Solder Joint Failures

Solder joints refer to the solder connections between a semiconductor package and the application board on which it is mounted. In unmounted devices, it may also refer to the package's solder connection features themselves, e.g., solder balls, solder bumps, solder studs, etc, in the context of their attachment to the package body. Since solder joints are one of the most fragile elements of a package (mainly due to the fact that they are small in size and used at high temperatures relative to their melting points), their reliability is of utmost concern to assembly engineers. A good understanding of solder joint failures is essential in the improvement of any package's solder joint reliability (SJR).

Solder joint failures occur for various reasons: 1) poor solder joint design; 2) poor solder joint processing; 3) solder material issues; 4) excessive stresses applied to the solder joints, etc.. In general, however, solder joint failures are simply classified in terms of the nature of the stresses that caused them, as well as the manner in which the solder joints fail.

Most solder joint failures fall under three major categories: 1) tensile fracture due to stress overloading, which is short-term; 2) creep failure due to the application of a long-term, permanent load; and 3) fatigue failure due to the application of cyclical stresses. Of course, more than one of these stresses can act on a solder joint in a given situation, so solder joint failure analysis can be challenging at times. Add to this the fact that solder joint degradation due to other factors such as corrosion can occur.

Solder Joint Fracture Due to Stress Overloading

Solder joint fractures attributed to short-term stress overloading are mainly those experienced by units subjected to gross mishandling or misprocessing, especially after these units have been mounted on the application board. The occurrence of mishandling or misprocessing subjects the parts to thermo-mechanical stress levels that exceed the fracture strength of the solder joints, resulting in solder joint failures.

Real-world examples of events that lead to solder joint fracture due to mechanical overloading include dropping of the application board (or final product) to the floor, 'force-fitting' of an improperly loaded application board into its module or enclosure, high-impact collisions involving the module containing the application board, and the like. These incidents subject the solder joints of the device to very high shear stresses that tend to rip them away from their board.

Solder Joint Failure Due to Creep

Solder joints that are subjected to permanent mechanical loading degrade over time and eventually fail. This failure phenomenon is known as creep, and is more pronounced at higher temperatures, although solder joint failures due to creep at room temperature can also occur.

A warped application board may not exert any significant stress on the devices mounted on it while it is in its uninstalled state, especially if the devices were soldered on the board that way. Once the board is screwed into position in its final assembly, however, it will be forced to 'straighten out', causing severe stresses to the solder joints of the mounted devices. Leaving the board in such a 'stressed' position will put the mounted devices under constant mechanical loading, which can eventually lead to creep failure.

Solder Joint Failure Due to Fatigue

Fatigue, or failure resulting from the application of cyclical stresses, is the third category of solder joint failures. It is often considered to be the largest and most critical failure category, since it is encountered in many different situations that are difficult to control. Solder joint fatigue failure is attributed primarily to stresses brought about by temperature swings and mismatches between the coefficients of thermal expansion (CTE's) of the mounted devices' solder joints and the application board.

Prior to the actual fatigue fracture, solder joints first undergo cyclic deformation from the cyclic stresses as the temperature alternates between its high and low values. Improper design of the solder joint aggravates the effects of this cyclic deformation, which can occur in large steps (in the order of 1%) especially in cases of low cycle fatigue. Low cycle fatigue, or failure from stress cycles that involve long periods wherein the cycle time is several hours, is a prevalent cause of solder joint failures in the field.

Examples of real-world events that can lead to fatigue failures include: 1) powering up of an equipment in the day and turning it off at night; 2) the frequently repeated cycle of driving a car and parking it, with the application board under the hood; and 3) the orbiting of a satellite that exposes it to the alternating direct heat heat of the sun and cold vacuum of space.

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