Technical Articles

And why waiting until the last process step is a bad idea.

BTCs, or bottom termination components, are a class of package referred to by a variety of acronyms and abbreviations. Different component and packaging companies may use different nomenclature (FIGURE 1), but almost all these components share one common, ugly characteristic: large pads that are prone to solder voiding. By design, these large thermal or ground pads require a defined percentage of contact with the solder and PCB to properly conduct heat and/or electricity. Excess solder voids can impact performance and reliability of the package.

The voiding problem has given both designers and manufacturers heartburn for the better part of two decades. Some of the drivers for this frustration are that voiding rates are highly variable, and numerous factors can impact their formation. While there is still...

Phase 2 of our study involved wetting balance, spread, DSC and mechanical tests.

We’re continuing our summary of a year-long experiment aimed at identifying lead-free solder alloys that mitigate tin whisker growth. This study cut right to the chase, beginning with an elimination round. The first phase knocked out two-thirds of the nine original candidate alloys based on their whisker production.

The results of the tests were both intriguing and enlightening, and forced us to rethink what we thought we knew about tin whisker propagation. The investigation focused on the conventional wisdom that tin whiskers are the result of compressive stresses caused by electroplating, bending, intermetallic growth, and thermal expansion mismatches.

It assessed the influence of these most commonly cited potential causes by creating...

A multi-part study found plenty of whiskers, but not where they were expected.

THE ELUSIVE TIN WHISKER is only 1/100th the width of a human hair, but this tiny, single filament protrusion can wreak havoc with all sorts of electronics, and has even been cited as the cause of some sudden acceleration failures in cars. Tin whiskering is not a new phenomenon, however. It has been documented as far back as the 1940s, so why the heightened concern now? Lead was a great mitigator of whiskers and its removal from electronic solder opened the door for increased whisker proliferation, perhaps with sometimes deadly consequences.

Despite years of study, a single root-cause mechanism on whisker formation has yet to be established. Common wisdom suggests that they are a form of stress-relief, compensating for the compressive stresses induced by...

Sn/Cu/Ni soldering performance at low temperatures.

Nickel-modified tin-copper solder, known commercially as SN100C®, is a leading lead-free alloy for PTH soldering, rework and hot air leveled PCB final finishes. Because it contains no silver, it is much more economical than SAC alloys containing 1, 3 or even 4 percent of the precious metal, and it produces smooth, shiny, easy-to-inspect solder joints. Why has it not gained major acceptance as an SMT alloy? In large part, fear. Fear of full compatibility with SAC reflow processes.

SnCuNi melts at 227°C. SAC305 begins to melt at 217°C, reaching its fully liquid state at approximately 220.6°C. Recommended reflow temperatures are typically at least 13°C higher than melting temperatures; hence the SAC305 peak temperature window of 233 – 255. Applying the 13-degree guideline to the...

Every seasoned SMT process engineer has at least one nightmare story about bad rework chemistry. Whether it’s the wrong flux sneaking into the operation, underprocessed flux remaining on the board, or improper residue removal, they all risk the same inglorious fate: field failure. While rework operators and process engineers rarely get a glimpse of the long-term damage of inappropriate materials or processes, the Failure Analysis engineers see it all. They report that the vast majority of solder-related issues are on reworked solder joints, and the lion’s share of those is due to the rework chemistry.

How does the wrong flux work its way into rework? Simple human error. In some cases, operators who experience difficulty using approved rework fluxes on parts with poor solderability bring in more active materials from their personal collections. In auditing rework stations, we’ve found all sorts of unapproved soldering fluxes...

Abstract.  In an effort to reduce volatile organic compound (VOC) emissions within our environment, policymakers have encouraged and/or mandated that electronics manufacturers change from alcohol-based VOC-containing fluxes to water-based VOC-free flux alternatives. As a result, the use of VOC-free fluxes is growing throughout North America, Asia and Europe.

The purpose of this study is to explain several factors relating to the use of a VOC-free flux in the soldering process and their impact on testing and product reliability. These factors include; the effect of varying types of acids used in flux formulations and their impact on Ion Chromatography (IC) and Surface Insulation Resistivity (SIR) test results and weak organic acids (WOA) solubility and their influence on the electrical integrity of assemblies. This paper shall provide valuable insight into the outcome of acid-solvent interactions.