Solder ball test
When Lotkugeltest ( English balling test or solder ball test , a defined quantity is called) solder paste with a predefined geometric distribution to a thermally highly conductive substrate applied and then remelted. After melting, the merging of the molten solder to form a solder ball and the remaining distribution of solder balls and solder residues on the printing surface are analyzed and evaluated.
Application of the test
This test can be used to check the quality properties of solder pastes. For example, brand-new pastes or stored pastes can be checked for their properties. This test can be performed with lead-free and lead-containing solder paste. With lead-free solder paste, higher temperatures tend to have to be used due to the higher liquidus temperature .
Application of the solder paste
The solder ball test is used to check the solder paste. For this purpose, solder paste is applied to a thin, non-wettable, but very good thermally conductive substrate (e.g. aluminum substrate ). For this purpose, a sample template is used to apply the solder paste. The thickness of the stencil should, if possible, be the same as that of the serial stencils (e.g. 80 µm, 100 µm, 120 µm, 150 µm, 180 µm, ...). Here, the solder paste is applied, for example, with a solder paste template and a squeegee as a circular area with a diameter of 2 to 3 mm with the respective template thickness.
Before the pastes are remelted, the printed test specimens can be stored for a defined period at defined temperatures. This can cause volatile paste components to evaporate. This allows the process to be simulated if there is a long time between the application of the paste and the reflow soldering in practical use and the properties of the solder paste therefore change.
Remelt the solder paste
The substrate is then placed on a soldering tape with liquid solder (solder bath temperature in the range from 250 ° C to 260 ° C) so that the applied solder paste on the substrate is heated by the heat of the liquid solder (the heat passes through the substrate) and melts. The substrate remains over the solder bath for about 3 seconds. Other test criteria are based on a temperature in the range of 215 ° C to 245 ° C with an exposure time of up to 5 seconds. As a variant, this test can be carried out twice, once at the lower temperature and once at the upper temperature.
Evaluation of the test results
After the sample has cooled down, the test is evaluated. The remelted solder and the remaining flux are assessed. The following points can be used as a criterion:
- The complete solder paste melts and contracts to form a single, geometrically neatly shaped solder ball.
- In the area of the application surface of the paste, no other balls form besides the main ball.
- There are no more traces of solder paste residues in the area where the paste is applied.
Variation of the test
As a variation of the test described above, the solder paste can be applied to a readily wettable substrate (for example a surface-clean substrate made of steel , copper or brass ) under the same conditions . After remelting, this test gives a different test result. Here, the solder paste does not contract, but remelted solder forms in the area of the printing surface of the paste. In order for a meaningful result to be achieved, the duration of the heat input must be increased to up to 20 seconds.
literature
- Reinard J. Klein Wassink: Soft soldering in electronics . 2nd Edition. Eugen G. Leuze, Saulgau 1991, ISBN 3-87480-066-0 .
- Wolfgang Scheel (Hrsg.): Assembly technology of electronics . Verlag Technik et al., Berlin et al. 1997, ISBN 3-341-01100-5 .