SMT007 Magazine

SMT007-Sept2019

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90 SMT007 MAGAZINE I SEPTEMBER 2019 operations, steam aging of PCBs before solder wick testing is seen as not only quick and inex- pensive but also a reliable shelf-life predictor. Eight hours of steam aging is reported to be equivalent to 12 months of shelf-life exposure for tin-lead (60/40) systems only [2] . There are many solderable surface finishes used on copper printed circuit boards that range from thin organic coatings to heavy metal plates, such as 1–3 microns of gold plate. Plated tin-lead or molten tin-lead coatings (HASL) are early predecessors of the diversi- fied finishes that are available today. These leaded alloys consistently show high resistance to steam aging and solder quality retention. Most other PCB coatings do not fare well after steam exposure. Currently, all other final sur- face finishes for PCBs fail to maintain accept- able solderability performance after one-hour steam exposure [2] . Of the many solderable finishes available, two are tested here. One—ENIG—has been available for over 20 years. It has a track record of robust soldering performance and is gener- ally ascribed a 12-month shelf life before its sol- dering performance begins to degrade. EPIG, on the other hand, is a new finish with EPIG test results showing very good solder wetting and force values. The overall shelf life has yet to be determined. However, test results in this article comparing ENIG performance to EPIG performance under steam exposure stress will shed light on this question. Steam exposure is not the only approach to determine how well a finish will hold up to environmental conditions. Other tests include thermal cycling, which shows the effect of multiple solder applications; real-time envi- ronmental exposure, which yields the most reliable data; and mixed flowing gas testing, a precise chamber test where gas type and con- centration are stringently controlled. Mean- while, steam exposure provides a quick, reli- able, and inexpensive indicator of soldering shelf life. Methodology A steam-aging preconditioning or stressing procedure was exacted upon both ENIG and EPIG coupons per IPC J-STD-002 and J-STD- 003. Using the stress of steam exposure helps to determine the durability and robustness of each solderable surface. Robustness is a dif- ficult term to define and measure accurately. Here, robustness is intended to mean that a PCB's shelf life vastly exceeds the typical 12 months, can withstand more solder reflows, and holds soldering properties regardless of the normal fabrication processes employed. Soldering test materials used include the application of a mild flux just before sol- der testing and the use of lead-free solder— Sn3Ag0.5Cu joining metal. These were limited to single non-variable options to simplify test- ing protocol. Both of these represent common commercial soldering practice. This assessment observes the solder wick- ing onto a PCB test coupon plated with either ENIG or EPIG. The testing is conducted on an as-plated condition and after exposure to steam. The duration of the steam exposure is in units of hours and continues until failure or a total of eight continuous hours of expo- sure. It is expected that failure to solder wet will coincide with the oxidation of plated films or the base copper to an extent whereby flux- ing is no longer able to clean and remove those oxides. The PCBs plated for testing consisted of standard solder wicking coupons, as shown in Figure 1. The ENIG film thickness used equals 3.75 microns of nickel phosphorus with 0.075 to 0.1 Figure 1: Solder test coupon used for solder wick testing.

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