Issue link: https://iconnect007.uberflip.com/i/1288481
SEPTEMBER 2020 I PCB007 MAGAZINE 79 has to be fulfilled to get a process to the mar- ket. These criteria include target requirements for the bath properties, process handling, as well as the performance of the final finish. The final finish parameters—such as appear- ance, etch resistance, thickness distribution, soldering and bonding performance, and IMC formation—are evaluated and judged accord- ing to internal pass/fail criteria and interna- tional standards such as IPC. Additionally, in the development of an ENIG process, the judg- ment of the corrosive attack of the nickel layer becomes one of the most important criteria to rate a sample as passed or failed. The challeng- ing task when implementing corrosion criteria is to find the best compromise between time ef- fort, statistical relevance, and objective rating. What is so critical? In general, there are dif- ferent options to evaluate an ENIG layer for corrosion: either the judgment is done in cross- section, or the evaluation is done on top view after stripping the gold. Table 1 gives an over- view of the general benefits or drawbacks of one or the other method. Comparing the two approaches, the analy- sis of the cross-section can give a better over- all picture of the amount and depths of the corrosion at the same time. Therefore, the cross-section analysis has been chosen as the preferred tool for internal bath judgment and development. To implement an evalua- tion system, which is independent of the sin- gle operator/evaluation person and allows the creation of comparable results, a rating ta- ble has been established internally to rate the amount and depth of the corrosion event at the same time. In this rating table, the corrosion is judged in terms of amount and depth. During the nu- merous evaluations, it turned out that corro- sion events are more likely in plated through- holes (PTHs) rather than on pads. The focus of the evaluation is put on rating corrosion in PTHs only. To allow a statistically relevant statement, it is recommended to do the inves- tigation on a minimum of two different areas at the PCB with a minimum of three PTHs per area. Another decision point is whether the in- vestigation should be done by a light micro- scope or SEM. Again, there are a number of benefits and disadvantages connected to the one or the other method (Table 2). Table 1: Benefits and drawbacks of different evaluation methods for ENIG corrosion. Table 2: Benefits and drawbacks of evaluating cross sections by light microscope or SEM.