Semiconductor Tales: Materials for Packaging and Saving Cost, Vol. 1

February 28, 2017 in Semiconductor Story by Gyuik Jeong
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Semiconductors and Reducing Cost

Hello to all dear readers of Amkor In Story, it’s already the end of February and March is coming. I am so eager for the warm spring to come and put the harsh winter in the past. A while ago, I took a look around the home electronics area in a big shopping center. Naturally, as a man, I had my attention on a large ultrahigh definition (UHD) TV and nothing much besides. The colorful scene filling the thin, wide screen mesmerized me for some time. It was cheaper than I expected, so I had a hard time refraining from buying it. Compared to just a few years ago, this new model has better performance and a bigger screen, but the price stayed about the same or even lower.

When I was in high school, one of my friends was showing off a cell phone that he secretly took from his dad. I was shocked to realize it was nearly $2,000 and although it was talk-only cell phone, it was bigger than a brick in size. Nowadays, the cell phones are cutting-edge and smaller but mysteriously the price keeps dropping. Why is that? There are many answers, but I will say, it’s the advances in technology and, today’s topic – reducing the production cost. The semiconductor packages used in flat panel TVs must have had many production cost saving steps to have a competitive edge in price. So, this volume will deal with the effort put into semiconductor packaging. There are many things to talk about, but today I will deal with the story of materials.

Semiconductor and Gold Wire

First, let’s look at the materials used in semiconductor packaging. Packaging needs several kinds of material. There are substrates like a leadframe or a printed circuit board (PCB), and there are the epoxy mold compound (EMC), die adhesive, gold wire, and more. The companies producing the materials put in a lot of effort to offer competitive prices. So, how can the company doing the packaging reduce the material cost?

First, there is the gold wire. This electrically connects the chip to external I/O sockets. You know the saying ‘the price of gold’? It is literally that. Expensive! The price of gold has been increasing steadily over the last 30 years. As a company making semiconductor packages, you wouldn’t want the price of the package rising as gold prices increase. It helps to reduce costs by using as few gold wires as possible.

▲ Change in Gold Price / Price Comparison Per Diameter of Gold Wire (Photo Source: (left) https://goo.gl/eIIPGG/ (right) https://goo.gl/2D2TkE)

The thickness of human hair is usually 50 to 100 microns. In contrast, the gold wires used in packaging are much thinner, typically less than 25 µm. Thinner wire means a more competitive edge to the price. However, it can’t get thinner indefinitely. The electrical characteristics get worse as the wire gets thinner, and the wire becomes susceptible to sweeping or movement during the molding stage, resulting in short circuits with adjacent wires.

▲ Wire Cost Savings Copper vs. Gold / Various Wire Materials

 

Gold is costly, but there are cheaper materials. The most competitive one is copper (Cu). How can copper ever be as expensive as gold? Because copper can be oxidized easily and harder than gold, you must pay more attention to the process and this can raise the total cost of using copper. There are many other materials with competitive prices, too.

Beside changing the material, you can use less gold wire. This can be done by connecting the wire as near to the chip as possible. In addition to wires, you need some type of adhesive to glue the chip onto the substrate. Epoxy is chosen for this kind of work, but some liquid epoxies will flow over to the outside of the chip during the curing stage. If the wire is bonded too close to the chip and some epoxy gets on it, then it might not be possible to achieve proper bonding. Using die attach film (DAF) instead of epoxy enables us to shorten the wire considerably. Also, reducing the thickness and width of the wiring can shorten the wire too. However, reducing those usually means higher cost for making the substrate, so you must design taking all these aspects into consideration.

▲ Location Comparison for Bond Finger Epoxy VS DAF / Comparison of Wire Length Per Bond Finger Pitch

Semiconductors and Substrates

The second topic I want to talk about is the substrate. One of the current packaging design trends is using a coreless substrate – one without a core. The traditional substrate design has a thick core with wiring layers on either side of it. If there is no core, both the price and the thickness of the entire package decreases, so quite a lot of interest is being shown in this technique. However, there’s no such thing as a free lunch. Not having a comparatively hard core can affect the warpage and can also cause issues with electrical properties. So, there have to be evaluations of package performance from various angles at the same time.

More wiring layers adds more effort to the production process, so the cost goes up. Reducing the number of wiring layers can also help reduce the production cost. One way to do this is with an embedded trace substrate (ETS), a coreless substrate technique. It uses a pre-impregnated or approach pre-preg instead of a core and can also reduce the number of wiring layers, so it has recently come into the substrate spotlight.

▲ Comparison of Cross-section between Cored and Coreless Substrate / Cross-sectional Structure of ETS (Photo Source: (left) https://goo.gl/8VNN1I/ (right) https://goo.gl/wy8RWd)

 

Once, I saw a whiteboard featuring the words ‘Save the Production Cost or Die’ in a material supplier’s office. It scared me a little, but then saving the production cost is not just an option but the only way to stay competitive. So, it must be conveyed as an expression of urgency and resolution. When the production costs are taken seriously, the price tag drops or allows end product manufacturers to add more features in home electronics products like an UHD TV as I noted earlier. It reminds me once again that reducing the production costs is a matter of grave importance in field work. I talked about materials in this volume, and for the next one we’ll take a look at techniques used during the manufacturing process. See you in the next issue!

WRITTEN BY Gyuik Jeong. It is already 10 years since my ambitions lead me to Amkor. They say 10 years is enough to change the face of the earth, but I wish for my mind to stay curious and amazed at each and every project like a new recruit, so the work is always enjoyable.