In semiconductor packaging, underfill plays a critical role in reliability. But have you ever wondered how the “dam” structure around it affects the detection of voids using C-SAM (Confocal Scanning Acoustic Microscopy)? Let’s take a closer look.
What Is a “Dam” in the Underfill Process?
In flip-chip packaging, a “dam” is a physical boundary—typically formed from epoxy resin—that surrounds the perimeter of a chip or bump array before underfill is dispensed. This dam controls the flow of underfill and creates a confined region, improving uniformity and reducing overflow.
But more than that—it becomes an essential element when it comes to void detection during C-SAM inspection.
Why Does the Dam Structure Matter for C-SAM?
C-SAM relies on acoustic impedance differences to detect delaminations or voids. When a dam is present:
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Clear Echo Boundaries: The dam structure acts as a reference frame. Any deviation in wave reflection within this confined zone signals a potential void.
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Localized Contrast: The interface between the underfill and the dam highlights areas where voids or delaminations might occur.
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Enhanced Signal Sensitivity: Because the underfill is more uniformly distributed within the dam, the likelihood of trapped air bubbles—common sources of voids—can be better evaluated.
Without a Dam: What’s the Risk?
If there is no dam structure:
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The underfill may spread unevenly.
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Voids can be less distinguishable during C-SAM inspection.
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The reliability of failure analysis drops due to poor echo clarity.
Put simply, the absence of a dam structure can mask critical defects.
Engineering Tip: Dam First, Underfill Second
In high-reliability applications (automotive, aerospace, medical), a “dam-first” approach is often used:
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Dispense the dam using jetting or stencil printing.
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Cure or semi-cure the dam to fix the boundary.
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Fill the confined area with underfill.
This layered approach not only improves flow control but also makes acoustic imaging (C-SAM) significantly more effective.
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