Solder paste shelf life extending approach for prototyping and small series activities

In case of prototyping and small series it can be identified a common practical issue for EMS (Electronic Manufacturing Services) companies, especially in case of prototyping and small series activities, regarding solder paste loss percent, production costs and solder joints reliability consequences. Although the solder pastes producers assure minimum 8 hours stencil life-STL (the warranted solder pastes in printing period usage) for printing or dispensing in the recommended ambient conditions: temperature 22deg to 28degC and 30%-70% relative humidity, and over six months shelf life-SHL (the shelf warranted storage period in the determined ambient conditions), the solder pastes loss percent goes over 20% in these cases. Process engineers allow to use solder pastes after SHL to and over STL limits, in order to reduce loses and implicit costs, taking (or not) into consideration possible solder joints reliability implications. Taking into consideration the solder pastes, as part of 4P Soldering Model for solder joints reliability analyze, in the paper will be presented the study done to find solution for prolonging SHL over producer data, maintaining their properties. The work will be used to develop acceptance methods for solder paste usage after SHL.