Solderability Testing Wetting

The adhesion of specimens subjected to environmental exposure tests was evaluated prior to and subsequent to the contact. Wet thermal shock testing consisted of five cycles each for IS minutes in boiling water and 2 minutes in ice water. The maximum transfer time between the two baths was 30 seconds. Thermal and temperature/humidity exposures were performed in controlled atmosphere chambers for 200 hours. Simulated solder tests comprised immersing the test. specimen in a silicone oil followed by solder flotation. The solder temperature varied from 232 to 288 C and the contact time was S or 10 seconds. In some cases, two flotations were performed on the same sample. The effect of a heat treatment at 135 C prior to solder testing was examined the heat treatment time varied from 0 to 16 hours. ... 

FIGURE 42.6 Possible outputs from a solderability test using a wetting balance. 

INTERPRETING THE RESULTS WETTING BALANCE SOLDERABILITY TESTING... 

The objective of the solderability test methods described in J-STD-003 is to determine the abihty of printed board surface conductors, attachment lands, and plated-through holes to wet easily with solder and to withstand the rigors of the PCB assembly processes. 

Takemoto, T. Miyazaki, M. Effect of excess temperature above liquidus of lead-free solders on wetting time in a wetting balance test. Mater. Trans. JIM 2001, 42 (5), 745-750. 

Standards will require revision to accommodate the potential for a variety of leaded and lead-free solder alloys. Acceptable test measurements including inspection criteria must be established including shear and tensile testing, wetting angle measurements, visual criteria defining the acceptable surface appearance of the solidified alloy and retraining will be required. 

FIG. 39 Comparison of the wettability of several lead-free solder baths and the eutectic Sn-Pb benchmark solder for various chip components and finishes. The lead-free alloys each show varying degrees of wettability with the various component electrode finishes, whereas the eutectic Sn-Pb solder completely wet for each test condition. (Courtesy of Panasonic.)... 

Generally, it is believed that many plastics classed as insulators become partial conductors when "hot and wet" (12). The UV and thermally cured solder masks in Table III show a dramatic loss of insulation resistance after seven days at 85°C and 95% relative humidity (RH). All the test boards recovered most of their initial insulation resistance after 24 hours at ambient conditions. Table IV shows that an additional exposure to 85°C at ambient humidity produces an increase in insulation resistance. 

Vaporization rates were determined utilizing the same basic container maintained at the full level of liquid oxygen. In this case, however, the vaporized liquid was vented through a wet-test meter which was equipped with a helical potentiometer and sufficient circuitry to record the volume boil-off rate. Unit area heat transfer rates were then determined from these data. The temperature of the container skin, Ts, was recorded with the use of a thermocouple soldered to the container wall during the length of the test. All tests of this nature were performed for times of 3 hr. 

Refer to Chap. 42 for detailed information on solderability of PCB surface finishes. Testing methods are contained in the comprehensive joint industry specification ANSI/IPC J-STD 003. Surface finishes are commonly tested in production at the board fabricator using a solder float or solder dip method. When qualifying the finish, an OEM or assembler uses more equipment-specific tests such as wetting balance, rotary dip, or paste spread methods. Solderability should be consistent from day to day and lot to lot, with very little degradation after exposure to at least three assembly thermal exposures. 

Figure 42.7 is a comparison of average wetting times for ENIG, tested at 235°C as a function of solder alloy used. The SnPb alloy wet the coupon after 0.75 seconds, SAC305 took more than 4 seconds and SnCu did not wet during the 10 seconds of the test. 

As per the JSTD, the test time for components is typically five seconds. The part is completely immersed into the solder bath and then removed, cleaned, and evaluated for dip and look testing. The part may have wet instantaneously or it may have wet at 4.999 seconds. This is something that can never be discovered via dip and look testing. This wetting time becomes critical if there are differential wetting times between endcap metallizations. When a chip with two endcap metallizations wets inconsistently endcap to endcap, the resultant forces of soldering will cause one end to rise up off the pad and the chip will exhibit the classic tombstone appearance, in which one end of the component becomes detached from the land area and the surface forces on the other end cause the component to stand on end, resembling a tombstone (See Fig. 42.15). 

FIGURE 42.15 This shows a test for some surface mount components that pass the five second dip and look test, but clearly show an issue with surface oxides prior to reaching a very acceptable final wetting force. The supplier of these parts, using dip and look testing, certified them to meet solderability requirements but the end... 

---