Solderability Testing Wetting balance

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

INTERPRETING THE RESULTS WETTING BALANCE SOLDERABILITY TESTING... 

Pinch and Sjostak utilized this type of equipment to characterize solderability of various Cu pastes on ceramic circuit boards. This type of test is particularly useful in distinguishing minor performance variations in materials that typically exhibit good solderabUity. There are numerous suppliers for this type of wetting balance test apparatus. ... 

Surface-tension Test —The tombstoning performance of solder pastes has been speculated to be related to the surface tension of respective solder alloys. In this study, a wetting balance was used to measure the surface tension of solder alloys following established conditions. A piece of alumina coupon with a dimension of 2.5 X 0.4 X 0.062 cm was used for the testing. The solder pot was maintained at 245°C for the SnPb alloy and 260°C for SnAgCu alloys. The measured force with respect to time was translated to a graph of force vs. dip depth based on dip speed. When the alumina piece immersed into the solder for a certain depth where the meniscus became stable, the measured force was proportional to the displaced volume of solder. This force may be described as follows ... 

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. 

FIG. 41 Wetting balance results for eutectic Sn-Pb and two lead-free alloys. The test consists of monitoring the time required to change the force exhibited on a coupon from compressive (solder is pushing on the coupon) to a tensile force (solder is pulling on the coupon). This change in force direction (or vector) is called the zero force time. (From Ref. 26.)... 

Wettability A wetting balance test assesses the force resulting when a copper wire is immersed in and wetted by a molten solder bath. A large force indicates good wetting, as does a short time to attain a wetting force of zero and a short time to attain a value of two-thirds of the maximum wetting force. Fmax > 300 pN, to < 0.6 sec, t2/3[Pg.669]

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