Semiconductor Back-End Process Episode 6: The Eight Steps of Assembling Conventional Packages

In an earlier episode, it was established that the two main categories of semiconductor packages are conventional and wafer-level packages. Going forward, this series will focus on these package types and their differences in assembly methods and functions starting with this article which will cover conventional packages.

Overview of Assembling Conventional Packages

Figure 1 shows the assembly process for plastic packages, which are a type of conventional package. Plastic packages are categorized into leadframe and substrate packages. The first half of the packaging process for these two packages is the same, but the second half differs in how the connection pins are applied.

▲ Figure 1. The steps of assembling leadframe and substrate packages (Source: Hanol Publishing)

 

Once the wafers are tested, they first go through backgrinding to become the desired thickness. Wafer sawing then follows so the wafers can be cut into chips. Afterwards, chips that are deemed to be of good quality are selected and attached to the leadframe or substrate through the die attach process. The chips are then electrically connected to the substrate through wire bonding before they are sealed with an epoxy molding compound (EMC) for protection. Both leadframe and substrate packages share these steps.

In the next stage, leadframe packages undergo several processes: trimming¹ that separates the leads, solder plating that applies solders to the ends of the leads, and, lastly, forming. The process of forming separates the packages into single units and bends the leads so they can be attached to the system board. As for substrate packages, they are molded before going through solder ball mounting where solder balls are attached to the substrate pads. This is followed by a process of cutting and forming individual packages called singulation. In the following section, the process of assembling conventional packages with an emphasis on the eight steps of producing substrate packages will be explained.

¹ Trimming: A process applied to leadframe packages that removes the dambar, which connects the space between the leads, using a cutting punch.

Step One: Backgrinding

The backgrinding process ensures a wafer is processed with the optimal thickness for its package’s characteristics. This includes processing the wafer’s back and mounting it to a ring frame, as shown in Figure 2.

▲ Figure 2. The four steps of the wafer backgrinding process (Source: Hanol Publishing)

 

Before grinding the backside of the wafer, a protective tape known as a backgrinding tape is laminated onto the wafer’s front. This is to prevent physical damage to the frontside where the circuit was formed. Next, grinding wheels are applied to the backside of the wafer to make it thinner. A rough grinding wheel is first used at high speed to remove most of the excess material before a fine grinding wheel grinds more delicately and accurately to reach the wafer’s target thickness. Afterwards, a fine pad is used for polishing to smooth the wafer’s surface. If the wafer’s surface is rough, cracks are more likely to occur when stress is applied during subsequent processes and result in the chip breaking. Therefore, it is crucial to reduce the chances of chip breakage by polishing that prevents the formation of cracks.

For packages consisting of a single chip, the wafer is generally grinded to a thickness of about 200 to 250 micrometers (μm). As for stacked packages, the chips—and essentially the wafers as well—need to be even thinner as multiple chips are stacked on the package. However, the residual stress from grinding the wafer’s backside causes shrinkage on the frontside and can potentially bend the wafer into the shape of a smile. Furthermore, the degree of bending becomes more severe as the wafer is thinned. To flatten out the wafer, mounting tape is first applied to the backside of the wafer and then it is attached to the ring frame. The backgrinding tape that was applied to protect the devices on the wafer’s front is then removed again, exposing the semiconductor devices to complete the backgrinding process.

Step Two: Wafer Sawing/Dicing

Wafer sawing is the process of cutting along the scribe lanes² of a wafer in order to break it into chips or dies. Also referred to as the dicing process, wafer sawing is a necessary procedure of the packaging process for chips or dies.

Figure 3 shows an example of a wafer being broken into chips through blade dicing, a method of wafer sawing that uses a wheel-shaped saw blade to cut and separate wafers. This saw blade with a wheel tip strengthened through diamond grit cuts along the wafer’s scribe lanes—the lattice-shaped lines of the wafer on the left of the figure. As the saw blade creates a working tolerance³ when it rotates, the scribe lane must be thicker than the wheel.

² Scribe lane: A space of sufficient width designated for cutting a chip or die from a wafer without affecting nearby devices while allowing for the distribution of the cut pieces.

³ Tolerance: The range of errors in space or values created from the difference of work capabilities.

▲ Figure 3. Sawing a wafer into chips through the blade dicing process (Source: Hanol Publishing)

 

One issue of blade dicing is that, as the blade physically contacts the wafers during the process, the wafers are more prone to breaking when they are requested to be made thinner. Laser dicing, another method of wafer sawing, resolves a lot of these issues as nothing physically contacts the wafer during the cutting process. Instead, a laser is shot from the back of the wafer during the dicing. Consequently, it is suitable for cutting thin wafers, while the chip remains robust due to the minimal damage to the wafer’s surface.

