

Keith Bryant – Chairman of SMART Group

PCB Surface Finish Defect Guide & Conformal Coating & Cleaning Defect Guide 2 launched FREE by SMART Group

The next SMART Defect Guide is in production and is entitled “Printed Circuit Solderable Finish Defect Guide” and will be released in February/March. This relates to different surface coating on PCB and the common process and assembly failures that may occur. It shows issues at good receipt and the typical assembly related problems. The guide provides example images of satisfactory surface finishes as a reference source and many common defect examples. It is well known the surface finishes can create joint failures and other reliability problems so good solderability is key to success

To be included as a sponsor/advertiser in the new Defect Guides contact Keith

“Conformal Coating & Cleaning Defect Guide 2″ The updated guide is also being produced and circulated to industry free. The colour guide provides examples of the most common process defects and common cures. The guide also features many of the common and less obvious defects seen during production. It will feature defects associated with components, printed circuit boards, design, materials, assembly and rework. It will also include some of the issues which may be seen on field returns.

SMART Groups first “Lead-Free Defect Guide” produced in 2007 and updated in 2008 was circulated worldwide with over 1000 free downloads. The guide is also provided to engineers to download free

Contact Keith Bryant by email or call him at SMART Group +44 07946133531

 

Tin Whisker Mitigation Methodologies Seminar & Table Top Exhibition

Since the introduction of the RoHS legislation in 2006, the threat of tin-whisker-related short circuit failure from pure tin finished components has remained a major concern within the high reliability electronics manufacturing industry. But how do we set about mitigating against such failure where the use of pure tin finished components is unavoidable?

A distinguished group of experts gathered at Loughborough University in the East Midlands of the UK to share their knowledge with a room-full of engineers from the defence, aerospace and high reliability electronics sector, at a seminar organised by SMART Group.

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SMART Group Chairman
Keith Bryant welcomed all present before inviting Technical Committee member Charles Cawthorne, Electronics Manufacturing Technologist with missile systems group MBDA, to moderate the proceedings.
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Cawthorne’s first presenter was
Dr Barrie Dunn, for many years Head of Materials and Processes Division with the European Space Agency, more recently Honorary Professor in the School of Engineering at the University of Portsmouth, and author of the newly published book: “Materials and Processes for Spacecraft and High Reliable Applications”. He began his presentation on problems associated with whisker growths with a remarkable time-lapse video recorded by researchers at Brown University, showing the initiation and growth of a tin whisker at a steady rate of about 1 micron per hour over a period of 40 hours.

The space environment presented particular challenges to electronic systems: high vacuum, low temperatures and thermal cycling as well as cosmic radiation, micrometeoroids and space debris. Remoteness and the inability to repair systems in situ obviously made the reliability issue even more critical, albeit for example Voyager 1 was still functioning after 39 years in operation. Although equipment designed to go into space was exempt from the RoHS Directive, the reliability of space projects clearly depended on the integrity of PCBs, components and assembly processes. Regarding whiskers, the aerospace industry standard GEIA-STD-0005-2 “Standard for Mitigating the effects of Tin Whiskers in Aerospace and High Performance Electronic Systems”, revised in 2012, defined three control levels. Level 3, which was relevant to equipment designed to be sent into space, prohibited the use of pure tin finishes.

Dr Dunn showed a selection of illustrations from his “Black Museum” of whiskering effects on a wide variety of tin-plated components and connectors. And he quoted instances of systems failure in communications satellites originating from tin whiskers. A whisker one micron in diameter could support a current of 10 milliamps. Although higher currents would cause instant burn-out, in high vacuum conditions a short circuit could result in a plasma discharge. Dr Dunn had collected data on whiskering effects over a period of 32 years, using the C-Ring test to introduce tensile and compressive stress, with different metal substrates, barrier layers and tin plating thicknesses, and the results were shortly to be published. A remarkable observation was that in some instances it could be several years before whiskers began to grow, depending on the substrate and stress level. Plated brass, tin-plated brass with a copper barrier layer, tin plated steel, tin-plated steel with a copper barrier layer, all displayed their own characteristics, sometimes predictable, sometimes not, but fused tin plating showed no whisker nucleation on any of  these substrates during the whole 32 years of the study.

