PCB manufacturer‘s solderability test method
Currently, there are two types of test methods that are basically different among PCB manufacturers:
(1) Unbalanced measurement, which studies the wetting rate problem;
(2) Measurement of the final result of the wetting process, which studies the degree of wetting that is ultimately achieved.
Both types of testing methods originated in the era of wave soldering. For the wave soldering process, the soldering time is often short, and the state of the wetting balance is still far away. Therefore, the first type of test method often measures the wettability closer to the actual situation than the second type of method
PCB manufacturers use analog methods more, such as plug-in prefer wave soldering, and surface-mount components using reflow soldering. Such evaluations are comprehensive, in addition to solderability, evaluate the effects of PCB process and storage on solderability. In the solderability test, the soldered metal material is brought into contact with the molten solder to be wetted by the soldered surface, and this time is called the wetting time (or soldering time). Wetting time is the result of the combination of the thermal requirements of the heated part and its surface wetting.
The test temperature is the temperature before the solder is in contact with the metal being soldered, so it is usually the same temperature as in the solder bath. The actual temperature of the wetted surface is, in almost all cases, lower than the temperature of the solder bath, and the actual temperature of the wetted surface is not constant and there is a significant change.
1.Wetting balance method
Wetting Balance Test is a solderability test method for identification that can quantitatively study the solderability of samples of any shape. Under normal circumstances, the solder terminals or pins of the components are tested, but can also be used for other aspects such as PCB, thick film substrate, solder, etc.
Dip Test, also known as dip soldering, is an inexpensive, fast, and qualitative solderability test method, but the test results are often subjective. Wetting balance methods can give quantitative results, but they are slower and more expensive. The dipping method is very simple, that is, the test sample (PCB or component lead or solder end) is immersed in a molten solder pot, and the degree of wetting – the percentage of the wetted area and the degree of non-wetting are visually observed. It can be used for testing the solderability of solder joints on PCBs and components.
The spreading method is also called the flow parameter method and the area expansion method. The spreading method is used to test both the activity of the liquid flux and the activity of the solder paste flux. Quantitative solder is placed on the copper surface of the controlled oxide thickness. After applying a certain amount of flux, the solder is hot melted. If the oxide layer is not removed, wetting will not occur. The stronger the flux activity, the more oxide removed, the lower the solder stack height.
Before performing the reliability test, the PCB manufacturer needs to aging the test sample because the just-produced element PCB, the solderability of the soldered surface is generally not problematic, but after storage for a while, It will be degraded by oxidation. The test must consider the effect of normal storage conditions on solderability. In the electronics manufacturing industry, components and PCBs often require a certain shelf life of storage, usually 6-12 months. Therefore, It is obviously impractical to wait for this period and then perform measurements, so it is necessary to adopt an accelerated storage aging method.
The common practice of PCB manufacturers is to use 8-24H steam aging for tin alloys, 115°C, 16H dry heat aging for coatings that are mainly caused by diffusion. For coatings with unknown quality degradation mechanism, steam ageing is used.
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