durosoneBasic Understanding of Solder Paste - Indium.pptVIP

SMT; Why Solder Paste ? Standard SMT Process Flowchart What constitutes a good soldering? Solder is an alloy used as a medium to bond the components and PCB substrate mechanically, thermally and electrically. A joint reliability is dependent on the remain flux residue, soldering time, thermal mismatch, etc. Paste Description Alloy Type - Depends on operating temperature e.g Sn63 Pb37 Mesh Size - Dependent on the application pitch size e.g -325/+500 for 20 mils QFP Flux - Depends on criteria such as probe testable, e.g NC-SMQ 92J Metal Load - Depends on flux type and application method. Packaging - Jar / Syringes / Cartridges Paste Flux Categories Functions of Flux in Soldering Remove surface oxides on the substrate as well as powder Act as oxidation barrier during reflow Lower the surface tension of the paste to form a more efficient wetting joint Flux Vehicle Chemistry Resin - modified / synthetic rosin - act as oxidation barrier - tacky and viscous Solvent - dissolved chemicals/constituents - evaporate during preheat - control viscosity and rheology Flux Vehicle Chemistry (Cont’d) Activator - cleaning agent during reflow - Dissolves oxides off the metal surfaces promote wetting Additives - regulate viscosity and paste stability Solder Alloys for PCB AssemblyTemperature Classification Common Solder Alloys Metal Load Metal Load is the weight percent of metal powder in paste. Typically 85 - 91% of solder paste is metal. In contrast, the volume percent of metal can be less than 50% of the solder paste due to the low density of flux vehicle. Metal Load Variables Flux Vehicle Alloy and Mesh Size Printing Vs. Dispensing Solder Powder Key Characteristics Alloy purity Oxide level Shape Spherical Low number of satellites Size distribution Low number of oversized particles Low number of undersized (fine) particles Powder Atomization Methods Water, Oil, or Gas Vacuum Rotating electrode Ultrasonic gas Rota