Avoiding electro-chemical failure in a lead-free age
AT A GLANCE
- The introduction of low-solids fluxes prompted concern about the effects of contamination and the possibility of dendritic growth and corrosion on high reliability “no-clean” electronics assemblies
- While Surface Insulation Resistance testing represented a considerable leap forward in establishing what level of contamination could compromise reliability, it has not been without its detractors
- Traditional SIR test methods make no allowance for the new materials and processing techniques introduced by lead-free soldering. Fortunately, IEC released new SIR test standards in August 2006, called IEC 61189-5. And IPC will also shortly release new SIR testing standards: IPC-TM-650 2.6.3.7 and IPC 9201A SIR Test Handbook
- The new procedures describe a test that drives the assembly-under-test to failure to establish at what point reliability is compromised by the reactions of ionic (and non-ionic) residues using actual process materials and representative samples
ARTICLE
High-reliability electronic assemblies are used in applications where failure could cause catastrophe. The potential consequences of an airbag not inflating during a road accident, or an aircraft’s navigational systems misbehaving, or a respirator in a hospital working intermittently due to malfunctioning electronics means lives could quite literally be put at risk.
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 their products won’t fail prematurely. But despite this comprehensive assembly and test regime, there is still one insidious danger threatening to compromise the electronics in the field: electro-chemical failure.
With the emphasis on multilayer, fine-pitch PCBs, “exotic” component packages and expensive capital equipment it’s not widely appreciated that electronic circuit production is actually dominated by chemistry - and pretty aggressive chemicals at that. Ionic residues can arise from a multitude of manufacturing process contaminants left over 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 (including those manufactured using no-clean processes). Then there are also other contaminants that may be non-ionic in nature and left behind, for example, by surfactants increasingly used to aid no-clean flux to do its job.
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 are familiar with these effects of course, and will have heard of Surface Insulation Resistance (SIR) testing – a method pioneered by the world-renowned UK National Physical Laboratory (NPL) in conjunction with Concoat Systems, the forerunner of my present company, Gen3 Systems. Following exhaustive research, 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, the standard SIR tests published as a result of the NPL’s work – defined by organisations such as International Electrotechnical Commission (IEC) and the US-based Association Connecting Electronics Industries (IPC) – have only characterised individual process chemistries such as flux or solder. However, more recent research by NPL has shown the test parameters used for these tests can yield grossly misleading data.
Worse, the introduction of lead-free solders – which the EU’s Restriction of Hazardous Substances (RoHS) legislation requires manufacturers to adopt – has completely changed the chemistry of electronics assembly and consequently the nature of the ionic contamination and how it may affect electronic circuits.
All of this means SIR testing must adapt to meet the new challenges of the lead-free age.
What is SIR testing?
Back in 1988 the electronics world was grappling with the problem of eliminating chlorofluorocarbons (CFCs) ahead of the Montreal Protocol’s formal introduction on January 1, 1989. One proposed solution to the problem caused by the elimination of these chemicals in electronics assembly (they were used for cleaning flux and other contaminants from assemblies) was the introduction of low-solids fluxes (based on carboxylic materials) that could “safely” be left on a PCB and thus eliminate the need for cleaning.
However, the introduction of low-solids fluxes prompted some engineers – who were concerned about the effects of contamination and the possibility of dendritic growth and corrosion - to question how clean a “no-clean” board would actually be. Dendrites are formed as metal dissolves at the “anode” and is subsequently electrodeposited at the “cathode”. The electrodeposited metal takes the form shown in figure 1. Dendrites can be silver, copper, tin, lead or a combination of metals and 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 can also take several months or more. The rate of growth is dependant on the applied voltage, the quantity of contamination and level of surface moisture.
Figure 1: Dendrites formed between conductors on a test coupon
The need to determine board cleanliness eventually led to Ionic Extract Cleanliness (IEC) testing - 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-ionised water, generally in a volumetric ratio of 75:25 to dissolve the contaminants, and 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 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 initially present.
In 1975, the US Military decided that there should be a threshold below which the assembly could be regarded as “reliable” and above, “unreliable”. This threshold was initially set at 22 µg/in2, later revised to 14 µg/in2 of NaCl equivalent and today to 1.54 µg/cm2. The problem with this reliable or “safe” threshold is that it’s anything but when working with modern fine-pitch components. Most experts agree that this “safe” value is too high for most modern, fine pitch assemblies.
The NPL took up the challenge of coming up with a method of determining what level of contamination is actually safe for a given process chemistry. After more than 6 years’ research into electro-chemical reactions in circuitry, the laboratory determined that measurements of changes to SIR was the way forward.
It should be noted that the fundamental purpose of the SIR test is to drive failures in order to establish at what point reliability is compromised. The SIR test doesn’t measure contamination directly; it measures the 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 with SEC and ROSE, it has not been without its detractors. And the experts continue to argue about such things as voltage gradients, coupon design, how long the test should be conducted for and at what humidity and temperature.
The “standard” test (there are several derivatives) is performed on a laminate and bare copper coupon featuring an interdigitated comb pattern (the exact design varies according to which standard is adopted, see figure 2). The coupon is subject to a tiny controlled amount of flux and the measurements are be taken over a period of seven days with the sample subject to 85% humidity at 85ºC and a bias voltage of typically 100 V is applied.