On behalf of ESA, a working group led by Dr Dunn had produced a comprehensive set of guidelines for creating a lead-free control plan, which described the problems, requirements and methods relevant to the preparation of a plan for companies to control against the use of lead-free components and to ensure that pure tin did not find its way into the manufacturing chain. He distributed copies to delegates, and it is available on-line for free download at:  http://esamultimedia.esa.int/multimedia/publications/STM-281/offline/download.pdf

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Charles Cawthorne then introduced
Dr Mark Ashworth of Loughborough University, who discussed the effect of plating methodologies, the first of two presentations describing research carried out at Loughborough into mechanisms and strategies for tin whisker mitigation. The factors influencing whisker growth were electroplating bath chemistry, whether pure tin or a tin alloy, bright or matt, electroplating parameters such as current density, temperature and agitation, and substrate, taking copper, brass and alloy 42 as examples. The deposit characteristics studied were thickness, grain size, morphology, orientation, intermetallic formation and elemental diffusion effects, under varying conditions of temperature, humidity, thermal cycling and applied external stress. A proprietary bright tin plating process was used.

The main conclusions were that bright tin electrodeposited onto copper did not always result in significant whisker growth, even after five years’ storage. Whisker growth was reduced by increasing deposit thickness and by deposition at higher current densities. The higher current densities tended to favour the formation of large eruptions, rather than filament whiskers. And it appeared that storage of tin deposits on copper for 5000 hours at 55°C and 85% humidity did not accelerate whisker growth, although these conditions favoured the formation of Cu3Sn intermetallic which was more planar than that developed at room temperature

Pulse plating could be used as a means of manipulating the grain structure and orientation of the tin deposit, and in some instances showed reduced whisker growth compared with direct current deposits. But higher pulse frequencies could result in greatly accelerated whisker growth, which was believed to be favoured by fine grained columnar structures. Dr Ashworth stressed that these observations related to the particular proprietary electroplating chemistry used in the study, and that other commercial formulations might demonstrate different relationships between current density, deposit microstructure and whisker growth.

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With
Professor Geoff Wilcox taking over as moderator, Charles Cawthorne began his own presentation by remarking that, since first observed in 1946, and despite many years of study, the mechanism of tin whiskering was still not fully understood and predictive modelling was not yet possible. But, because of RoHS legislation, increasing numbers of components were only available with pure tin finishes. Consequently, manufacturers of high-reliability electronics with RoHS exemption were left with no alternative but to manage tin-lead obsolescence by avoidance or mitigation. He discussed the evolution of tin whisker mitigation methods via technical standards, with reference to the updated GEIA-STD-0005-2 standard already mentioned by Barrie Dunn, and the IEC/TS 62647-2 technical specification “Process Management for Avionics, Aerospace and Defence Electronic Systems containing Lead free Solder, Part 2: Mitigation of Deleterious Effects of Tin”, published in November 2012.

In order of preference, common mitigation practices included non-tin plating, for example nickel-palladium-gold finish, as used by Texas Instruments but not many other component manufacturers, adding lead or bismuth to the tin plating, using a nickel underlayer or annealing tin plating at 150°C for 1 hour within 24 hours of plating, although this would be acceptable only when accompanied by supporting test data. Other viable mitigation practices were hot-dip tinning for structural steel parts, or hot-oil fused tin plating. Finishes to be avoided were silver, plated tin-copper, any tin plating over brass without a copper or nickel barrier, and bright tin.

