8 XUE Young --- WBC General Introduction and Capability Set-up in China
Stencil
For thicker coating (>1mil), it is typically recommended to use open cut stencils. Open cut is big opening in the metal stencil.
Stencil Printing Screen Printing Spin Coating
Ablestik has three methods to achieve wafer backside coating process.
7 XUE Young --- WBC General Introduction and Capability Set-WBC General Introduction and Capability Set-up in China
WBC Process Introduction
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XUE Young --- WBC General Introduction and Capability Set-up in China
WBC Merits -2
Discrete/Small SO Applications
----Eutectic and soft solder is high stress. WBC can improve reliability by reducing cracks ----Eliminate N2 or H2 forming gasses due to lower temperature cure
July 15, 2008
Background
Ablestik introduces a new line of specially designed adhesives that can be applied to the back of a wafer and dried. Conductive and nonconductive wafer backside coating materials have been developed for die sizes 3mm or less and enable such package types as COL, SO, TSOP and QFN. High thermal materials have been developed that are suitable for use in discrete and small power IC devices. The advantages of the wafer backside coating process are: * Bond line thickness and fillet formation can be controlled * Resin bleed is eliminated * Moisture resistance and adhesive strength are improved * WBC materials can achieve higher reliability levels
WBC Training Notes Wafer Backside Coating (WBC)
Project: TWIR-7FPB92 --- Introduction and Capability Set-up in China
Tony Winster , Ablestik U.K. XUE Young 21 June 2008 Service Engineer Sr. Technical
4 XUE Young --- WBC General Introduction and Capability Set-up in China
WBC Merits -1
IC Application
Lead frame standardization
WBC Process
No need for dispensing ; glue is already on 100% of the chip at the desired thickness
Improved reliability regardless of LDF type
L1,260c possible on Ag, Cu, PPF and Aolly42
Improved yields
Reduced die tilt; No bleeding
Improved design flexibility
Tighter die bond to pad ratios; Ability to choose thickness
WBC can offer significant process benefits at least 20-30% cheaper than film.
Paste Process
5 XUE Young --- WBC General Introduction and Capability Set-up in China
Eutectic
----Reduce and standardize backside coating layer
Soft Solder
----Increase line speeds by 2-3 times ----Pb-free die attach solution (Typically soft solder is 95% Pb)
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XUE Young --- WBC General Introduction and Capability Set-up in China
Squeegee
Squeegee Stencil
Wafer back side material wafer
Metal Shim
Roll bar squeegee (or metal squeegee with reinforced rod at the edge) is recommend to reduce the scooping during printing with big opening stencils. Roll bar squeegee is not recommended for screen printing. Rubber squeegees or metal blade are recommended for screen printing.
Stencil printing
Squeegee
Palette
Vacuum tube
Palette
Stencil printing is mainly wafer backside coating method recommended to customer. Stencil printing is able to provide smooth surface (good performance in roughness) and has good thickness control. Moreover, its process is stable and easy to control. Stencil printing has a strict request to shim and stencil thickness uniformity. In addition, the flatness of palette and squeegee are key factors for printing performance.
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XUE Young --- WBC General Introduction and Capability Set-up in China
Palette
Squeegee
Palette
Vacuum tube
Palette
Printing back side coating on wafers requires vacuum chuck to make the wafer stable. Palette needs to be very smooth and flat so as not to damage the front face of the wafer. Besides, the flatness of palette will cause print thickness variation. Granite palette usually shows better performance than metal palette. But its weight is really high and it may cause negative effect on printer.
WBC can be dropped into existing soft solder and eutectic lines.
6 XUE Young --- WBC General Introduction and Capability Set-up in China
WBC Wafer Back Side Coating
Metal Opening
Frame
Stencil opening can selected based on the print area requirements. Typically opening will be 2 mm less than the wafer diameter. (Example: 198 mm opening for 200 mm wafer). Stencil thickness depends on the coating thickness requirement. For thinner coating (≤1 mil), screens are recommended.