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Avoid Electro-chemical Failure Using SIR
December 31, 1969 |Estimated reading time: 7 minutes
Surface insulation resistance (SIR) testing has formed the mainstay for high-reliability electronics manufacturers that must underwrite the long-term field reliability of their products. But the move to lead-free solder has led to a review of this technique. This article describes how SIR testing has been adapted to meet the challenges of RoHS-compliant manufacturing.
By Graham Naisbitt, Gen3 Systems
High-reliability electronic assemblies are used in applications where failure could result in a catastrophe. Manufacturers of high-reliability electronics take their responsibilities seriously. They may use, for example, high-quality components, ISO-certified assembly procedures, thorough in-circuit and functional test routines, and stress and vibration testing to ensure that products won’t fail prematurely. Despite this comprehensive assembly and test regime, there is one insidious danger threatening to compromise electronics in the field: electro-chemical failure.
With the emphasis on multi-layer, fine-pitch PCBs, exotic component packages, and expensive capital equipment, it is not widely appreciated that electronic circuit production is dominated by chemistry and fairly aggressive chemicals. Ionic residues can arise from a multitude of manufacturing-process contaminants remaining from chemicals used at all stages - from bare board to loaded PCB fabrication. These include unreacted plating residues, improperly cured solder resists, soldering fluxes, and inadequately cleaned assemblies. There also are other contaminants that may be non-ionic in nature and left behind, for example, by surfactants that are increasingly used to aid no-clean flux.
Figure 1: Dendrites formed between conductors on a test coupon.
On their own, these contaminants are relatively harmless. But when combined with moisture (for example, from a humid atmosphere) and an electrical potential, they are a recipe for dendritic growth that can cause shorts and corrosion, and ultimately, electro-chemically induced disaster. Many engineers have heard of surface insulation resistance (SIR) testing, a method pioneered by the UK National Physical Laboratory (NPL). Following exhaustive research, the NPL determined that measurements of changes to SIR would be a valuable, if not essential, metric in measuring the susceptibility of electronic circuits to electro-chemical failure. To date, standard SIR tests published as a result of the NPL’s work, defined by organizations such as International Electrotechnical Commission (IEC) and the IPC, have characterized individual process chemistries such as flux or solder. However, recent NPL research has shown that test parameters can yield grossly misleading data. Worse, the introduction of lead-free solders, as part of the EU’s RoHS legislation, has completely changed the chemistry of electronics assemblies, and consequently the nature of ionic contamination and how it may affect electronic circuits.
In 1988, the electronics world was grappling with the problem of eliminating chlorofluorocarbons (CFCs) as required by the Montreal Protocol. One proposed solution was the introduction of low-solids fluxes (based on carboxylic materials) that could remain on a PCB, eliminating the need for cleaning. However, the introduction of low-solids fluxes prompted engineers concerned with the effects of contamination and the possibility of dendritic growth and corrosion to question how clean a “no-clean” board would be. Dendrites are formed as metal dissolves at the anode and is electro-deposited at the cathode. The electro-deposited metal takes the form shown in Figure 1. Dendrites can be silver, copper, tin, lead, or a combination of metals, and can cause failures by creating short circuits. Dendrite growth can be very rapid; failures have been known to occur in a few seconds to a few minutes, but also can take several months . The growth rate depends on the applied voltage, the quantity of contamination, and the level of surface moisture.
The need to determine board cleanliness led to ionic extract cleanliness (IEC) testing, which is commonly referred to as solvent extract conductivity (SEC) and resistivity (or resistance) of solvent extracted (ROSE) testing. In its simplest form, IEC testing involves washing a component or assembly with a test solution of isopropanol and de-ionized (DI) water, generally in a volumetric ratio of 75:25 to dissolve the contaminants, and then measuring the resistivity of the collected washings. The test is based on the fact that all fluxes - including no-clean versions - leave residues. The salts can be dissolved in the water, and the fluxes are broken down in the alcohol. The test was based on measuring the resistivity (or conductivity) of the clean mixture, then immersing the assembly for a minute, removing the assembly, and then re-measuring the resistivity. The change in resistivity relates to the surface contamination in µg/in2 (or µg/cm2) that was present initially.
In 1975, the U.S. military determined a threshold at which an assembly should be regarded as reliable or unreliable. This threshold was initially set at 22 µg/in2, and today stands at 1.54 µg/cm2. The problem with this threshold is that it is anything but safe when working with modern fine-pitch components. Most experts agree that this value is too high for such assemblies.
