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Thin Film Enhancement for Laminate Substrates Disclosure Number: IPCOM000005047D
Original Publication Date: 2001-Aug-01
Included in the Prior Art Database: 2001-Aug-01
Document File: 2 page(s) / 40K

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Burt Carpenter: AUTHOR [+1]


Thin Film Enhancement for Laminate Substrates

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Thin Film Enhancement for Laminate Substrates

Burt Carpenter, Bill Stone

Flip chip packaging on laminate substrates is currently pitch limited by the thickness and tolerances of the solder mask materials used to protect top metal circuitry and define the metal substrate pads. In the case of interconnects utilizing eutectic bumps joined directly to the metal pads of the substrate this issue is even more pronounced, as compared to high Pb bumps with eutectic solder deposits on the substrate.

The thickness of the solder mask creates an aperture that a eutectic bump must pass through to reach the metal substrate pad (Figure 1). When the solder mask is thick relative to the diameter of the bump, the bump can have difficulty passing through the aperture, which can result in the bump not wetting to the metal pad. If the diameter of the solder mask aperture is increased the pitch between metal pads is limited due to the possibility of exposing adjacent circuitry in the aperture.

If the diameter of the bump is reduced to accommodate clearance of the solder mask aperture, the stand-off between the die and substrate in reduced (Figure 2). The thick solder mask itself also minimized the stand-off between the substrate and the die, even without reducing the bump diameter. Reduction of the stand-off between the die and substrate is undesirable because it minimizes the effectiveness of the underfill material and increase the stress in the interconnect.

Minimizing the thickness of conventional solder mask materials is difficult, due to the need to assure coverage of the entire substrate surface. By using a non-conventional material such as a thin film polyimide or BCB to replace the solder mask on the laminate substrate, finer pitches can be accommodated. A thin film such as polyimide or BCB is also a good replacement for conventional solder mask materials because it has relatively good adhesion, moisture, and resolution characteristics.