Semiconductor Back-End Process Episode 9: Exploring the Roles of Materials in Conventional Semiconductor Packaging

Reliability and stability are key requirements of semiconductor products to ensure their smooth operation. Semiconductor devices must therefore be protected from physical, chemical, and thermal damage by their packages, and the materials used to make these packages must offer certain qualities. As semiconductor products are increasingly required to perform at higher speeds, packaging materials need to have enhanced electrical properties such as low permittivity¹ and low dielectric loss² for substrates. Materials used for semiconductor memory, as well as logic chips such as CPUs and GPUs, are also required to possess good thermal conductivity for quality thermal dissipation. This highlights the importance of ensuring the advancement of packaging materials to keep up with industry demands. The next two episodes will discuss the characteristics of materials used in the two major package types, starting with this article which covers the materials of conventional packages.

¹Permittivity: The sensitivity to an external electric field, or the degree to which its internal charge responds when the electric field is applied to an insulator.
²Dielectric loss: The conversion of electrical energy when a dielectric is placed in an alternating electric field.

Primary and Subsidiary Packaging Materials

Packaging materials can be broadly categorized into primary and subsidiary materials. Primary materials make up the package itself and have a direct impact on the product’s quality and reliability. Subsidiary materials, on the other hand, are not part of the product’s structure as they are solely used during the packaging process and then removed.

▲ Figure 1. Materials used in the different stages of the conventional packaging process (Source: Hanol Publishing)

 

Figure 1 shows the different types of materials used in a typical conventional packaging process. There are six types of organic composites used as primary materials in conventional packaging: adhesives, substrates, epoxy molding compounds (EMC), leadframes, wires, and solder balls, with the latter three being metals. As for subsidiary materials, some examples include tapes and fluxes³. These small but fundamental materials will be explained with consideration of their crucial roles.

³Flux: A water-soluble and oil-soluble solvent that makes solder balls adhere well to the copper of the ball land.

Leadframes: Metal Alloys for Internal Electrical Connections

Leadframes electrically connect the chip inside the package with the printed circuit board (PCB) located outside the package. The metal plate used to form a leadframe is usually made of Alloy 42⁴ or alloys made of copper. Etching and stamping are the two methods used to produce a leadframe. In the etching process, the leadframe is created by applying photoresist to the metal plate along the pattern of the leadframe and exposing it to an etchant⁵ so the areas not covered by the photoresist are removed. This method is typically used when a fine leadframe pattern is required. For stamping, a progressive die⁶ is mounted on a high-speed press to create the leadframe.

⁴Alloy 42: An iron-based alloy with a coefficient of thermal expansion similar to silicon.
⁵Etchant: A general term for corroding substances such as chemical solutions and gases in the etching process.
⁶Progressive die: A mold technology that compresses several stages of processes into one continuous process.

Substrates: Copper, Glass Fabric & More Used in the Device’s Foundation

▲ Figure 2. A side view of the substrate after going through the packaging process (Source: Hanol Publishing)

 

Just like leadframes, substrates also play a role in electrically connecting the chip inside the package to the PCB outside the package. Consequently, substrates are key components of semiconductor chips in ball grid array (BGA) packages that use solder balls instead of a leadframe. Figure 2 shows the side view of the substrate’s structure after the packaging process. A solder ball is attached to the bottom and a wire is connected to the top. The center of the substrate is formed with a material called the “core” that consists of copper foil bonded to sides of a glass fabric impregnated⁷ with bismaleimide triazine (BT)⁸ resin that possesses thermal stability. Metal wiring is formed on the copper foil while solder resist is applied on top of it to expose the metal pad that serves as a protective layer.

⁷Impregnation: The process of filling gaps that are formed during the casting process to reduce coating failures during the plating process.
⁸Bismaleimide triazine (BT): A type of synthetic resin used in PCBs that is made by reacting heat-resistant bismaleimide and triazine.