Figure 2: Interdigitated comb test schematic
Unfortunately, recent research by NPL, backed by an EU grant, 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, not to mention uncontrolled flux volumes – have a significant impact on SIR yet aren’t even tested by the standard methodology.
Consider an example: no-clean fluxes contain less than 2% solids (compared with the 40% solids of traditional fluxes) to ensure that they can be “safely” left on the board. Unfortunately, these low solids versions don’t “stick” to or wet the board without the aid of surfactants (materials than modify surface tension). These surfactants are a form of contamination that may well influence electro-chemical behaviour.
Further, to remove oxides with a no-clean flux it’s sometimes necessary to increase the volume of flux or the pre-heat temperature (in order to boost the material’s “activity”). When the pre-heat is turned up PCB expansion increases. This causes voids in the substrate to open up encouraging the PCB to act like a sponge, sucking up the flux. All this increases the propensity for dendritic growth below the PCB surface.
Worse still, the traditional SIR test methods make no allowance for the new materials and processing techniques introduced by lead-free soldering – now the dominant manufacturing technique thanks to the introduction of RoHS.
For example, high-tin alloys such SAC305 (Sn96.5% Ag3.0% Cu0.5%) –recommended as tin/lead solder replacements by the IPC and IEC – melt at 219ºC, which is much higher than eutectic solder’s (60% tin(Sn) and 40% lead(Pb)) 183ºC. This changes the process environment significantly. For example, all fluxes leave residues, but at the higher processing temperatures typical of lead-free assemblies, these residues are likely to be absorbed into the substrate increasing the ionic contamination and are more likely to vitrify. The problem is that none of these effects or contaminants are picked up by the standard test.
Time for a new standard
Fortunately for high reliability manufacturers there is some good news. The industry has addressed the need to urgently revise the SIR standards to make them applicable to practical production conditions.
IEC released new SIR test standards in August 2006, called IEC 61189-5. And IPC will also shortly release new SIR testing standards: IPC-TM-650 2.6.3.7 and IPC 9201A SIR Test Handbook. Because of the mass introduction of lead-free solder the timing of the releases couldn’t be better. These standards comprise new SIR tests for both solder flux characterisation and now process characterisation where you can examine the synergistic behaviour of your assembly process materials selection.
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; that’s dendrites to you and me) that I briefly described above. The new standards 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. Many of the numbers in these documents relate to an older German DIN standard and are consequently backed-up by lots of data.
Lets consider a process characterisation example. My company manufactures the Auto-SIR (see figure 3). This is an automated SIR test system that was originally developed in conjunction with the NPL. The Auto-SIR now tests an assembly put together according to IPC B-52/IEC TB57 and using the intended process chemistry mix and true dummy components representative of those on the production assembly. (Note, it’s important not to use reject components for these tests) (see figure 4). The parameters for the new test 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 not less than 72 hours. Whilst the NPL research showed that dendrites appeared in all cases within the first 72 hours of test, it is known that some fluxes take longer to react so the test duration might extend to more than 1000 hours according to research carried out by HP. A typical output from the Auto-SIR equipment operating under these test parameters is shown in figure 5.
Figure 3: Auto-SIR test equipment
Figure 4: Typical SIR test coupon for assembly characterisation
Figure 5: Auto-SIR test output
One point worth a further mention is the bias voltage. Many have suggested that 5 V is far too low, but the NPL’s work has actually shown that a lower bias voltage actually increases electro-chemical activity. It’s counterintuitive, but true.
A realistic test
Now that the SIR testing procedure has been redefined by the recently released IEC and IPC standards it can be used to characterise a process using a test vehicle assembled with actual process materials. This compares with the somewhat artificial test procedure of old that neither tested all the materials used in actual assembly nor mimicked assembly processes. Perhaps it’s not surprising that the traditional SIR test has been shown to give flawed results.
The new procedures describe a test that drives the assembly-under-test to failure to establish at what point reliability is compromised by the reactions of ionic (and non-ionic) residues using actual process materials and representative samples. With the relative lack of knowledge about processing with lead-free solders (compared to decades with traditional tin/lead alloys) and a continuing revolution in process chemicals, more precise SIR testing will come as a tremendous relief to manufacturers faced with underwriting the reliability of their critical electronic products.
FOR MORE INFORMATION
If you can’t find the information you need at Gen3 Systems’ website at www.gen3systems.com, then please contact Gen3 Systems direct on:
Telephone: +44 (0)1252 521500,Fax: +44 (0)1252 52 1112, or e-mail.
ABOUT THE AUTHOR
Graham Naisbitt (e-mail) is Managing Director of Gen3 Systems Limited (founded on Concoat Systems) and is a member of the IEC’s TC91 WG3, the working group that formulates test standards for the assembly industry. Naisbitt is also Leader of Solderability Testing Standard IEC 60068-2-69, Co-leader of Solderability Testing Standard IEC 60068-2-54, and Member of IPC-J-STD 002 and IPC-J-STD 003.
Download the PDF (2171 KB)7th Aug 2008