Electronics reliability company, Gen3 Systems, has announced the release of revised surface insulation resistance (SIR) standards designed to improve reliability and integrity testing of products assembled with lead-free solder.
The IEC published a new SIR test standard (IEC 61189-5) on 31 August, 2006. Additionally the IPC will also shortly release new SIR testing standards: IPC-TM-650 220.127.116.11 and IPC 9201A SIR Test Handbook. In addition, the IPC recently published a standard, employing SIR techniques, to determine the influence of sub-surface reactions known as CAF [cathodic (or conductive) anodic filamentation].
The new standards, for both individual materials and production process characterisation, adopt different test parameters for humidity, temperature, test duration, voltage bias, measurement frequency and test coupon, to take into account the effects of new production processes when using lead-free assemblies.
Lead-free assemblies are rapidly becoming the norm. Any manufacturer now wishing to import electronic assemblies into the EU must ensure the PCBs, components and solder contain no lead. Similar RoHS legislation is likely in many US states and China in the near future.
"What is not widely appreciated is that modern lead-free electronic circuit production is dominated by chemistry, and pretty aggressive chemicals at that," says Gen3 Systems' MD Graham Naisbitt. "Ionic residues arise from a multitude of manufacturing process - contaminants such as unreacted plating residues, improperly cured solder resists, soldering fluxes and inadequately-cleaned assemblies (including those manufactured using no-clean processes). Other contaminants may be non-ionic in nature, left behind, for example, by surfactants increasingly used to aid no-clean flux do its job."
These ionic contaminants (in the presence of electric potential and humidity) can cause failure due to electro-chemical reactions.
Exhaustive research by the National Physical Laboratory (NPL) in the UK, and Concoat Systems (the forerunner of Gen3 Systems), determined that measurements of changes in SIR would be a valuable, if not essential, metric in determining the susceptibility of electronic circuits to electro-chemical failure. However, to date, SIR testing has only been used to characterise individual process chemistries such as solder. In addition, NPL's research has now shown the test parameters used for these tests have yielded grossly misleading data.
The revised standards now define SIR testing procedures that can be used to characterise a process using a test vehicle assembled with the actual process materials used for modern lead-free, no-clean technology. In the new procedure, the assembly under test is driven to failure to establish at what point reliability is compromised by the reactions of ionic (and non-ionic) residues. This is important because of the differences in lead-free assembly compared with production using eutectic solder (60% tin and 40% lead).
The IPC is now recommending lead-free alloys such as SAC305 (Sn 96,5% Ag 3,0% Cu 0,5%) for electronics assembly. These solders require much higher reflow and flowsoldering temperatures (219°C compared to 183°C for eutectic solder).
"The higher processing temperatures typical of lead-free assemblies change how contamination forms. For example, all fluxes leave residues, but at elevated temperatures, these residues are more likely to be absorbed into the substrate increasing the overall ionic contamination," says Naisbitt.
Gen3 Systems manufactures the Auto-SIR insulation resistance testing system. The Auto-SIR tests to all major international standards, including the revised versions specifically recommended for lead-free assemblies.
SIR testing was established as a method for evaluating electronic assembly materials and processes as one measure of reliability. The objective of SIR testing is drive test coupons to failure to determine at under what conditions failure-inducing electrochemical reactions occur so that these conditions can be avoided during routine assembly.
Electrochemical failure occurs in the presence of electrical potential, moisture (from a humid environment) and ionic residue. If any of these is not present, electrochemical reactions cannot occur. The ionic residue can come from unreacted plating residues, improperly cured solder resist or from flux. These residues can cause corrosion, metal migration or dendritic growth leading to early failure.
SIR testing is performed with industry standard test coupons etched with patterns designed for the purpose (typically interdigitated combs). The insulation resistance of a test pattern is monitored at intervals as temperature and humidity are varied. Revised tests recommend test coupons complete with components.
There are several standards from organisations such as IEC, IPC and ISO that largely replace the now defunct Bellcore specification. Each defines different humidity, temperature, test duration, voltage bias, measurement frequency and test coupon. It is recommended that technicians refer to the latest version of the ‘IPC-9201 The SIR Handbook’ for detailed guidance on the subject.
More information on SIR testing is available from Gen3 Systems’ website (www.gen3systems.com).
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