Shifting domestic/global economics and technological advancements have proven one thing as fact: Electronics
manufacturing is about change. It is becoming unusual for an electronics manufacturer to produce the same product for the same customer while utilizing the same equipment, same processes and materials for an extended period of time. Surface insulation resistance (SIR) testing is a powerful tool electronics manufacturers can use when adapting to changes in processes, materials, and manufacturing equipment.
It is commonly used to assess process and design changes, qualify new and existing materials, and compare like materials or processes. Once product performance and reliability have been established using SIR, testing results can be used as equally powerful marketing and quality assurance tools.
What is SIR testing?
In the electronics manufacturing industry, SIR testing is an extremely sensitive test method for evaluating the reliability of materials used for board assembly. SIR testing evaluates the propensity for assembly failure caused by shorts or current leakage between metal conductors. These failures can be induced by material interactions, inadequate process control, or poor material performance. SIR is an electrical test that measures a change in current over time and is typically performed at elevated temperatures and humidity levels. Surface insulation resistance is defined as electrical resistance between two electrical conductors. Sheet resistance, bulk conductivity, and electrolytic contaminant leakage are all factors that affect the insulation resistance. For the electronics manufacturer or producer, surface insulation resistance can be thought of as a system's ability to resist surface shorting of leads or traces.
It is well known in the electronics manufacturing industry that the cleanliness of a printed wiring board (PWB) is crucial to the performance and reliability of an assembly. Monitoring and quantifying the degree of cleanliness is necessary in order to ensure that the final cleanliness of an assembly is acceptable. Performance of SIR test samples are directly related to cleanliness. It is the reason why bare PWB manufacturers, flux and conformal coating producers, and assemblers embrace SIR as a process and product development tool.
SIR test results can be used for a variety of purposes, including:
- Classifying fluxes
- Qualifying fluxes and pastes
- Improving a cleaning process
- Qualifying and comparing conformal coatings
- Comparing cleaning materials
- Comparing flux materials
- Qualifying bare-board cleanliness
Contamination can severely diminish the ability to resist shorting of leads or traces. There are specific types of contamination that are indigenous to the electronics manufacturing process prior to or after product assembly. Harmful residues and contaminants are separated into two main categories; ionic and non-ionic.
Ionic residues can be described as residue that contains molecules or atoms that are conductive when in solution. Some of the most common sources of ionic residue include:
- Plating chemistries
- Flux activators
- Ionic surfactants
Non-ionic residues are not conductive and are usually organic species that can remain on the PWB after board fabrication or assembly.
- Hand lotion
While both ionic and non-ionic contamination can impact the operation and reliability of the device on which they are present, the effects of ionic contamination are of greater interest to the most PWB manufacturers. A higher number of failures are associated with ionic contamination than its non-ionic counterpart.
Electrochemical migration is an occurrence of a conductive metal bridge forming between conductors when they are subjected to a DC voltage bias. Metal conductors, like lead from a tin-lead HASL coating, grow from a positively charged conductor (cathode) to a negatively charged adjacent conductor (anode) creating a short circuit between the conductors. The growth takes the tree-like form of a dendrite as seen in Figure 3-1.
The added presence of moisture can cause ionic residues to disassociate into either negatively or positively charged species and create conductive solutions, known as electrolytic solutions. When there is a voltage bias between a cathode and anode, an ionic species will grow from one conductor to another. Chloride and bromide, commonly found in fluxes and PWB substrates, are two of the most common dendrite forming substances. Metallic salts from copper based metals, which conduct electricity and create shorts across the leads or traces, can also be formed in the process.
Chloride and other halides commonly found in fluxes often form acidic solutions that attack the metal surface. Simply put, the higher the concentration of the halides, the stronger the acidity of the solution. The acidic electrolyte solution corrodes metallic conductors such as copper and tin-lead traces and leads. Voltage bias will severely accelerate the corrosive process and free-up metallic ions. Once in the solution, the metallic ions either form dendrites or form conductive salts like copper chloride or copper sulfates. Dendrites and conductive salts both have the effect of lowering the insulation resistance and ultimately result in short circuits or current leakage.
It is the function of conformal coatings to protect a board or assembly from harsh environments. In water condensing environments, conformal coatings are expected to prevent moisture from interacting with any process residues thus preventing dendrite or conductive salt formation. Permeability is one key factor that determines how well a coating performs. Despite having excellent permeability characteristics no conformal coating can proscribe water indefinitely.
The mechanics of SIR testing SIR test samples utilize specific patterns similar to those shown in Figure 3-2 to assess insulation resistance.
The growth of dendrites or the presence of conductive solutions between the conductors of the patterns affects the resistance between them. Higher SIR values are the result of cleaner boards that do not form dendrites. Lower values are a result of the presence of conductive dendrites or salts. A preferred method of testing involves the use of specialized SIR measuring equipment that applies voltage and records DC resistance at programmed intervals. Increasing temperature and humidity can have the effect of accelerating dendrite growth and artificially age the samples to simulate the service life of a product. Having precisely controlled temperature/humidity chambers and SIR measuring equipment are the most reliable and efficient ways of performing an accurate SIR test.
Standards and Test Vehicles
There are several industry standards developed by the IPC, Bellcore, and Japanese Industrial Standards used to classify fluxes, residues, and conformal coatings. The standards used, test conditions, and layout of the SIR pattern are dependent upon the materials to be tested. There are industry accepted test patterns that can be ordered through the IPC or Bellcore as Gerber file data. Many bare board manufacturers will be familiar with SIR test pattern Gerber data thus reducing the cost of producing test samples. Table 1-1 shows some of the typical uses for many of the popular test patterns. One of the most common tests is conducted using IPC TM-650-126.96.36.199. This test is intended to characterize fluxes using a standard comb pattern at 85% relative humidity and 85ºC.
Is SIR testing right for you?
If you plan to make changes to your manufacturing processes
then SIR testing should be seriously considered, particularly
if you intend to do any of the following:
- Alter board design or layout
- Change cleaning materials or processes
- Change fluxes and/or pastes
- Perform radical changes in reflow or wave profile
- Use new conformal coating materials or processes
- Qualify bare board suppliers
- Market a product based on reliability
At the EMPF, SIR testing is relied on to improve the reliability of electronic components used in commercial and military applications, and to enhance the overall productivity of electronics manufacturers.