Inside the Back End of Power Semiconductor Manufacturing: A Tour of Shiima Electronics

Today, to see the back-end process of power semiconductor manufacturing, we came to Shiima Electronics in Yamanashi, Japan. Last time I visited Toshiba Device & Storage's Kaga Plant, we saw the front-end process of power semiconductor manufacturing. This is primarily the process for making the chips inside power semiconductor packages. What we will be looking at this time is called back-end processing. We will look at the process of putting finished semiconductor chips into the package shapes seen in actual products.

Shiima Electronics is a company that mainly handles semiconductor prototyping and evaluation. This time, we were given a special opportunity to show you how the back-end process for power semiconductors is done. So we're going to look at the back-end process of power semiconductor manufacturing. But first, let me introduce our sponsors. Go ahead.

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Here at the semiconductor plant, there are wafers that have passed through the front-end process. The devices on the wafer are already finished. The back-end process involves processing this wafer in various ways: backgrinding, dicing, die attach, wire bonding, resin encapsulation, and singulation.

The back-end process for power semiconductors that we will look at here involves prototypes in small quantities, unlike mass-produced products. Therefore, many of the processes are done by hand. You can actually see what is being done inside the black box of semiconductor manufacturing equipment.

The back-end process of semiconductors starts with polishing the backside of the wafer. This is called backgrinding. Before starting the polishing process, first apply backgrinding tape to the wafer. We use this device. This is for holding the wafer by suction and protecting the circuits built on it from shavings during polishing. We will apply it like this.

This tape uses a UV-curable adhesive. After backgrinding is complete, it is irradiated with ultraviolet light so it can be peeled off. By the way, for mass-produced products, the process of applying and removing this tape is generally done entirely automatically.

When you have finished applying the backgrinding tape, next we will actually grind the backside of the wafer. The thickness of a wafer cut from an ingot is about 725 μm for an eight-inch wafer. The thickness after grinding changes depending on the semiconductor type. For example, power semiconductors are ground to about 100 μm. Memory and similar devices are ground even thinner. As wafers get thinner, there is a risk of chip breakage, so handle them with care.

Now let's actually do it. Set this wafer here. Set this in place as well, and the rest of the backgrinding starts automatically.

The backgrinding process includes three steps: rough grinding, fine grinding, and cleaning. After rough grinding to approximate dimensions, the wafer is polished to the target thickness in the fine-grinding process. Then we clean it. Processing time varies depending on conditions and other factors, but it usually takes only a few dozen minutes.

Looking inside the backgrinding device, it looks like this. This is the part where the wafer is ground. There is a grinding stone in here. The grinding stones look like this. After grinding is complete, the wafer comes out cleaned. The backside of the wafer after grinding looks like this.

Polishing has just finished. Take it out of the equipment. Handle it with care, as it is quite thin. It looks like this. I say "grinding," but it's actually more like polishing. This surface is quite smooth. This surface roughness is related to bonding to the leadframe later in the process and to reliability.

Here is the wafer after backgrinding. Once we get to this point, we move on to the next process, dicing. Dicing is the process of dividing and cutting a wafer into individual chips called dies. Before starting dicing, we attach something called dicing tape to the wafer.

But before we go any further, we apply UV light to the backgrinding tape we just applied to weaken its adhesion. This makes it easier to peel off. Here is the device. You put it in here like this, then close it. Now the ultraviolet irradiation begins, making the tape easier to remove.

We've finished irradiating the backgrinding tape with UV light. From here, we apply the dicing tape. I've changed the type of tape in the same device we used earlier. Put it in this way, and then we can apply the tape. I've attached the dicing tape like this.

Here is the one we just applied the tape to. Remove the backgrinding tape. Since it was irradiated with UV light, its adhesion is much weaker. It comes off cleanly. When you're done here, proceed to dicing.

Dicing is done using this equipment. Inside, it contains an ultra-thin grinding blade shaped like this. It spins at high speed and cuts by contacting the wafer. This one is really thin when you look at it. It uses synthetic diamond abrasive grains. Basically, it cuts in a straight line while spraying water.

You have to be careful about chipping when dicing. Chipping is the phenomenon of chips and cracks forming along the edges of the silicon chip. When chipping occurs, the electrical characteristics of the finished semiconductor may deteriorate, or its mechanical strength may be reduced, greatly affecting reliability. It also lowers yield in mass production.

