Clean and mean, the road to reliability

By Dave Mohr

With Revisions by Dave Mohr

 

Contaminants remaining on an electronics assembly as a result of the product’s manufacturing process can contribute to unexpected field failures. Learn which test methods are commonly used to test for ionic contamination and how to catch ionic residues before they cause problems.

 

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When electronic assemblies fail

... Heavy rain and turbulence shook the aircraft as it began its descent toward the landing field. The shiny new avionic screens flickered feebly and suddenly went dark—forcing the lone and increasingly desperate pilot to land his passengers manually in the storm ...


This chilling scenario illustrates the potentially serious consequences of circuit board failures. Robust electronic assemblies are crucial for many industries—including personal transportation—and unexpected failures can carry life-threatening consequences.

 

The “fire triangle”: three factors that contribute to electronic assembly failures

A common threat to the reliability of transportation system and other electronics is sometimes referred to as the fire triangle: contamination, available atmospheric moisture, and bias differential. Together, the presence of these three factors can result in electrical leakage and failure of the assembly.

 

Culprit #1: contamination

Because there are no universal limits for ionic contamination that apply to all electronics, individual electronics assemblers must determine through testing the type and degree of ionic content that is acceptable for their products. This blog will discuss three common tests used to identify contamination of electronic assemblies that have been standardized in industry guidance published by IPC (IPC TM-650).

 

ROSE testing

Resistivity of Solvent Extract or ROSE testing (TM-650 2.3.25) is one of the oldest tests for cleanliness. ROSE testing is accomplished by exposing the product to an extraction solution by manual (rinsing), dynamic, or static extraction methods. Measurement of the fluid’s conductivity is an indication of the amount of ionic contamination present.

 

There are two notable drawbacks to ROSE testing. First, the ionic content measured is an average of all components and board surfaces. Second, the types of ionic contamination present are not identified, and thus cannot be specifically remedied. IPC WP-019, a very detailed white paper, describes the history of ROSE testing and outlines why it should no longer be used the way it is typically used today (e.g. for bottom terminated surface mounted parts using no-clean solder paste).

 

Ion chromatography

Ion chromatography (IC) is the most accurate method to determine the ionic content present on a product. The standard or “total board” extraction method is described in TM-650 2.3.28, Ionic Analysis of Circuit Boards, Ion Chromatography Method.

 

Using the standard method, a sample is subjected to a mixture of isopropyl alcohol and deionized water at 80oC for one hour. This causes soluble ionic surface contaminants to dissolve into the extraction solution which is then processed by the chromatograph. IC analysis identifies the type and precise amounts of anions and cations, as well as the general weak organic acid total.

 

The importance of IC analysis findings cannot be understated. Each ion-contributing material that goes into the assembly process has a chemical signature that can be matched with the IC results. It is this information that is crucial in determining the risk of electrical leakage. Using IC analysis, Ionic content can be traced to flux activators, plating chemistries, wash signatures, bare board fab, handling, etc. Once the offending process or material has been identified it can be quantitatively optimized to minimize active residuals.

 

IC analysis by standard/ total board extraction shares one disadvantage with ROSE testing, in that the extracted solution contains contaminants collected from all exposed board and component surfaces. To address this, Foresite’s C3 localized extraction system was developed to target individual components and/or board surfaces. With the C3, the effects of specific processes like wave solder, hand solder and localized cleaning effectiveness can be assessed.

 

In our opinion, C3 testing with subsequent IC analysis of the effluent is the best tool for rapid assessment of the potential impact that processing residues have on reliability.

 

 

Foresite C3 tester

 

Foresite C3 Tester for localized extraction

 

 

IC spectrum acquired from C3 effluent

 

IC spectrum acquired from C3 effluent

 

 

SIR testing

The last test method highlighted is surface insulation resistance—or SIR as it is commonly referred to (TM-650 2.6.3.7). SIR testing is used for material and process qualification utilizing unpopulated test coupons with known spacing comb patterns in an elevated heat (40oC) and humidity (90%) chamber with constant voltage application. Although SIR testing was originally designed to use IPC B-24 test boards, in the past few years more and more companies are using the IPC B-52 test board, as it incorporates components commonly used in today’s manufacturing.  

 

Another SIR test option, TM-650 2.6.3.3, uses test parameters of 85oC and 85% relative humidity, but these can alter the chemical composition of weak organic acids and produce false results. Because of this, the TM-650 2.6.3.7 method is typically preferred. TM-650 2.6.3.7 tests run for no less than 72 hours with most running up to 168 hours. Measurements are taken every 20 minutes to monitor the effect elevated heat and humidity have when combined with the residues present on the sample. Much like the IC test, the idea is to determine whether the residues present will facilitate electrical leakage and/or electrochemical migration. The acceptance criteria for this test are found in the J-STD-004 standard, section 3.4.1.4.1.

 

Although a performance test like SIR does not identify ionic contaminants, it can reveal the assembly house’s ability to process a chosen set of materials with their equipment. However, consideration should be given, as test results may not necessarily translate to the actual product being built.* For that, temperature, humidity, and bias (THB) testing is recommended—but that is for another blog!

 

*Because test coupons are typically used in SIR testing instead of actual product due to expense and timing, the results don’t always represent the actual product with 100 percent accuracy.

 

Determining which reliability tests are right for your products

While contamination testing alone isn’t enough to ensure a product’s reliability, identifying the right tests for ionic contamination make a world of difference. Our team of certified reliability experts can talk you through the most important factors to consider.

 

 

Have questions? We would love to help.


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About the Authors

Eric Camden  is our lead investigator for ionic cleanliness and microelectronics reliability. Dave Mohr has been employed with Foresite for over three years as a metallurgical engineer and has over 20 years’ experience in the industry.