Solderability and joint design: thermal solderability demands solder not cool below its melting point during joint formation.

Successful wave soldering requires that components to be joined by solder withstand the process without damage. This sounds obvious, but it is not always an easy requirement to satisfy considering that the components must have good wettability (or surface solderability) to form good solder joints, combined with adequate dwell times in the liquid solder to form these good joints.

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The design of the joint must be such that the solder will stay liquid during the soldering process and will not cool below its melting point during joint formation. This is termed thermal solderability. Only when the demands for surface solderability and thermal solderability are met can solder fill the holes in a joint completely.

R.J. Klein Wassink provides several industrial standards that give directives for the measurement of (surface) solderability. (1) In these documents, test conditions such as test time, temperature and flux type are prescribed. Also, test criteria are provided.

In a thermal solderability test, we must deal with both the surface

and thermal aspects, but to achieve a clear understanding, it is useful to separate solderability aspects into these two categories. The thermal aspect in soldering becomes important if the heatsink effect during joint formation becomes so strong that good wetting within the process settings is obstructed by solidifying solder.

Preventing thermal damage. During the soldering process, sufficient heat must be applied to the joint area to create a sound solder joint; conversely, part of that heat will be transported toward the component body. Most components cannot withstand excessively high temperatures; therefore, it is important to know some ways to prevent thermal damage during soldering.

The most effective method for wave or dip soldering, as well as selective soldering, is to use a standoff or spacer between the component body and joint area. This extra lead length acts as a heat resistor and thus inhibits temperature rise in the component body.

Specific soldering distance is the minimum space needed between a component body and the solder joint to reduce the heat sinking effect of the component body and to guarantee good solderability for a specific component. This can be expressed as the minimum distance from the component body where good wetting can be achieved within two seconds of dwell time. The specific soldering distance is measured as termination length minus immersion depth. The test is done on a wetting balance. Before this test, the lead should be dip soldered over its entire length, to ensure that the surface solderability is excellent. (1)

PCB solderability. The measure of the solderability of a circuit board is always a combination of surface and thermal solderability. For the PCB, there are not separate tests for both aspects. The solderability test of a PCB with plated through-holes is actually a destructive test: the PCB cannot be used afterward for mounting components because all the holes are filled with solder after testing. Often, only small coupons of a PCB are tested. In such cases it is important to use those parts for the test that will be most critical from a thermal point of view. If, after the test, even the most critical thermal layout will give completely filled holes, then the solderability fulfills both requirements.

Optimal gap between lead and hole. For good hole filling, the difference between lead and hole diameter should be at least 0.4 mm for component leads up to 0.8 mm diameter. The optimal gap is 0.7 mm. For larger lead diameters, the diameter should be at least 1.5 times the lead diameter. A hole diameter up to twice the lead diameter will normally not create problems for solder quality. In the case of PCBs thicker than 1.6 mm, or multilayer PCBs with more than one innerlayer connected to the barrel, larger holes may assist in better hole filling.

Pb-free? If Pb-free solder is used for soldering, test procedures and requirements should be modified to accommodate this solder. This means that higher test and process temperatures are involved due to the higher melting temperatures and requisite higher soldering process temperatures of Pb-free solders relative to SnPb solder.

It is good practice to have the test temperature at a setting 10[degrees] to 15[degrees]C lower than the solder temperature used in the solder process. This way, a good process window is guaranteed.

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References

(1.) Reinard J. Klein Wassink, Soldering in Electronics, second edition, 1989.

Ursula Marquez is a process and research engineer at Vitronics Soltec's Technology Group (vitronics.com), [e-mail].