Chipping is related to the grain size of the grinding wheel, the cutting conditions, and the tape. Because silicon itself is a fragile material, the slightest thing can cause chipping. Something like this is chipping. Therefore, there is a lot of expertise involved in the blade's rotation speed, movement speed, and so on.

Here is the wafer after dicing. The cut chips are still stuck to the dicing tape. It's like this. Can you see?

Next, we remove the chips from the dicing tape and mount them on leadframes, substrates, and so on. This process is called die attach or die bonding. What I'm holding right now is a TO-247 package leadframe. You might recognize the legs of this power semiconductor. The actual power semiconductor will be placed on this flat area called the die pad.

The bonding materials depend on the purpose. We use solder preforms, solder paste, sintered materials, and so on, like these. In most power semiconductors, the drain or collector is connected to this die pad area.

This is a wafer after dicing. Solder will not adhere to this wafer. Normally, after backgrinding, metal is deposited on the wafer for connection with solder and so on. This is called back metal. In the die bonding process, the leadframe, solder, and chip with back metal already applied are stacked in that order. When heated, die bonding is complete.

Now let's look at the procedure using this equipment. First, let's look at the case of using solder paste. With this solder paste printing equipment, we manually print solder paste on the leadframe. Because this is small-quantity, high-mix prototyping, we use something like this. Put solder paste on the stencil like this, then use a spatula to print it onto the leadframe. This is almost the same as the process of printing solder paste onto printed circuit boards. By the way, for the convenience of filming this time, we're doing this process outside the clean room.

Here is the leadframe after solder paste printing. After this, a device called a die bonder is used to place a chip on it. Here is a view of a chip being placed with a die bonder. The package being used is completely different. The chip after dicing is picked up by suction at the tip of the machine and placed on a substrate or leadframe.

If you do it manually, it looks like this. The chip is placed on top of the TO-247 package with solder paste already printed on it. Manual placement requires very careful, precise work.

We've finished placing the solder and chip on the leadframe. After this, it passes through a reflow oven to melt the solder and bond the chip to the die pad. Let's send it through. It travels like this. By the way, the inside of the reflow furnace has a nitrogen atmosphere. This is also the same as with printed circuit boards.

The leadframe with the chip bonded to it has now come out of the reflow oven. I think they are well attached, but we're not finished yet. Flux remains. If we proceed to the next step like this, the flux can cause defects and so on. I'd like to move on to the cleaning process to remove the flux. Before that, I'll explain a little more about the reflow process.

The method we've just seen uses solder paste containing flux. When flux is included, cavities called voids can form in the chip joint where the solder has not bonded. Voids can form because, as the flux is heated in the reflow oven, solvents in the flux evaporate and generate gas. When that gas gets trapped in the molten solder, voids can form.

If voids form, the paths for heat dissipation from the chip to the leadframe become smaller. Heat does not dissipate well, and joint strength is also lower. This results in a loss of reliability. You might think it would be better not to use flux, but that won't work here. Without flux, the oxide film on the metal surface cannot be removed, which lowers the bonding strength between the leadframe and the chip.

There is a chip mounting method called formic acid reflow that solves these problems. I can't go into detail here, so I'll explain it in a different location.

We have moved inside the clean room. Formic acid reflow is a solder bonding method that does not use flux. It reduces oxide films and oxidized metals, eliminating the need for flux. The solder used also changes from solder paste to solder in pellet form. With formic acid reflow, there are no flux-derived voids, and bonding can be performed with reduced void formation overall.

Especially for power semiconductors, the heat generated by the chip needs to escape efficiently to the leadframe. Voids cause poor heat dissipation, so joints with minimal voids are required.

I'm going to try performing formic acid reflow. We heat it in a special device while maintaining a formic acid atmosphere. Here is the completed formic acid reflow. Even though no flux is used, the solder has wetted and spread well. There is no flux residue like there was during the earlier reflow soldering.

I've rambled on a bit. Let's return to removing the flux after reflow. There are many ways to remove flux. This time, we'll use a cleaning solution. Insert the leadframe that just went through the reflow oven into this jig. I've brought the one we just set up over to this cleaning machine. When you open the lid, the liquid is simmering inside. Put it in like this.

The degree of cleaning depends on the type of chemicals, soaking time, and temperature. It takes roughly a few minutes to a few hours. This is how we remove the flux.

We've finished cleaning with this chemical solution. Now we'll remove the chemical solution. This is called a rinse. Open it like this. Here is the one with chemical solution on it. Put it in here with a splash. We'll rinse it this way.

