Flip-Chip Assembly

         

The term “flip-chip” refers to an electronic component or semiconductor device that can be mounted directly onto a substrate, board, or carrier in a ‘face-down’ manner. Electrical connection is achieved through conductive bumps built on the surface of the chips, which is why the mounting process is ‘face-down’ in nature. During mounting, the chip is flipped on the substrate, board, or carrier, (hence the name ‘flip-chip’), with the bumps being precisely positioned on their target locations. Because flip chips do not require wirebonds, their size is much smaller than their conventional counterparts.

   

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The flip-chip concept is not new, having been around as early as the 1960’s when IBM used them for their mainframes.  Since then, various companies have developed the flip-chip for use in thousands of different applications, taking advantage of the size and cost benefits offered by this assembly method. Flip chips have likewise eliminated performance problems related to inductance and capacitance associated with bond wires.

           

Fig. 1. Structure of a Flip Chip BGA

      

The flip chip is structurally different from traditional semiconductor packages, and therefore requires an assembly process that also differs from conventional semiconductor assembly. Flip chip assembly consists of three major steps: 1) bumping of the chips; 2) ‘face-down’ attachment of the bumped chips to the substrate or board; and 3) under-filling, which is the process of filling the open spaces between the chip and the substrate or board with a non-conductive but mechanically protective material. Given the many different materials and technologies used in the bumping, attachment, and underfilling steps, the flip chip now comes in a vast array of variants.

 

Flip-chip Bumping

 

Physically, the bump on a flip-chip is exactly just that – a bump formed on a bond pad of the die.  Bumps serve various functions: 1) to provide an electrical connection between the die and the board or substrate; 2) to provide thermal conduction from the chip to the board or substrate, thereby helping dissipate heat from the flip chip; 3) to act as spacer for preventing electrical shorts between the die or chip circuit and the board or substrate circuit; and 4) to provide mechanical support to the flip-chip.

              

There are many known processes for flip-chip bumping. Solder bumping consists of placing underbump metallization (UBM) over the bond pad by sputtering, plating, or a similar means. This process of putting UBM removes the passivating oxide layer on the bond pad and defines the solder-wetted area.  Solder may then be deposited over the UBM by a suitable method, e.g., evaporation, electroplating, screen-printing, needle-depositing, etc.

  

This entire process of solder bumping is done at wafer level. Solder-bumped wafers are sawn into individual flip-chips that get mounted on a board or substrate by subjecting the assembly to a temperature that’s high enough to melt the solder, forming the interconnection.

    

Another type of flip-chip bumping is what’s known as plated bumping. Plated bumping removes the oxide layer on the Al bond pad through wet chemical cleaning processes.  Electroless nickel plating is then employed to cover the Al bond pad with a nickel layer to the desired plating thickness, forming the foundation of the bump.  An immersion gold layer is then added over the nickel bump for protection. 

        

Stud bumping is another flip-chip bumping process. This technique is very similar to gold ball bonding in the sense that it starts by melting the end of the wire to form a free-air ball or sphere, which is then attached to the bond pad.  Unlike wirebonding though, the wire is broken off the ball bond after the latter has been attached to the bond pad. Gold stud-bumped flip chips may be mounted on a board or substrate using conductive adhesives or by thermosonic gold-to-gold interconnection.

   

Adhesive bumping is a flip-chip bumping process that stencils electrically conductive adhesive over an underbump metallization placed over the bond pad.  The stenciled adhesive serves as the bump after it has been cured.  Mounting of adhesive-bumped flip-chips also uses conductive adhesives.

         

  

Fig. 2. Example of a Solder Bump Structure

      

Flip-Chip Underfilling

  

The open spaces between the flip chip surface and the board or substrate is filled with a non-conductive adhesive ‘underfill’ material to protect the bumps and the flip chip surface from moisture, contaminants, and other environmental hazards.  More importantly, this underfill material mechanically locks the flip chip surface to the board or substrate, thereby reducing the differences between the expansion of the flip chip and the substrate.  This prevents the bumps from being damaged by shear stresses caused by differences between the thermal expansions of the chip and the substrate.

    

Flip-chip underfilling is achieved by needle dispensation (Fig. 5) along the edges of the flip-chip. Capillary action then draws the dispensed underfill inwards, until the open spaces are filled.  Thermal curing is then performed to form the permanent bond.

        

Fig. 3. Photos of an underfill dispensing machine (left)

and an underfill dispensing tool (right)

   

Front-End Assembly Links:  Wafer Backgrind Die Preparation Die Attach Wirebonding Die Overcoat

Back-End Assembly Links:  Molding Sealing Marking DTFS Leadfinish   

       

See Also:  BGA CSPIC Manufacturing Assembly Equipment

 

 

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