by Jon Roth (AcousTech, Inc. President)
When a surface-breaking feature in a plastic encapsulated microcircuit (PEM)
, such as a lead finger or tie bar, is delaminated, water can enter along the delaminated interface during the course of the scanning acoustic microscopy
(SAM) inspection. When this occurs, the delamination often becomes obscured in the SAM image. The reason is that as water fills the delaminated interface between the mold compound and the feature of interest, the water provides acoustic coupling across the gap. Instead of reflecting the entire acoustic signal as an air filled delamination would, the water permits transmission of a portion of the acoustic signal. In many cases, this can completely obscure the delamination in the SAM image. This phenomenon is categorized by JEDEC J-STD-035
, Acoustic Microscopy for Nonhermetic Encapsulated Electronic Components, as a Potential Image Pitfall in Annex B.
The images show the effect of water ingress during scanning of a group of 8-pin SOIC devices. The image on the left shows the original scan. Areas of lead finger delamination are identified by the arrows. The image on the right shows a second scan obtained less than two minutes after the first. Note that the delamination is no longer visible at three of the lead fingers. This effect can be seen in real-time on a different group of parts in the video below.
Also, be sure to check back for PART II of this blog on water ingress to learn what steps AcousTech suggests to help deal with this issue.