Forcing the Issue of Solderability

By Graham Naisbitt, Gen3 Systems Limited

With the imminent arrival of brand new IPC standards
(J-STD-002C, J-STD-003B) documenting how to quantitatively assess solderability via force measurements, the wholesale shift to lead free is an ideal opportunity for both assemblers and suppliers to upgrade their solderability testing and move away from less scientific manual dip-and-look procedures.

Like it or not, ready or not, a new European Union (EU) law effectively came into force on Saturday, 1 July 2006, named RoHS — Restrictions on Hazardous Substances. This includes a stipulation that the lead content of all homogeneous electrical and electronic equipment has to be limited to less than 0.1%.

This will have a profound effect upon the electronics industry around the world — as it raises a number of issues that have previously lain dormant. One of these issues relates to solderability: Namely, how to make accurate and repeatable solderability assessments of components and circuit boards.

Central to the issue is the industry’s reliance on the decades-old “dip-and-look” test method for assessing solderability, formalized in standards such as J-STD-002 and 003.

In this test (which is at least satisfactorily defined), technicians dip representative component samples into a molten solder bath and observe how far the meniscus climbs. With experience, this provides a qualitative measure of solder wetting and hence solderability. As such, dip-and-look is quick, easy and cheap to implement.

The major problem with dip-and-look, however, is that, as a test method, it lacks “Gauge Repeatability & Reproducibility” (Gauge R&R). In other words, two people conducting the test at different times are likely to interpret the results differently. Yet the whole point of solderability testing is to gauge the likelihood that the surface finish on a bare board or component is satisfactory to encourage wetting and the formation of a robust solder fillet. What kind of basis is there for quality control if one person says the component is fine but another says it’s not, depending on the time and place of the test?

This is particularly true for assessments that take place between dispatch at board and component suppliers and goods-in at assemblers; or after pro-longed periods of on-site storage at either — a key issue affecting solderability as numerous studies by the UK’s National Physical Laboratory have revealed (fig. 1). [please contact author for full details].

Figure 1 (Click to enlarge)

As a result of such issues, the two IPC task groups on solderability testing (5-23a and 5-23b) agreed to undertake a “round-robin” test program that would not only study the characteristics of lead free alloys and fluxes but also finally include solderability force measurement methods. The standards, which are due to be published imminently, also aim to address a perceived confusion within the industry about the difference between solderability and “soldering ability.”

To explain this, I will first quote from the IPC J-STD-002C and -003B documents: “Solderability evaluations are made to verify that the solderability of component leads and terminations meets the established requirements and to determine that storage has had no adverse effect on the ability to solder the component to the board.”

Good measurement practice minimizes the number of variables involved in the exercise and solderability is no exception. However, many assume that the test should be performed with their own process materials, introducing a huge number of variables (many continuously fluctuating).

Figure 2a (Click to enlarge)

Although this kind of testing has its place for qualitatively checking if your process flux and solder alloy are working well, it’s not scientifically controllable enough to form the basis of a benchmark standard for solderability testing.

For a precision solderability test, the standards recommend that the test flux, for instance, is carefully maintained and kept free of contamination during force measurement.

This includes either covering the flux when not in use and discarding it after eight hours or maintaining it to a specific gravity of 0.843 ± 0.005 at 25 ± 2 C (77 ± 3.6 F) and discarding it after one week of use.

Figure 2b (Click to enlarge)

The solder in the solder bath used for solderability testing should also be chemically or spectrographically analyzed or replaced each 30 operating days according to strict standard-defined contamination limits. This includes the composition of the lead free solder (including maximum contamination levels) being maintained during testing with the silver and copper element levels adjusted for alloy requirements.

All this should help the industry finally move away from the very common but poor discipline of tweaking a soldering processes to “force” component pads or the surface finish on boards to solder (e.g. by adding more flux to compensate for poor wetting) and hence to enhance soldering ability.

This practice also does nothing to aid long-term field reliability as the graphs in figures 2(a) and 2(b) illustrate (courtesy: NPL). These show the drop in SIR resistance as the flux volume is increased on a standard test coupon. The drop in the curves correspond to electrically conductive areas of the bare board formed by the flux residues after evaporation of volatile components.

How force measurements work

Solderability force measurements use what’s known as a wetting balance that measures force to an extremely high level of accuracy (milli-Newtons). Some instruments even measure to levels of better than 0.1μN/BIT. Although the type of wetting balance used for plated-through hole (PTH) and surface mount (SM) components differs, both are based on the same physical principles.

Namely: if a metallic body is dipped into molten solder, the weight and speed with which the solder meniscus climbs upwards on the body’s immersed surface indicates how well the solder wets it and thus its solderability. The greater the solderability, the higher the meniscus will climb, which can be measured as a change in the vertical force action on the suspended specimen.

For certain TH components and circuit board coupons, the specimen device is immersed in a bath of molten solder and the forces of buoyancy and surface tension action upon it are measured. For smaller SMDs, a higher resolution method is required: the Microwetting Balance procedure (fig. 3) that employs a solder globule. Here the solder bath is replaced by a globule block of 4, 3.2, 2 or 1-mm size employing 200, 100, 25 or 5-mg pellets of solder alloy (depending on specimen size) allowing individual leads to be performed on a multi-leaded component.

Figure 4

Assemblers need to know for sure that they are using boards and components of known good solderability. Force-based solderability testing is intended to quantitatively measure the robustness of a given surface finish under test and remove any scope for opinion or “manual” judgment calls that typically occur with dip-and-look.

With the market now awash with brand new lead free solder alloys, pastes, pad geometries and surface finishes, now is the time to make the switch to 21st century solderability testing.

Graham Naisbitt is managing director of Gen3 Systems Limited (founded on Concoat Systems, see www.gen3systems.com) and is a member of the IEC’s TC91 WG3, the working group that formulates test standards for the assembly industry. Naisbitt is also leader of Solderability Testing Standard IEC 60068-2-69, co-leader of Solderability Testing Standard IEC 60068-2-54, and a member of IPC’s 5-23 Solderability Committee.