For companies specifying
GEIA-STD-0006 for retinning of component leads using solder dip (also known as
tin whisker mitigation), confusion often exists regarding the scanning acoustic microscopy (SAM) requirements within this standard. Specifically, users are often unsure of how to deal with deficiencies noted during the baseline SAM inspection. This uncertainty arises from the assumption that new components will be defect free. When delamination of the mold compound from the internal features of a component is discovered prior to the solder dip process, frustrating delays often ensue while decision makers attempt to sort out the ramifications of SAM rejects in as-purchased parts. When attempting to determine the significance of a baseline SAM reject prior to solder dipping, consider the following.
First, remember that the purpose of the SAM inspection in
GEIA-STD-0006 is to verify the integrity of the solder dip process. By generating “before” and “after” SAM images, any internal changes resulting from the solder dip process can be observed. A dipping process that is not properly controlled may result in excessive heat within the package body, leading to internal delamination or package cracking that is readily visible in the
SAM image. Delamination or cracking can also occur from a properly controlled process if inherent weakness is present within the component. Regardless of whether or not any pre-process delamination is present, the SAM inspection functions as a process monitor by providing evidence of any internal changes that occur during solder dipping.
Second, understand that a SAM rejection during the baseline inspection is not a statement that the parts will likely fail in the user’s application.
GEIA-0006 Method 300 requires acceptance to the
J-STD-020 acoustic microscopy criteria and prohibits any delamination increase of more than 10%. Obviously, when inspecting components prior to solder dipping, the requirement prohibiting a delamination increase is not applicable. That leaves the
J-STD-020 criteria by which to conduct a baseline evaluation. Not all defects rejected by
J-STD-020 are equal in their potential reliability impact. It is important to consider the nature of the specific defects identified by the SAM inspection and to understand the potential failure mechanisms associated with each. In fact,
J-STD-020 allows for acceptance of parts with delamination if the reliability impact can be shown to be acceptable.
Third, realize that it is not uncommon for plastic-encapsulated components to have some degree of internal delamination. These are primarily commercial components being subjected to
GEIA-0006 solder dip in order to upgrade them for a high reliability application. Creating a manufacturing timeline that assumes all components to be solder dipped will be defect free in the baseline condition inevitably leads to issues that are difficult to resolve without incurring delays.
Scanning acoustic microscopy offers a double benefit when performed as part of a
GEIA-0006 retinning process. Not only does it provide validation of the solder dip process (and expose problems when they occur), SAM also
identifies anomalies in the as-manufactured condition that may have a reliability impact in the end product. Over time, as the user gains familiarity with the SAM data being provided, the review process can be shortened as the impact of particular SAM defects is understood and categorized.