Process development for flux-free soldering with preforms (Part 3)

In the third part of the article series process development for flux-free soldering we deal with temperature measurement, profiling, wetting and soldering tests.

Temperature measurement: measurement dummy and data logger

After defining the holding times and temperatures of the soldering profile, the next step is to ensure that ensure that the product reaches the temperature within a defined amount of time. Therefore, a measurement dummy is needed. The dummy must fulfill the following requirements:

  • Be as close to the real product as possible
  • Temperature measurement must be repeatable
  • Temperature measurement must be reproducible
  • Temperature measurement must be independent from the operator
  • The difference between the dummies must be as little as possible
  • The lifetime of the dummy must be high

The mounting of the thermocouples and a measurement setup that is as realistic as possible are crucial for tests with the dummy. Moreover, the dummy should posses a long lifetime and a high reproducibility. A description of how to perform datalogger measurements can be found here. Lastly, a measurement system analysis (MSA type 2 or 3) must be performed to determine a Gage R&R value.

Soldering profile and fixture development

After developing a measurement dummy, the next step is the development of the soldering profile. By means of the dummy and a data-logger the temperature trend and distribution are recorded. Through iterations, the process is optimized until the desired result is achieved. As the thermal mass of soldering jigs or fixtures affects the soldering process and temperature homogeneity, their performance must be measured as well.

Wetting tests

Once the soldering profile is set, wetting tests are recommended. Preforms without dies or substrates will be placed on the baseplates or interconnect devices to observe the wetting behavior during the soldering process (see animation 1). The aim is always a perfect wetting with sharp edges. No wetting or de-wetting must be avoided. If either one is the result of wetting tests, the cause should be analyzed and eliminated. A common reason for de-wetting is for instance an oxygen content of >5ppm.

A detailed explanation of wetting tests can be found at the Budapest University of Technology and Economics, Department of Electronics Technology.

Soldering tests

After a successful wetting test it is time for the soldering tests. During the soldering test phase, the quality requirements are checked one by one. The void rate and solder splashes are usually the first quality requirement to focus on. Once these requirements are fulfilled, the testing continues with the other requirements. A, B, and C samples are often performed and complemented by reliability.

The following test sequences are commonly used for non-distructive tests:

For destructive tests:

  • Shear tests
  • Cross sectioning
  • Scanning electron microscope SEM
  • Passive temperature cycling
  • Active temperature cycling (Power Cycling)
  • Further reliability tests (e.g. corrosion testing)

Video 1: Influence of vacuum during reflow soldering, Source Youtube

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