Adhesives: Epoxy-Based Polymers for Bonding Key Components

Adhesives are in either liquid form like pastes or solid form such as films. They are primarily composed of thermosetting, epoxy-based polymers and are used to bond chips to either a leadframe or substrate. They can also be used to bond chips together in chip stacking. For an adhesive to achieve high reliability in testing, it must have high adhesion, low moisture absorption, appropriate mechanical properties, and low ionic impurities. In addition, to ensure the quality of the process, an adhesive must have excellent material flowability and wettability of the bonding interface during the high-temperature and high-pressure bonding process. Voids⁹ must also be suppressed to achieve high interfacial adhesion. This requires optimization of rheological properties such as viscosity, thixotropy¹⁰, and hardening traits, as well as strong adhesion between the chip and the leadframe or substrate surface.

⁹Voids: Hollow holes or air pockets that form within a material. They are a defect that occurs during material manufacturing or processes involving heat treatment.
¹⁰Thixotropy: The property of a liquid substance to decrease in viscosity when a shear force, such as stirring, is applied and to increase in viscosity when no shear force is applied.

Liquid adhesives include epoxy and silicone adhesives. Solid adhesives include lead-on-chip (LOC) tapes that are used for leadframes, spacer tapes used to keep chips apart when stacking chips of the same size, and die attach films (DAFs) that are used for chip stacking or to attach chips to the substrate. DAFs are also called wafer backside laminate (WBL) films because they are applied to the backside of the wafer.

Epoxy Molding Compounds (EMC): Thermosetting Polymers for Protection & Heat Dissipation

EMCs are encapsulants¹¹ used in the semiconductor packaging process and are composites of inorganic silica and thermosetting epoxy polymer that form a three-dimensional bonding structure when heated. As EMCs encapsulate the chip, they must protect it from external physical and chemical damage and effectively dissipate heat that is generated when the chip is in operation. Moreover, EMCs should be easily moldable to achieve the desired package shape. Since they interface with other packaging materials such as substrates and chips, EMCs must strongly adhere to these materials to ensure the reliability of the package.

¹¹Encapsulants: They consist of thermosetting polymers that form a three-dimensional structure which is hardened by external heat. Their function is to protect the contents from heat, moisture, and shock.

▲ Figure 3. Types of EMCs

 

Figure 3 shows the types of EMCs and the processes in which each type is used. Tablet-type EMCs are mainly used for transfer molding, while powdered EMCs are commonly used for compression molding or to mold a large wafer. On the other hand, liquid EMCs are used for molding a wafer that is difficult to mold. Recently, vacuum lamination using film-type EMCs has also been used for fan-out wafer-level chip scale packages (WLCSP) and large-sized panel-level packages (PLPs). There are also EMCs for molded underfill (MUF), a process where underfilling and molding are performed simultaneously in the flip-chip process.

Solders: From Tin to Unleaded Alloys for Mechanical & Electrical Connections

Solder is a metal that melts at low temperatures, and this property makes it widely used for electrical and mechanical connections in various structures. It is used to connect the package to the PCB in semiconductor packaging, as well as to connect the chip to the substrate in flip-chip packaging. The solder for connecting the package to the PCB is mainly in the form of balls that vary in size from 30 micrometers (μm) to 760 μm. Today, the number of pins connecting a package to the PCB is increasing to improve electrical properties, leading the solder balls to get smaller.

Solder balls must be uniform in alloy composition when made from solder alloy as a lack of uniformity can compromise reliability in its drop impact or temperature cycle tests. It is also important that they have good oxidation resistance as excessive oxide buildup on materials or during the reflow process can result in non-wetting defects where solder balls will not adhere and fall off. For this reason, fluxes are used to remove oxide film buildup during soldering, and an inert atmosphere needs to be created with nitrogen gas during reflow. In addition, there should be no voids as the solder volume will be insufficient due to voids and possibly lead to the reduction of the solder joints’ reliability. The size of the solder balls is also crucial because uniformly sized solder balls increase the efficiency of the process. Lastly, the surface of the solder balls must also be free of contamination and dendrite¹² growth. These can increase the failure rate and reduce the reliability of the solder joints.