The NPL established a method to determine a safe level of contamination for a given process chemistry. After more than six years of research, they determined that changes in SIR were needed. It should be noted that the purpose of SIR testing is to establish the point at which reliability is compromised. This test doesn’t measure contamination directly; it measures electro-chemical reactions caused by contaminants in conjunction with electrical potential and moisture.
Lead-free Challenges for SIR
While SIR testing represented a considerable leap forward in establishing what level of contamination could compromise reliability compared to SEC and ROSE, it has not been without its detractors. Experts continue to argue about voltage gradients, coupon design, for how long the test should be conducted, and at what humidity and temperature. The standard test is performed on a laminate and bare-copper coupon featuring an interdigitated comb pattern - the exact design varies according to which standard is adopted (Figure 2). The coupon is subject to a small, controlled amount of flux, and measurements are taken over seven days, with the sample subject to 85% humidity at 85ºC and a bias voltage of typically 100 V.
Figure 2: Interdigitated comb-test schematic.
Unfortunately, recent NPL research has revealed this methodology yields dubious results - sometimes incorrect by an order of magnitude, and described as “grossly misleading” by some experts. Moreover, materials used in actual production, for example, solder resists and unreacted plating residues, have a significant impact on SIR. For example, no-clean fluxes contain less than 2% solids (compared with the 40% solids of traditional fluxes) to ensure that they can safely remain on the board. Unfortunately, these low-solids versions do not stick to or wet the board without the aid of surfactants. These surfactants are a form of contamination that may influence electro-chemical behavior. Further, to remove oxides with a no-clean flux, it sometimes is necessary to increase the volume of flux or the pre-heat temperature. When the pre-heat is raised PCB expansion increases, causing voids in the substrate to open up - encouraging the PCB to act like a sponge and soak up the flux. This increases the propensity for dendritic growth below the PCB surface. Worse yet, traditional SIR test methods make no allowance for new materials and processing techniques introduced by lead-free soldering. For example, high-tin alloys such SAC 305 melt at 219ºC, which is much higher than the melting temperature of eutectic solders, which melt at 183ºC. This changes the process environment significantly because all fluxes leave residues, but at the higher processing temperatures of lead-free assemblies, these residues are likely to be absorbed into the substrate and vitrify.
New Standards
There is some good news. The industry has addressed the need to revise SIR standards to make them applicable to practical production conditions. IEC released new SIR test standards in August 2006, called IEC 61189-5. IPC also will release IPC-TM-650 2.6.3.7 and IPC 9201A SIR Test Handbook, which comprise new SIR tests for both solder-flux and process characterization where you can examine the synergistic behavior of your assembly-process-materials selection. IPC also recently published a standard to determine the influence of sub-surface reactions known as cathodic (conductive) anodic filamentation (CAF) - dendrites.
Figure 3: Automated SIR test output.
Consider this process characterization example: using an automated SIR test system* developed in conjunction with the NPL, the system tests an assembly put together according to IPC B-52/IEC TB57, and uses the intended process chemistry mix and dummy components representative of those on the production assembly. Test parameters are a voltage gradient of 25 V/mm, equating to 5-V bias, and measurement on 200-µm conductor width - a measurement frequency of 10 to 30 minutes for a period of no less than 72 hours. While NPL research showed that dendrites appeared in all cases within the first 72 hours, some fluxes take longer to react. Therefore, the test duration might extend to more than 1,000 hours. A typical output from automated equipment operating under these parameters is shown in Figure 3. Many also have suggested that 5 V is too low for the bias, but the NPL has shown that a lower bias voltage can increase electro-chemical activity.
Conclusion
Now that the SIR testing procedure has been redefined, it can be used to characterize a process using a test vehicle assembled with actual process materials. This compares with the somewhat artificial test procedure of the past that neither tested all the materials used in actual assembly, nor mimicked assembly processes. New procedures describe a test that drives the assembly-under-test to fail. With the relative lack of knowledge about processing with lead-free solders, and a continuing revolution in process chemicals, more precise SIR testing will come as a relief to manufacturers faced with underwriting the reliability of their critical electronic products.*Auto-SIR, Gen3 Systems, Farnborough, U.K.
Graham Naisbitt, managing director, Gen3 Systems, may be contacted via e-mail: graham.naisbitt@gen3systems.com.