The first rinse is finished. There is a second rinse. Drain it well like this, then put it into the next rinse tank. After the second rinse, let it dry, and you can proceed to the next step.

Next is the wire bonding process. This is the process of making electrical connections with wires between the power semiconductor chip and the external electrodes. It connects the remaining gate terminals and source or emitter terminals that were not connected by die attach.

Wire bonding includes methods called ball bonding and wedge bonding. Ball bonding uses gold, silver, and copper wires a few tens of microns in diameter. Ball bonding is often used in logic ICs.

Next is wedge bonding. Aluminum and copper wires are often used for this. Unlike ball bonding, the wire diameter is quite thick. The thickest is about 500 μm. Because large currents flow through power semiconductors, wedge bonding with thick wire is used.

Behind me is the wedge bonding equipment. In this wedge bonding process, ultrasonic waves are applied while placing a load on the aluminum wire to bond it. The semiconductor chip electrodes and leadframe are wired together. For video recording, it is operating at a much slower speed setting. They say wedge bonding on mass production lines moves faster.

Wedge bonding is finished. Here it is. We've finally made it this far, and it's starting to look like a power semiconductor. After bonding, it undergoes visual inspection, then proceeds to the molding process.

While we were here, I also got to see ball bonding. It uses gold wire. Compared with the wedge bonding described earlier, it is moving much faster.

Here are some parts that have completed wire bonding. Before starting the next process, they are cleaned with plasma. The next process is mold encapsulation. Plasma cleaning beforehand improves adhesion between the resin and leadframe.

Open it like this, and put this inside. We place them quite carefully. If you let go and set them down with a bang, the bonding wires may come off. So we'll be quite careful.

Setup is finished. Press and hold the button here. Plasma cleaning begins. We'll wait for a while until cleaning is complete. Plasma cleaning has finished. It's hard to tell with the human eye, but it's clean.

Next is the mold encapsulation process. Encasing the IC chip in epoxy resin or a similar material protects it from the outside environment and provides durability and reliability. This black material is the molding material, epoxy resin. Encasing it in molding material like this physically protects the chip and also ensures electrical isolation.

There are two main resin encapsulation methods: transfer molding and compression molding. This equipment uses transfer molding. Place the leadframe with the chip and completed wire bonding into the mold, close the mold, and inject heat-melted resin into it.

The material comes in pellets like these. It's rare to see molding material at this stage, so it's a fresh impression. The heated and melted pellets are pressurized and injected into the area where the leadframe fits. In this video, I've omitted much of the preparation. In practice, preparations such as applying a mold release agent to the mold are also necessary.

Here is what has hardened in the mold. Actual mass-produced power semiconductors are made using this same transfer molding method.

Next is laser engraving. If you look closely at a power semiconductor, you see the model number and manufacturer's marking on the surface. Those are engraved with a laser. For mass-produced products, this is done at this stage. For prototypes, it is sometimes done after singulation. This time, we engraved "Ichiken × Shiima Electronics."

Next is the tie bar cutting process. We've come this far, and now we're almost there. This is the process of cutting away excess leadframe material. We cut it using this hydraulic machine. Put it in the die like this and cut it.

Here is what it looks like after cutting. It now has the power semiconductor shape you all see all the time.

After this, various inspection processes will be carried out. I'm talking about the case of mass-produced products. Mainly visual and electrical inspections will be performed.

First, in visual inspection, we will look for any defects in appearance. For customers who are automotive-related or demanding in terms of appearance, we inspect every product. For consumer goods, for example, sometimes a spot check is all that's needed.

Electrical testing checks whether the semiconductor is working properly. We inspect all products even if they are consumer products.

Regarding yield, the yield itself in the back-end process of power semiconductors is quite good. It's in the high 90s, almost 100%.

Today, I've seen what the back-end process of power semiconductors looks like. Unlike a factory for mass-produced goods, I hope you were able to see it quite openly.

If you are interested in Shiima Electronics or the front-end process of power semiconductors, you can find the link in the description. Please take a look there! Thank you for watching the video until the end.

Lastly, an announcement from Ichiken. I've been on these factory tours this time. Requests such as wanting us to introduce a company's factory or make a work experience video for recruiting purposes are becoming more and more common these days. We at Ichiken welcome the production of such videos, so please contact us using the inquiry form in the description. We would love to hear from you! Please feel free to contact us!