In the past, solder balls were often made of the tin alloy (Pb-Sn) which has good mechanical properties and electrical conductivity. However, as lead became subject to environmental regulations such as the EU’s RoHS Directive¹³ after it was found to be harmful to human health, unleaded solder with a lead content of 700 parts per million (ppm) or less is now mainly used.

¹²Dendrites: Crystals formed in the shape of tree branches; a type of fractal found in nature.
¹³RoHS Directive: The EU’s Restriction of the Use of Certain Hazardous Substances in Electrical and Electronic Equipment (RoHS) Directive aims to protect the environment and human health by substituting hazardous materials for safer alternatives.

Tapes: PSA For Permanent & Temporary Bonding

There are two types of tape that will be explained in this section. The first is adhesive tape used to permanently bond solid surfaces to homogenous or heterogenous surfaces. The other type is temporarily adhesive tape such as dicing and backgrinding tapes that are able to bond and be removed through cohesion and elasticity. The material used in these tapes is called pressure-sensitive adhesive (PSA).

Backgrinding tape is applied to the front of a wafer to protect the devices on the wafer during the backgrinding process. After the backgrinding process is complete, the tape must be removed so no adhesive residue is left on the wafer.

Dicing tape, also known as mounting tape, is used to fix a wafer in a ring frame and ensure that the chips on the wafer do not come off during the wafer dicing process. Thus, dicing tape must have good adhesion during wafer dicing, but it must also be easily detachable. Therefore, the tape is irradiated with ultraviolet light before the chips are removed as dicing tape contains PSA that reacts to the ultraviolet light. This weakens the adhesion and makes it easier to remove the chips. In the past, wafers were attached to the dicing tape after backgrinding, but with the wide use of WBL as an adhesive for chips, the wafers that have completed the backgrinding process are attached to the tape that combines WBL and dicing tape.

Wires: From Gold to Copper for Electrical Chip Connections

▲ Figure 4. Gold (Au) wire

 

The wire that electrically connects a chip to a substrate or leadframe, or a chip to a chip, is typically made of high-purity gold. Gold has good malleability so it can be spread thin, as well as ductility to allow it to be stretched into a thread. These attributes are highly beneficial in the wiring process. In addition, gold’s oxidation resistance makes it highly reliable, while its excellent electrical conductivity provides it with good electrical properties. However, gold increases manufacturing costs due to its high price. For this reason, thinned gold wire is sometimes used, but it is more susceptible to breakage if stretched too much, limiting its use. Therefore, other metals such as silver are added to make alloys with gold, while gold-coated silver, copper, palladium-coated copper, and gold palladium-coated copper are also used.

Copper wires are increasingly used instead of gold wires due to their price competitiveness and the fact that copper wires also have good electrical conductivity while having slightly less malleability and ductility than gold wires. However, as copper is susceptible to oxidation, copper wires can be oxidized during and after the wiring process. Therefore, unlike gold wires, the wiring equipment for copper wires are sealed and the inside of the equipment is filled with nitrogen gas to prevent the copper wire from being exposed to air and being oxidized.

Packing Materials: Preparing for Shipment With Tape & Reel

▲ Figure 5. Tape and reel packaging (above) and a tray (below)

 

After packaging and the package testing, semiconductor products are shipped to customers. Tape and reel (T&R), or tray packing, is used to pack the products. In T&R packing, the packages are placed on a tape with pockets that are the same size as the package. The tape is then rolled onto a reel and the package is packed for shipment to the customer. In tray packing, the packages are placed in a tray, which is then stacked and packed for shipment.

A Look Ahead to the Materials of Wafer-Level Packages

Following this introduction of the materials used in the various processes that complete conventional packages, the next episode will look at the materials used in wafer-level packages. In addition to seeing what types of substances they are made of, the article will cover the crucial roles they play in ensuring the quality and durability of semiconductor products.

 

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