Even the best electronics manufacturing process using high-end equipment cannot succeed if the boards and/or the components used in the fabrication will not solder. The inability to form proper solder joints in an SMT or through-hole operation can result in a host of non-ideal and defective conditions on the final assembly. Having acceptable solderability contributes to forming better solder joints, which in turn leads to better first-time yields and less rework. As a result, being able to measure the solderability of board pads and through-holes as well as component leads and terminations is a key capability to sustaining a successful electronics manufacturing operation. Two analytical pieces of equipment used to quantitatively measure solderability are the wetting balance, used for leads and terminations, and Sequential Electrochemical Reduction Analysis (SERA), used for board pads and plated through-holes. The basic design, operation, and applications of each technique are described in this article.

Wetting Balance
One example of a wetting balance is the Multicore MUST System II (Figure 1). The general operation of any wetting balance is the same. A specimen is attached to a very sensitive microbalance, which can measure very small forces that act on the sample. Flux is applied to the sample of interest and the bath or solder globule is raised in a controlled fashion so that the sample is immersed into the solder. Test parameters, such as immersion speed, immersion angle, and immersion depth can be found in standards such as J-STD-002A and IEC 68-2-69. Wetting balance data is typically displayed in the form of a wetting curve. A typical curve from a solderable sample is shown in Figure 2. Upon solder contact, the sample is initially pushed upwards due to the surface tension of the solder and the buoyancy force. Such upward forces are recorded as negative values on the wetting curve plot. Ideally, soon after immersion, the solder will wet up the lead to some extent, which is measured as a positive force by the microbalance. Quantitative data can be obtained from the wetting curve. Quantitative analysis of the wetting curve includes data such as those listed in Table 1.

Evaluation criteria for acceptable solderability are given in various standards. The J-STD-002A requires that the t (buoyancy) be £ 1 second. This is considered the onset of wetting. Another criterion states that the force at 2 seconds be ³ 50% of the maximum theoretical force. This theoretical force is calculated by considering the ideal contact angle of 0 o as well as the perimeter of the sample/solder/air interface and the volume of sample immersed in the solder. Overall, the faster the wetting curve becomes positive and the greater the magnitude of the maximum force achieved, then the better the solderability of the sample.

SERA
A Sequential Electrochemical Reduction Analysis (SERA) unit (shown in Figure 3) uses a gasket to create a seal on the board pad, PTH or metal coupon to be tested. A pump circulates an electrolytic solution through a tubing system so that it comes into contact with the sample under the sealed gasket. A probe external to the gasket is used to form a complete circuit with the solution under the gasket. Samples must be such that the gasket can seal around the area of interest and have a point exterior to the gasket that is connected electrically to the area under the gasket. Asmall constant current is applied at the test point. The voltage is measured against a reference electrode as a function of either time or charge density. As oxides are reduced from the surface, a voltage response is measured. Oxides, such as SnO and SnO2 are reduced at different voltages (or reduction potentials). The voltage at which an oxide is reduced is directly related to the oxide present on the surface. Knowing the surface area under analysis, current applied, and time to reduce the oxide, an approximate thickness of the oxide can be determined.

SERA is a relatively new technique, and therefore the established standards typically do not include pass/fail criteria. An experienced user can use this technique to evaluate a surface with respect to a fabrication (e.g. flux type, etc.) process.

Applications
As stated above, a wetting balance can be used to quantitatively measure the solderability of component leads and terminations and evaluate according to established pass/fail criteria. One use of this instrument is to study a set of components that are difficult to solder. This test will determine if there is a problem with the components. If not, then the solderability problems are more likely process related. This test is also useful to evaluate the performances of different fluxes, solders, or solder/flux combinations with respect to one another. This would be of interest if one is considering a new product such as a Pb-free solder.

SERA can identify oxide type and thickness. The user can then relate these readings to solderability. This type of analysis is useful if any board related solderability problems are encountered or suspected. Also, the solderability of different surface finishes after environmental conditioning can be evaluated. Wetting balance and SERA are two important instruments used to evaluate solderability of components and boards. Knowledge from these techniques is vital to the electronics manufacturer in order to identify, eliminate, and prevent solderability problems. This will ultimately lead to less rework, better yields, and a better overall quality product. Using either or both of these techniques can aid in process problem solving or incoming inspection and qualification. If you would like more information about these techniques please call the EMPF Helpline at 610-362-1320.