Insulation resistance tester meets new standards

Revised surface insulation resistance (SIR) standards are designed to improve reliability and integrity testing of products assembled with lead-free solder.

Revised surface insulation resistance (SIR) standards are designed to improve reliability and integrity testing of products assembled with lead-free solder.

The International Electrotechnical Commission published a new SIR test standard (IEC61189-5) on 31st August 2006.

Additionally the US-based IPC will also shortly release new SIR testing standards: IPC-TM-650 2.6.3.7 and IPC9201A SIR Test Handbook.

In addition, the IPC recently published a standard, employing SIR techniques, to determine the influence of subsurface 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 coon, to take into account the effects of new production processes when using lead-free assemblies.

Lead-free assemblies are rapidly becoming the norm following the EU's Restriction of Hazardous Substances (RoHS) legislation that passed into law on 1st July 2006.

Any manufacturer now wishing to import electronic assemblies into the EU must ensure the printed circuit boards (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 nonionic 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 electrochemical reactions.

Exhaustive research by the world-renowned 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 electrochemical 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.

Now the revised standards 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 nonionic) 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 flow-soldering temperatures (219C compared with 183C 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 system has been developed in collaboration with the NPL.

The Auto-SIR tests to all major international standards, including the revised versions specifically recommended for lead-free assemblies.

The Auto-SIR employs frequent monitoring trend analysis (FMTA).

This can be used to monitor long-term process condition trends.

Naisbitt is a member of the International Electrotechnical Commission's (IEC) TC91 WG3, the working group that formulates test standards for the assembly industry.

He is also Leader of Solderability Testing Standard IEC60068-2-69, Co-leader of Solderability Testing Standard IEC 60068-2-54, and Member of IPC-J-STD 002 and IPC-J-STD 003.

Back to top