As wafers have become thinner, there have been proposals to use dicing before grinding (DBG) which reverses the sequence of processes to reduce chip damage during wafer cutting. While the conventional process involves thinning the wafer by backgrinding before it is cut, DBG is a different method that partially cuts the wafer before it goes through backgrinding and completely cuts it off through mounting tape expand⁴.

⁴ Mounting tape expand (MTE): Expansion of the mounting tape that is attached to the wafer after stealth dicing, a method of creating cracks in a wafer with a laser. Physical force is then applied to the relevant areas to break the wafer into chips.

Step Three: Die Attach

As shown in Figure 4, die attach is the process of picking up the chips that have gone through the wafer cutting process from the mounting tape and attaching them to a substrate or leadframe that has been coated with an adhesive.

▲ Figure 4. The die attach process (Source: Hanol Publishing)

 

During the wafer cutting process, the chip that has been cut should not fall off the mounting tape. However, in the attach process, the chip must be peeled off the mounting tape. As damage might be caused during the removal of the chip if the adhesion of the mounting tape is too strong, the adhesive should maintain a strong bond during wafer cutting and then weaken when it is exposed to ultraviolet light before the attach process. At this time, only chips that pass the wafer test are detached from the mounting tape.

While the removed chips must be reattached to the substrate with adhesive, there are differences depending on the type of adhesive used. If a liquid adhesive is used, it must be applied to the substrate in advance using a syringe-like dispenser or through stencil printing⁵. On the other hand, solid adhesives are usually in the form of a tape. Also known as die attach films (DAF) or wafer backside lamination (WBL) films, solid adhesives are especially preferred when chips need to be stacked. After backgrinding is complete, a DAF is attached between the mounting tape and the back of the wafer. When the wafer is cut, the DAF is cut along with it. As the DAF and the chip attached to its back will fall off, the DAF can be glued on top of the substrate or chip.

⁵ Stencil printing: A method of printing using a stencil mask to apply paste-type materials to devices such as substrates.

Step Four: Interconnection

Interconnection refers to the electrical connections between chips, chips and substrates, and other combinations within a package. The following section will introduce two interconnection methods: wire bonding and flip chip bonding.

▲ Figure 5. The seven steps of the wire bonding process (Source: Hanol Publishing)

 

Wire Bonding

Wire bonding uses heat, pressure, and vibration to electrically connect chips and substrates with metal wires. The wires are usually gold (Au) as they have good electrical conductivity and ductility. Wire bonding can be compared to sewing where the thread is the wire and the needle is the capillary⁶. The wire is rolled up onto a spool like a yarn and equipped to the machinery before it is pulled out and passed through the center of the capillary to form the tail at the end of the capillary. When the electronic flame-off (EFO)⁷ gives a strong electrical spark to the wire’s tail, that part melts and solidifies to form a free air ball (FAB) that is essentially caused by surface tension.

After the FAB is created, it is attached to the chip’s pad with force to form ball bonding. When the capillary is moved toward the substrate, the wire comes out like a thread to form a loop. Stitch bonding⁸ is formed by pressing the wire against the bond finger—the part of the substrate that will make the electrical connection. The wire is then pulled back even more to form a tail, and the connection between the chip and the substrate made with wiring becomes complete after the tail is cut. This procedure is repeated on the other chip pads and the substrate’s bond fingers during the wire bonding process.

⁶ Capillary: A tool used in wire bonding machines to connect chip electrodes and lead terminals with wires.

⁷ Electronic flame-off (EFO): A process which melts a wire tip by an electrical spark to form a FAB.

⁸ Stitch bonding: The bonding of wires to a pad during the semiconductor packaging process by pressing and attaching the wires.

Flip Chip Bonding and Underfill

Flip chip bonding creates a bump on top of the chip to make an electrical and mechanical connection with the substrate. Therefore, it has better electrical properties than wire bonding. There are two types of flip chip bonding: the mass reflow (MR) process and thermocompression. MR attaches the chip with the substrate by melting the junction’s solder at a high temperature. The thermocompression process, on the other hand, applies heat and pressure to the juncture to make the connection between the chip and substrate.

Since the stress caused by the difference in the coefficient of thermal expansion⁹ (CTE) between the chip and the substrate cannot be handled by the bump alone, an underfill process that fills the space between bumps with polymer is necessary to ensure solder joint reliability. There are two main underfill processes to fill up the space between bumps: post-filling, which fills the materials after flip chip bonding, and pre-applied underfill, which fills the materials before flip chip bonding. Additionally, post filling can be divided into capillary underfill (CUF) and molded underfill (MUF) depending on the underfill method. After flip chip bonding is applied, CUF fills in the gaps between bumps by using the capillary to inject underfill material into the side of the chip. As for MUF, it allows EMC to function as an underfill by using it to fill up the spaces between bumps.