Cawthorne reviewed the definitions of the mitigation control levels defined in GEIA-STD-0005-2, commenting that the required level would normally be a function of the design authority in consultation with the customer, and that military applications would typically expect mitigation to at least Level 2B – risks managed primarily through mitigations, including design rules, and more likely 2C – risks managed more by avoidance and less by mitigation. Level 3 managed whisker risks through complete avoidance. He went on to discuss the detail requirements of Levels 2B, 2C and 3, with particular reference to the use of conformal coatings to form a physical barrier. The defining standard for tin whisker susceptibility of tin and tin alloy surface finishes was JEDEC JESD201A, an accelerated test used by component manufacturers, but there was some doubt as to the reliability of results since tin whiskering was such an unpredictable phenomenon.

In summary, as the standards had evolved, they had become increasingly definitive with regard to component-to-component spacing design rules. There had been a move away from component termination material type, structure and processing as specific mitigation strategies. Strategies were now based on protective barriers, re-finishing of component terminations, and automatic coverage of pure tin finished surfaces by tin-lead solder during the assembly process.

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A series of collaborative projects had been carried out at the National Physical Laboratory to evaluate conformal coating as a tin whisker mitigation strategy for printed circuit assemblies.
Martin Wickham reviewed previous findings and gave an update on current work. Using a tin plating process deliberately chosen for it high propensity to whiskering, NPL had developed a parallel plate test vehicle that had been enabled electrical detection of failure. One observation was that failure predominantly occurred at the edges of plates, where full coating thickness was not maintained around the right angle bend, and this geometry was a characteristic feature of component leads. An additional test vehicle had been designed, based on a PCB with 24 SOIC14 packages, assembled using range of techniques, to enable testing for short circuits between adjacent leads on individual component. Each batch of eight boards was delivered to consortium partners for coating and returned to NPL for testing. The assemblies were constantly monitored at a test voltage of 15 volts with a series resistor to limit current to 15 milliamps.

Nine batches had been built to date, together with control assemblies with no coating, all of which had shown extensive whiskering before any evidence was seen on the coated examples. Using the analogue of two sword-fencers for illustration, Wickham explained different failure modes: intermittent shorts involving more than one whisker, and longer shorts penetrating out through the coating in one position and back in through the coating elsewhere. He showed several examples of actual failures, and other instances where whiskers had grown but not yet been detected electrically. It was intended to continue ageing the test vehicles and review failures after a further six months, and to visually inspect the assemblies again after twelve months, also to build control assemblies to investigate the effect of temperature during coating. Possible future work would investigate the effects of vibration and forced air cooling.

SMART Group Steering Committee member Ian Fox, from Rolls Royce Control Systems, discussed practical aspects of tin whisker mitigation from the viewpoint of a manufacturer of aero engine control electronics. A proper control plan was fundamental to lead-free component risk mitigation, and he described how to prepare one in accordance with IEC/TS 62647-1 and to carry out system assessment using the decision tree from IEC/TS 62647-2 Annex A. The Rolls Royce control plan fitted into an area between Levels 2B and 2C defined by GEIA-STD-0005-2. Their management plan for components was based on IEC/TS 62239-1 and referenced a database of approved lead-free components which listed all the components used, with an approved lead-free finish defined for each component. Any components not lead-free approved were verified by X-ray fluorescence testing on receipt. All product change notifications received were reviewed against the company’s requirements for lead-free component acceptability, and actions were taken to verify their continued acceptability. Alternatively, the component change could be rejected and remedial action taken. The PCN review procedure was extremely useful in indicating industry assembly and material trends.

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Fox showed a range of component package styles, together with the acceptance tests and mitigation strategies for each type, including PCB design rules defining minimum component lead spacing. He also discussed the attributes of a range of conformal coatings: acrylic, polyurethane, silicone and parylene. Returning to the subject of component finishes, he commented on the debate as to whether the reflow process reduced or increased the propensity of tin to whisker, and concluded that if there was any doubt or there was a contractual obligation not to use tin, then refinishing of components was justified, using a controlled solder dip process that met the requirements of GEIA-STD-0006. “The only way to avoid whiskers is to avoid tin!”

