What are the methods for testing the solderability of SMD SMT PCB components?

The solderability inspection of SMT components is mainly aimed at the solderability of solder terminals or pins.
The primary cause of solderability problems
The reason is that the surface of the component soldering end or pin is oxidized or polluted,
It is also the primary factor affecting the reliability of SMA soldering.

Oxidation or contamination on the surface of solder ends or pins of components is more likely to occur,
To ensure soldering reliability,
It is necessary to pay attention to the packaging conditions and environmental conditions of components during transportation and storage.
It is also necessary to take measures to prevent components from being exposed to the air for a long time before soldering,
and prevent its long-term storage together,
Pay attention to the solderability test before welding, so as to find problems in time and deal with them.
There are many ways to detect the solderability of components,
Some of the more commonly used test methods are introduced below.

1. Solder bath moistening method

Solder bath moistening method is one of the most original component solderability testing methods.
It is a test method that is assessed visually (or through a magnifying glass),
The basic test procedure is: take out the sample after dipping it in flux,
After removing the remaining flux, immerse in the molten solder tank and take it out after about twice the actual production soldering time.
Then perform a visual assessment.

This test method is usually carried out with a dip tester.
It can control the sample immersion depth, speed and residence time according to the regular parameters,
It is more convenient and quick to get test results.
This test method can only get a qualitative conclusion of visual estimation,
Not suitable for test occasions with quantitative precision requirements,
However, it is more suitable for the rapid and direct inspection requirements of the SMT assembly production site.
Still relatively common.
The qualification criteria for the solderability test using the solder bath wetting method are:
All test samples to be tested shall exhibit a continuous solder coverage,
Or at least 95% of the solder coverage area of ​​each sample is qualified.

2. Solder ball method

The solderability test of the solder ball method is also a relatively simple qualitative test method.
Its fundamental principle is:
Select solder balls of appropriate specifications according to relevant standards and place them on the heating head to heat to the specified temperature;
Place the sample to be tested (leads or pins) coated with flux horizontally,
and dipped straight into the solder ball at a regular speed,
Record the time until the lead wire is completely wetted by the solder ball,
The solderability is measured by the length of this moment.
The qualification criteria for solderability testing using the solder ball method are:
The time when the lead wire is completely wetted by the solder ball is about 1S, and it is unqualified if it exceeds 2S.

3. Wet method

Apparatus and test principle for solderability test by wet weighing method
Its fundamental principle is:
Suspend the sample of the component to be tested on the beam of the active scale;
Immerse the part of the sample to be tested into the molten solder at a constant temperature to a regular depth;
together with this,
The resultant force of buoyancy and surface tension acting on the submerged sample in the vertical direction is measured by the sensor and converted into a signal,
And recorded by the high-speed characteristic curve recorder into a force-moment function curve;
Compare this function curve with an ideal moisture weighing curve obtained from an experimental sample of the same nature and size and capable of being thoroughly wetted,
to obtain test results.

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The moisture balance tester is an instrument for solderability testing using the moisture weighing method.
When the test sample is immersed in the molten solder to a predetermined depth,
At first the solder liquid level is pressed down into a meniscus shape,
This is because the cohesive force of the solder is greater than the adhesive force between the molten solder and the test sample (pin),

Make wet angle 8 greater than 90 degrees
The straight weight Fr of the surface tension Fr of the meniscus under the solder is in the same direction as the buoyancy Fb,
Both estimate the test sample upwards.
When the sample reaches the soldering temperature,
The adhesion of the sample to the molten solder is greater than the cohesive force of the solder,
Molten solder starts to wet the sample,
Make the meniscus gradually bend up until 6 is equal to 90°,
The test sample is only affected by buoyancy.
Then, the molten solder continues to wet the sample to present an upper meniscus, 9 less than 90°,
A downward pulling effect occurs until the vertical weight Fr of the surface tension Fr is equal to the buoyancy Fb,
A moisture balance is reached.

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