Type of Solder for Screen Resistors

Resistors are attached to printed wiring boards in a process similar to silkscreening.

Soldering screening creates a standardized mass-production process for laying solder down on printed wiring boards and circuit card assemblies. Components like resistors are mounted to the solder left by the screen printing before being soldered to the board. In printed wiring board assembly, resistors can receive additional treatments to protect them from environmental degradation.

Types of Solder

Tin-lead solder is commonly used to attach resistors to circuit boards.

Tin-lead solder is widely used in circuit board assemblies and screen printing. "Surface Mount Technology: Principles and Practice" states "60 SN/40 Pb (are) widely used tin-lead solders for surface mounting and general circuit assembly." The ratio of tin and lead alters the melting point of the solder. Lead-indium and tin-lead-indium solders exist. Gold-tin solder is used to bind components to gold surface mounts. These solder materials are used to attach resistors during surface mount technology (SMT) assembly of components to printed wiring boards.

Surface Mount Resistor Networks

Individual resistors can be installed using screen printing. Resistor networks called R-packs are also used in surface mount technology (SMT) circuits. R-packs replace discrete resistors, saving space on the printed wiring board. Resistors for SMT assemblies come in thin film and thick film designs. "Surface Mount Technology: Principles and Practice" states "thin film resistors are made for very high precision circuits that require very close tolerances (<1%)."

Screen Printing

The printed wiring board is etched to expose the copper etching or pads to which electronics are attached. Solder mask may be spread around the printed wiring board connection locations to prevent solder from adhering where it is not wanted. A screen is placed on top of the printed wiring board with holes where solder for electrical connectors is to be placed. Solder is spread across the screen using a squeegee. The squeegee is pulled across the locations where components will be soldered. The screen is removed while the solder remains in place. Electrical components are placed on top of the soldering material. The printed wiring board is then heated to the melting point of the solder, binding the electrical components to the printed wiring board and creating electrical connections between the circuit paths.

Screen Printing Solder for Resistors

The basic process of screen printing is used to attach resistors to printed wiring boards. However, resistors require additional steps when screen printing is used. When the passivation layer on the outside of the resistor is damaged or degrades, the resistive layer is exposed. If it becomes wet or degrades, the resistor's resistive value is affected. "Basic Engineering Circuit Analysis" states that "a second layer of glass is screen printed and fired to seal and protect the resistor." Solder under resistors may need to be trimmed using a laser to ensure equal height. According to "Electronic Systems Maintenance Handbook, Second Edition", "overglaze material has long been used to stabilize thick-film resistors after laser trim." The overglaze protects the resistor from moisture and foreign object debris. Additional layers of silicone resin or epoxy may be applied to the whole circuit board for additional protection.

References

• "Basic Engineering Circuit Analysis"; J. David Irwin, Robert M. Nelms; 2011
• "Electronic Systems Maintenance Handbook, Second Edition"; Jerry C. Whitaker; 2001
• "Electrical Engineer's Reference Book"; M. A. Laughton, D. F. Warne; 2002
• "Surface Mount Technology: Principles and Practice"; Ray P. Prasad; 1997

Resources

• "Lead-Free Solder Process Development"; Greg Henshall, Jasbir Bath, Carol Handwerker; 2011
• "Electronic Packaging and Interconnection Handbook"; Charles A. Harper; 2004
• "Solders and Soldering"; Howard H. Manko; 2001