Evaluating Desorption Baking Cycles for Moisture Sensitive Integrated Circuits
Baking moisture sensitive ICs requires matching package thickness and carrier heat ratings to balance desorption speed against intermetallic lead oxidation risks.
Tin Oxide Growth
Oxidation Mechanism
A surface oxidation process on tin-plated metals creates a non-conductive layer that grows thicker over time and impairs the solderability of electronic leads. During storage, tin oxide growth occurs on the surface of components, forming a barrier that prevents solder from wetting the metal below. This reaction accelerates in environments with high humidity and elevated temperatures.
Growth Rate
The progression of the oxide layer depends on the temperature and humidity to which the components are exposed. At room temperature, the layer grows slowly, but exposure to higher temperatures or moisture can cause rapid thickening. This layer can reach thicknesses that prevent standard soldering flux from performing its cleaning action during the reflow process.
Solderability Impact
An excessively thick oxide layer resists the wetting action of molten solder, resulting in weak joints or incomplete electrical connections. Fluxes applied during the soldering process are designed to remove light oxides, but they cannot break down heavy oxide layers. This results in soldering defects such as non-wetting or dewetting, which can lead to intermittent electrical open circuits.
Mitigation Strategy
To minimize this oxidation, components are stored in sealed bags with desiccant packs or in dry cabinets. These storage methods keep the relative humidity low, which significantly slows down the chemical reaction rate.