Solder balls

Computers, microprocessors, and other microelectronic devices could not exist as we know them today without the technology of depositing thin metal or alloy films with fine lithographic patterns. For example, in a computer, the individual transistors that make up an integrated circuit must be electrically interconnected by a complex network of conducting lines and vias that are deposited above the semiconductor layers. Furthermore, the chips are joined to multi-chip packaging modules, a process in which many electrical connections are simultaneously established by solder balls. 

Fig. 14.3 Process steps involved in the C4NP technology. Wafers with BLM and solder-fiUed molds are processed in parallel and joined together. Solder bumps are thus transferred to the wafer. Inset Solder balls formed by C4NP process. Ref. [23] reproduced with permission of the Electrochemical Society...

After attaching the BGA packages to a PWB and reflowing the solder, an underfill adhesive similar to that used for flip chips may be dispensed and allowed to fill the spaces between the solder balls, thus reinforcing the connections and relieving stress in the entire structure. 

Bare die such as integrated circuits may be assembled in plastic or ceramic carriers, called chip-scale packages, whose dimensions are slightly larger than the chip. CSPs are often defined as packages that are no larger than 1.5 times the area of the die or no more than 1.2 times the width or length of the die. If the carrier is a BGA type, an alternate definition is that the solder-ball pitch be less than 1 mm. The pitch of... 

This CSP type uses a flexible circuit having solder balls or metal bumps as an interconnect interposer between the chip and the next circuit board level. The bare... 

Chip is attached face down and wire bonded to the interposer. A thin elastomer, sandwiched between the chip and interposer, cushions the chip and the solder-ball interconnects, relieving stresses (see Fig. 1.13). The interposer generally consists of a metallized, flexible polyimide tape on which are formed electrical connections by photolithographic processes. As a final step, the exposed wire bonds and edges of the chip are molded with epoxy. 

The absorption of moisture in underfill adhesives induces a tensile hygrothermal stress on the solder-ball connections causing electrical opens in the connections and cracking in the adhesive. These tensile stresses offset the compressive stresses that underfill adhesives provide in improving the reliability of flip-chip and ball-grid-array devices. 

Silver-filled epoxies and other electrically conductive adhesives are widely used to electrically connect chip devices or packaged components to interconnect substrates or printed-circuit boards. Chip capacitors, resistors, transistors, diodes, and magnetic components may be attached with silver-filled epoxies whose volume resistivities range from 1 x 10 " to 3 x 10 " ohm-cm or with gold-filled epoxies whose volume resistivities are approximately 8 x 10 ohm-cm. Conductive adhesives are also finding use as replacements for solder balls in flip-chip devices. In all cases, to achieve reliable connections, initially low-contact resistances or volume resistivities must remain low on aging and on exposure to operational stress conditions, such as humidity, temperature, vibration, shock, and power. 

Keywords Discrete polydisperse spray Electric droplet charging Extension nozzle Ink-jet printing Monodisperse droplet stream Monodisperse spray Multihole orifice Modulated jet excitation Nozzle hole shapes Rapid prototyping Rayleigh-type jet break-up Solder ball production... 

The drop generator established by Yim et al. for producing solder balls is shown in Fig. 26.5 as an example for a device suitable for use with melts [20]. This device combines a heatable reservoir for the metal melt with a solenoid-driven vibrator, which transmits oscillations by a disk mounted at the end of a shaft to the molten metal bath. The orifice piece is manufactured from ruby. The solidification of the solder drops is controlled by immersing them into a silicone oil bath. Particles produced with this device are nicely spherical, with typical diameters of 780 pm and a standard deviation of 26 pm. Apart from the spherical particles, irregular shapes are also observed, which are due to coalescence of the drops, either in the drop stream before entry into the oil bath, or in the oil bath prior to formation of a solid shell on their surface. 

Fig. 26.5 Drop generator for manufacturing solder balls by solidification of melt droplets [20] (Reprinted from [20]. With kind permission from Prof. Chun, MIT, 2010)...

The CSP was mounted on the PWB by using eutectic solder balls (37Pb/63Sn) and paste. The height of solder joints was approximately 0.45 mm. 

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