Nordson ASYMTEK's products enable precision dispensing processes used in semiconductor packaging such as flip-chip assembly and lid attachment for heat dissipation.
Flip chip is the most popular packaging technology for advanced semiconductors such as MPU, FPGA, application processors, higher speed memory, and wireless devices. Flip chip has been produced in full swing since early 2000 and more devices will adopt it in the future because of its superior performance, small form factor, and reasonable costs. Several fluids are required for its raw materials and for its processing during the flip chip assembly process.
A flip chip is attached to an organic substrate with bumps for interconnections. The gap between chip and substrate must be filled by underfill, an epoxy with fillers. The underfill is critical to make the flip chip devices reliable by preventing flip chip devices from failure because of the coefficient of thermal expansion (CTE) mismatch between silicon chip and organic substrate. Filling the gap with underfill material has several options: capillary underfill (CUF), non-conductive paste (NCP), non-conductive film (NCF) and molded underfill (MUF). CUF is the most popular one since flip chip technology emerged, while there are different benefits and disadvantages among the techniques.
CUF works very simply: when underfill fluid is dispensed next to the flip chip, it spreads under the die with capillary force, which is caused by the narrow gap between the die, bumps, and substrate. Underfill material should be dispensed close to the die edge because the underfill fillet should be as small as possible. The keep-out-zone (KOZ) is quite critical because of denser component layouts; therefore, jet dispensing is the standard technology for this application. Jetting is non-contact dispensing, shooting small fluid dots from the dispenser tip, which does not touch the device or substrate. Jet dispensing thus contributes to smaller KOZ.
Another important process for flip-chip assembly is flux. It removes the oxidized layer from the circuit pads and bump surfaces for better solder wetting and reliable solder bonding. Before flip-chip bonding, flux must be applied on the bumps of the flip chip or the substrate surface. Many die bonders use flux dipping in which the flip-chip bumps are dipped into a thin flux coating just before attachment to the substrate. This method has two challenges: 1) Too much flux tends to be applied and 2) Bump height variation causes the flux to be applied non-uniformly. The viscosity of the flux used for dipping is higher than for spraying, so the flux sometimes climbs up the die edge to the backside of the die, resulting in too much flux and contamination to the top of the die. The extra flux residue can cause device failure. In addition, because the flux isn't applied uniformly, some bumps do not get fluxed.
A better method of applying flux is to use a dedicated system designed to apply the flux with a spray in a thin and uniform coat onto the substrate surface. The thinner the flux the better because only a small amount of flux is needed to remove the oxidized layer, and also because too much flux leaves a residue that can prevent underfill from filling the gap between the die and substrate, causing the device to fail. Spraying the flux over the substrate ensures a more uniform application than applying it to the bump side of the die.
Lids are attached to the top of die used in logic devices to dissipate the heat that the die generate during their use. Thermal interface material (TIM) is dispensed on the top of the die, filling the gap between the top of the die and the lid. The cut edge of the lid is attached to the substrate surface with a solder paste sealing line that forms an adhesion between the lid and the substrate.
Dispensing TIM is a very exacting process. If there is too little fluid dispensed or it's not dispensed accurately, a gap can form between the die and the lid, and the heat won't dissipate adequately. Therefore, maintaining volume control during dispensing is a major challenge. Along with Calibrated Process Jetting (CPJ), some systems provide mass flow calibration. Working in tandem, they provide automatic calibration to consistently deliver user-specified amounts of fluid, minimizing set-up and line-to-line variations, while completely eliminating the need for operator interaction.
Solder paste sealing lines are applied to attach the lids in semiconductor packaging in a process very similar to MEMS and printed circuit board assembly (PCBA). However, unlike MEMS cap sealing, the semiconductor lid attachment is most commonly used for heat spreading from the semiconductor chip to a larger heat sink. Although the purpose of the lid is different, many of the challenges in solder paste dispensing are the same for these applications. As package sizes are reduced and board densities increased, these sealing lines become thinner to accommodate smaller package sizes. Yet there is constant pressure to increase the dispenser's productivity by moving to higher line speeds to dispense the solder paste and to use multiple valve systems to increase productivity.
Key dispensing applications to serve semiconductor packaging are:
● First-level underfill dispensing for flip chip
● Flux dispensing for flip chip
● Sealing for lid attachment
● TIM dispensing for heat dissipation
We offer detailed information on other pages for these specialized processes that are related to semiconductor packaging:
● 3D Packaging and Wafer-Level Packaging
● Chip Encapsulation and Dam and Fill