⁹ Coefficient of thermal expansion (CTE): A material property that indicates the extent to which a material expands upon heating.

Step Five: Molding

Once the chip is wire bonded or flip chip bonded, it needs to be encapsulated to protect the structure from external impact. Such protection processes include molding, sealing, and welding, but only molding is used for plastic packages. The process of molding encloses EMC, which mixes thermosetting resin¹⁰ with several inorganic materials, around parts including chips and wires to protect them from physical and chemical external impacts and to create the desired package size or shape.

¹⁰ Thermosetting resin: A stable polymer material that undergoes a polymerization reaction when heated to harden and form a polymer compound. It is primarily used for EMC that protects the electronics and electrical properties of semiconductor circuits by preventing thermal and mechanical damage in addition to corrosion.

The molding process takes place in a mold. For transfer molding, a substrate with chips connected by wire bonding is placed on both molds while an EMC tablet is placed in the middle and heat and pressure is applied. This liquidizes the solid EMC to flow into both molds and fill up the space. Transfer molding faces challenges when the gap between the chip and the top of the package gets smaller as it becomes more difficult to be filled with a liquid such as EMC. Furthermore, when the substrate gets bigger, the mold has to increase in size accordingly and it therefore becomes harder for EMC to fill the space.

In recent years, the process of transfer molding reached its limits. As the number of chip stacks has increased while a package’s thickness has generally decreased, the gap between the chip and the top of the package has continued to shrink. The size of the substrate is also growing as more chips are being processed in large batches to lower manufacturing costs. For this reason, compression molding has emerged as the solution to filling the small gap. In compression molding, the mold is pre-filled with EMC powder. When heat and pressure are applied after the substrate is placed in the mold, the EMC powder filled in the mold liquidizes and is eventually molded. In this case, the EMC immediately becomes liquid and fills the space without flowing, so there is no problem filling the small gap between the chip and the top of the package.

Step Six: Marking

Marking is a process of engraving product information such as the semiconductor type or manufacturer, in addition to patterns, symbols, numbers, or letters requested by the customer, on the surface of semiconductor packages. This proves to be important when a semiconductor product fails to operate after it is packaged as the markings can assist in tracing the cause of the product’s failure. Markings can either be engraved by burning materials such as EMC with a laser or by embossing using ink.

For plastic packages, they need to be molded before the requested information is displayed on the surface. Since laser marking is simply the act of engraving, a black EMC is usually the preferred choice as it increases the legibility of the markings. This is because color cannot be applied to the engraved characters or symbols, so it is more visible to have engravings on a black background. The remaining two steps will cover the final stages of packaging substrate packages. This is where the difference lies between the processes of substrate and leadframe packages.

Step Seven: Solder Ball Mounting

Solder balls in a substrate package do not only serve as an electrical pathway between the package and external circuitry, but they also provide mechanical connections. Solder ball mounting is the process of attaching a solder ball to a substrate pad. In the first step of the process, flux¹¹ is applied to the pad and then the solder balls are placed on the pad. Then, the reflow process melts and attaches the solder balls before the flux is washed and removed. The role of the flux here is to remove impurities and oxides from the surface of the solder balls during the reflow process. This allows the solder balls to melt uniformly and provides a clean surface. When these melted solder balls flow into a stencil on the substrate, they fill each hole in the stencil. Then, the substrate and stencil are separated but the solder balls remain on top of the substrate due to the adhesion of the flux. As there will be flux that has already been applied to the pad, the solder balls will be temporarily adhesive and attach to the pad.

¹¹ Flux: A water-soluble and oil-soluble solvent that makes solder balls adhere well to the copper of the ball land.

▲ Figure 6. The temperature profile applied during the reflow process (Source: Hanol Publishing)

 

The solder balls attached to the substrate pad with flux melt through the reflow process. Figure 6 shows the temperature profile applied during this process. The flux is activated in the soak zone before the solder reaches its melting temperature, removing oxides and impurities from the surface of the solder balls. While the solder balls melt and attach to the pad when it is above the melting temperature, they do not flow off completely. Instead, they form a globular shape caused by surface tension in all areas except the parts where they adhere to the metal part of the pad. As the temperature decreases, they retain their shape and solidify again.

Step Eight: Singulation

Singulation is the final process of creating a substrate package. The process involves using a blade to cut the finished substrate strips into individual packages. Once the singulation process is complete, the packages are placed on a tray for package testing and the rest of the process steps.

 

The various steps involved in assembling conventional packages highlight how factors such as precise alignment, optimal electrical connections, and robust protection against external damages are integral to their formation. In the next episode, wafer-level packages—the other main type of semiconductor packages—will be explored in detail.

 

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