Surface finishes Reliability test

In a search for reliable accelerated test methods for determining coating performance, electrochemical techniques have often been explored. The corrosion resistance of a coated steel panel is a composite of the steel quality, its surface finish and the quality of the coating. For this reason. Bonderite 40 coated steel panels were included in our work. They were employed primarily to aid in the interpretation of the electrical measurements for the nitrile-based photocured samples. 

Electrical testing is another class of reliability testing. The function of a solderjoint formed with the surface finish can be measured by burn-in testing, electrical functional tests, and/or high-current-density application for extended periods. The contact resistance of an unassembled surface finish is an important measurement. The PCB finish, when exposed to heat and humidity metrics, as well as environmental exposure, may need to pass a sensitive four-point probe contact resistance check. 

Contact Lands. The PWB is much like any other board. It has lands to accommodate electrical interconnect of the LGA socket to the board. The lands correspond in size and shape to socket requirements. Most often, the LGA lands on the PWB are electrolytic nickel covered by electrolytic gold. Hardened gold is often used for added contact durability and reliability. Other surface finishes can also be used, but should be tested for reliable and long-lasting contact. 

There are a number of other surface finishes used in the industry, such as Electroless Nickel Electroless Palladium Immersion Gold (NiPdAu), Immersion Silver, Immersion Gold, Immersion Tin, OSP and Electrolytic Nickel Gold. There are reliability and process trade offs with each surface finish. That is why it is recommended that strain/strain rate characterization and thermal cycling be performed for each set of surface finish before it is selected for the specific end-use conditions in which it will be used. The industry test methods used to evaluate different surface finishes are outlined in detail in the next chapter. 

For large boards, excessive board flexure, during PWB manufacturing, testing, assembly, and use, often can cause solder interconnect failures. Establishment of the strain rate-strain limit for lead-free solder interconnection on different PWB surface finishes can greatly help safeguard the reliability of the products (Ref 23). 

Organic-coated copper provides a consistent, flat, solderable metal finish. Exposed copper after printed circuit assembly has been a persistent reUabiUty concern, because it is generally not permitted on HASL boards. While exposed Cu on HASL boards is associated with poor solderabUity, which may be due to contaminants that were not removed before the HASL process, there is little evidence that exposed Cu on a properly processed OCC board causes reliability problems. Surface insulation resistance (SIR) testing shows that OCC boards have comparable or better performance than HASL boards in high-temperature, high-humidity storage tests. 

Manufacturability and reliability assessments were performed with a PWB Test Vehicle (RTV) utilizing a variety of surface mount components, as listed in Table 42. The test vehicle is shown in Figure 30. An imidazole-finished copper surface was used on the PWB and the component leads were hot-solder-dipped in the same alloy as the paste used for assembly. Thermal cycling was performed from —55 to 160°C with a ramp rate of 10°C/min and temperature dwell of 10 min at each end of the cycle, for a total cycle duration of 1 hr. For each alloy, approximately 27 boards each containing 28 components were tested. 

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