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Dr Mark Ashworth
returned to deliver his second presentation on research at Loughborough, this time focused on post-plating mitigation methods. He commented that whatever might be achieved by way of optimisation of the tin electroplating process, whisker growth was so unpredictable that even the ‘best’ electroplated tin coatings could only be considered ‘whisker-resistant’ rather than ‘whisker-proof’, and additional precautions were required to further suppress the growth of whiskers. Three different techniques had been investigated.

The WHISKERMIT 2 research programme had set out to develop novel conformal coatings specifically designed to mitigate whisker growth by incorporating nano-fillers in the polymer formulation. Brass coupons electroplated with 2 microns of bright tin had been spray-coated with a proprietary acrylic formulation, modified by the addition of a nano-filler at 3%, 5% and 7% loading, and stored in an environmental chamber at 55°C and 85% humidity. The coupons had been periodically inspected for whisker growth using a stereo microscope. The modified coatings showed significant improvement in whisker mitigation compared with unmodified coatings, with the 5% nano-filler addition appearing to offer the optimum balance between mechanical strength and ductility.

Research published in 1994 suggested that tin whisker growth initiated at cracks in the surface oxide layer, so a second approach had been to investigate the effect of increasing the thickness of the oxide by electrochemical oxidation in borate buffer and potassium carbonate/bicarbonate solutions. An order of magnitude reduction in whisker growth had been demonstrated and the increased-thickness oxide layer had been observed to be still effective at mitigating whisker growth after three years.

The third approach reported by Dr Ashworth was atomic layer deposition, a proprietary thin film coating method using a self-limiting gas-phase chemical reaction to achieve thicknesses at the nanometre level. A range of pre-treatments and process conditions had been investigated. Coated and control samples had been inspected over a 12 month period using optical and scanning electron microscopy and significant reductions in whisker growth had been observed.

There had been several mentions of component refinishing during the seminar, in the context of the requirements of the GEIA-STD-0006 standard. Addressing the issues of continuation of supply and development of best practice, Mark Walmsley described how Micross Components had set out to develop and qualify an automated process for hot solder dipping electronic components that was compliant to the GEIA specification. The outcome was a seven-axis, multi-functional robotic machine with controlled pre-heat and the capability to manipulate and position its robotic arm to within 0.1mm. The machine offered a choice of conventional wave, side wave or flat pot soldering, with control of depth, dwell, entry and exit speed, solder angle and exit angle, followed by controlled cool-down and in-line washing.

The qualification of process and equipment to the GEIA standard was undertaken in partnership with the University of Greenwich for mathematical modelling of package reliability, and National Physical Laboratory to investigate terminal finish and reliability. Greenwich investigated 10 different component types and concluded by non-parametric statistical testing that the refinishing process had no significant impact on the electrical performance of the components and that the null hypothesis, that un-refinished parts were the same as the refinished parts, could not be rejected at the 5% significance level. And confocal scanning acoustic microscopy detected no degradation after hot-solder-dip processing. NPL selected 25 devices from RoHS-compliant sources, representing QFP, BGA and through-hole package types, and these were characterised before and after re-termination by X-ray fluorescence spectroscopy, micro-sectioning and optical microscopy, solderability testing, BGA ball shear measurement, scanning electron microscopy and scanning acoustic microscopy. The re-terminated components showed solderability equal to or better than the original components. Ball shear results for BGA components were acceptable, scanning acoustic microscopy did not locate any differences between original and re-terminated components, and thermal cycle solder joint reliability was improved for re-terminated components compared with tin plated originals

Micross had undertaken a two year exercise with six industrial partners and two academic institutions to develop a European source for hot solder dipping. A number of technical papers had been published, and the overall conclusion was that hot solder dipping components was not a major risk to electronic components.

This SMART Group seminar was notably interactive. The presenters were happy to be interrupted with questions and many interesting points of discussion were raised. And the coffee and lunch breaks presented abundant networking opportunities. A few one-liners worthy of recording: “Whiskers are as trustworthy as politicians!” “People have been looking at whiskers for 70 years and we’re still looking!” “Anyone who tells you it’s whisker resistant is taking a bit of a chance!” An excellent day all round.

Pete Starkey I-Connect007
November 2016