Reflow soldering in a heterogeneous device environment for Aerospace applications

The limited availability of certain military devices, the obsolescence in technology and the long lead times associated with the procurement of electronic devices has construed the electronic package fabricators to deploy the available versions-lead, lead-free and combination of both outright. This has motivated the authors to investigate the suitability of usage of both versions at their end through a large number of experiments and thus identifying the ways to overcome the issues. Different factors affecting soldering have been considered. The study also concentrates on selecting the type of solder paste best suited for reflow soldering of the printed wiring board. Towards this, a set of test vehicles were designed and experiments conducted to assess the solder strength of different class of devices with different pastes. Miniscule chip capacitors of size 0402, 0603 and 0805 were selected as the candidate devices. The printed wiring boards and device metallic terminations were screened as per IPC/EIA J-STD-003 for solderability. Those which have passed the tests were populated with lead and lead-free versions of chip capacitors and reflow soldered with both RoHS and non-RoHS types of pastes. Two Ball Grid Array devices were also populated on the printed wiring boards as bench-in-bench. The populated boards were further screened for solder coverage (IPC-A/610G) using non-invasive X-ray technique. Further assessments were carried out with thermal soak, thermal cycling test and shear test. The conclusions were validated with another set of printed wiring boards fabricated with a mix of devices. It is concluded that a heterogeneous device population meets all the requirements for aerospace application. It is studied that RoHS paste can be used for fabricating boards populated with a mix of devices. The solder paste (96.5 Sn 3Ag 0.5Cu) is a good choice for fabricating boards for the above.