Book Excerpt: The Printed Circuit Assembler’s Guide to... Low-Temperature Soldering, Vol. 2, Chapter 4

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Chapter 4: Challenges in Maintaining the High Electrical Reliability of Low-temperature Solder Pastes

With the rapid advancement of technology and changes in end-user expectations, portable electronics such as mobile and computing devices are becoming increasingly thinner while requiring enhanced processing power. To meet these requirements, a new generation of electronic assembly materials is needed. Low-temperature solders with a melting temperature of 138°C are widely preferred by many semiconductor package manufacturers for assembling packages onto printed circuit boards. Most of these packages are very thin and require high performance.

As mentioned in previous chapters, lower peak reflow temperatures are being increasingly adopted in surface mount applications as package densities continue to increase in mobile and wearable devices. Lower peak reflow temperature helps to prevent warpage-induced defects in large and thin components. However, reducing the peak reflow temperature poses a potential issue to electrical reliability as lower peak reflow temperatures may promote electrical failure. Performance matrix: An SIR value greater than 1E+08 Ohm throughout the test duration (168 hours) is considered as pass.

The residue remaining after SMT reflow is almost always responsible for compromising the electrical reliability properties, such as surface insulation resistance and electrochemical migration under a given temperature with humidity. There is a need for a flux that passes all the given electrical reliability properties as specified by industry standards such as IPC and JIS.

In general, the design of a solder flux consists of many organic chemicals such as rosins, solvents, weak organic acids, amines, rheology modifiers, and other additives. These chemicals combine to form a homogeneous mixture called flux. The main function of the flux, when mixed with metallic powder, is to remove the oxide layer on the surface of each powder particle and prepare the adhering surface for soldering. The final critical properties of the solder paste, such as wetting, non-wet opening, and head-in-pillow, are extremely dependent on the type of flux and its constituents. Low-temperature reflow solder alloys create unique challenges for chemistry design. By considering the requirement and the critical factors involved in achieving higher electrical properties of low-temperature solders, much time and resources have been spent solving the challenges of engineering a new flux chemistry platform with advanced chemical molecules.

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