ADVANCED SIR TESTING FOR TRAPPED SOLDER PASTE FLUX RESIDUE

摘要

One of the biggest challenges to producing electrochemically reliable electronics is the trapping of flux residues under Bottom Termination Components (BTC), such as QFN's, MLF's and some LGA's. BTC use has become extensive in electronic assembly due to low cost and small size with improved thermal and electrical performance, making these components very attractive for many applications. Electrochemical reliability of the BTC assemblies has been addressed in many publications for both cleaned and unclean electronic assemblies in the recent past. Many no-clean solder pastes that are very reliable in standard SMT reflow assembly processes with non-BTC configurations exhibit electrochemical failures under BTC"s, especially with a larger ground pads/heat sink design. Solder paste flux activators and solvents are unable to evaporate during reflow; they remain behind in the flux residue and allow electrochemical failures, such as low Surface Insulation Resistance (SIR) and Electro Chemical Migration (ECM) shorting. A second reliability challenge is the ever decreasing spacing between components, leads and pitch reduction overall. Mobile and communication devices present the worst combination of these reliability challenges because of their high board density. There is a need to develop solder pastes that not only allow printing small stencil apertures but also provide better electrochemical reliability when flux residue is trapped under low-standoff components with closely spaced IO's. To meet this need, an SIR/ECM test methodology has been developed using 200μm spacing 1 mm trace comb patterns covered with thin, light glass slips that float on top of the solder paste during reflow that prevent the volatilization of most of the paste activators and solvents and keep them within the flux residue. These test vehicles are then exposed to 85°C and 85% RH at 10V. Most commercially available solder pastes showed some degree of ECM within hours of exposure. The development of this test method has allowed better differentiation between the reliability of various solder paste and facilitated the development of pastes that are much more reliable reliability under BTC's and take a significantly longer time to fail.