|
|
Cleaning under Bottom Terminated Components - The Importance of Good Rinsing
Production Floor |
|
Authored By:Vladimír Sítko, Ing. PBT Works s.r.o. Czech Republic Mike Bixenman, DBA Magnalytix LLC TN, USA SummaryThe demand for faster and more compact electronic assemblies requires that the interconnections are as short as possible. Bottom-terminated components offer that option. However, using such components can be challenging in achieving the perfect signal quality and robustness against a harsh environment. Small distances between poles and large areas under packages cause incomplete solder flux degassing and higher ion volatility in such an environment under components. With small distances between poles, the risk of electrochemical migration (ECM) is much higher than at older types of components. Experience shows that cleaning is the best option in such a situation. Also, it is often proven that cleaning must be thorough, including the most hidden places under components and any spaces between poles of the assembly. Even the small amounts of visible residues between poles can cause electrochemical migration. Also, the invisible ionic active compounds on the surface can damage the result. Cleaning under the bottom-terminated components is challenging due to the large skinny gap under the package. Both the first phase – washing (dissolving of flux residues) and the final phase – rinsing must be perfectly controlled to achieve acceptable results. Our work targets the one deciding parameter of successful rinse – the cleanliness of rinse water. We want to show that not only the ionic active pollution but also the non-ionic contamination can be crucial for a perfect result in the cleaning process. We compare the values of the test boards' surface insulation and surface energy in terms of the different statuses of a cleaning machine contamination during the long-lasting cleaning process. We demonstrate the possibility of continuously detecting the proper quality of rinsing water and the status of chemical filters, which provide the required parameters. Such a study should be helpful for these users of the cleaning process, which recirculate fully or partially the deionized water used for rinsing. ConclusionsThe gradual saturation test of the cleaner yielded some surprising information. We expected some decrease in the washing speed due to the dropping of the concentration gradient between flux residues and cleaner emulsion. However, we were surprised by the massive effect of saturation increase. According to the washing results, we should increase the time for washing to 250% of the original optimized value ( for a fresh cleaner) at the saturation of 3,2%. We know that the saturation effect may differ for different cleaner-to-flux residue combinations. In any case, by such high influence, we think it is essential to conduct further investigations of such an effect and generate a test method that could give exact numbers for actual combinations of flux residues and cleaners. We have already taken fundamental steps in such an investigation. We have developed a method for fast – online measuring of saturation, and we are just finalizing the version of the measuring system, fully integrated into the cleaning machine. We have (for a longer time already) the Chip Glass Test boards and automatic optical tester of washing results under components. Data from this test can be directly used to calculate necessary washing time corrections. Initially Published in the SMTA Proceedings |
|
Comments
|
|
|
|
|