THE INFLUENCE OF TEMPERATURE AND HUMIDITY ON PRINTED WIRING BOARD SURFACE FINISHES - IMMERSION TIN VS ORGANIC AZOLES

Substitution of lead-free solders in electronic assemblies requires ch anges in the conventional Sn:Pb finishes on substrates and component l eads to prevent contamination of the candidate lead-free solder. Optio ns for solderability preservative coatings on the printed wiring board include organic (azole or rosin/resin based) films and tin-based plat ed metallic coatings. This paper compares the solderability performanc e and corrosion protection effectiveness of electroless tin coatings v s organic azole films after exposure to a series of humidity and therm al cycling conditions. The solderability of immersion tin is directly related to the tin oxide growth on the surface and is not affected by the formation of Sn-Cu intermetallic phases as long as the intermetall ic phase is underneath a protective Sn layer. Thin azole films decompo se upon heating in the presence of oxygen and lead to solderability de gradation. Evaluations of lead-free solder pastes for surface mount as sembly applications indicate that immersion tin significantly improves the spreading of Sn:Ag and Sn:Bi alloys as compared to azole surface finishes.