Just like it says “on the tin”, wafer level chip scale packaging (WLCSP) is a technology that shrinks the substrate down to a size that is quite close to that of the actual, silicon, gallium arsenide or whatever material makes up the die. Rather than calling it a substrate, the WLCSP material is known as a redistribution layer or RDL for short. It’s a subtle but important distinction.
By definition, WLCSP devices would exclude wire bonding leaving flip chip technology as the method of die attach. That means that there is no die cavity where a solid ground plane on the bottom of the die would normally act as the mating surface. Instead, the chip is mounted face down with BGA style balls on a pitch that is typically less than 0.5 mm. Right there, the challenge can be to maintain a good thermal path through the tiny connections.
Figure 1. Image Credit: Author - One of many iterations of a test board for a WiFi/Bluetooth/FM combo device in a wafer level chip scale package.
The solution to this problem is to have numerous ground balls to help dissipate thermal energy. The ground balls can be distributed around the device or gathered into a central square or rectangle, maybe both. Either way, it’s best if every one of the ground connections gets a dedicated via rather than combining them to share a via.
Chip On Board vs. Wafer Level Chip Scale Packaging
This is the main operational difference between chip on board (COB) and WLCSP technology. When it comes to COB, you can mount either type of die on the PCB and then create the wire bond cage as required before potting the device within an encapsulant known as a glob top.
Handling a bare die is tricky to start with. The coefficient of thermal expansion (CTE) of the chip is going to be different from the CTE of the printed circuit board. The encapsulant spreads out from the chip and holds the board in place against the local effects of thermal expansion.
So, while the bare die is marginally smaller than the WLCSP, the glob top generally has to expand well beyond the limits of the chip. That means that the COB solution ends up taking more space that ends up being a circular keep-out around the die. The fragility of the bare die makes shipping and handling more likely to cause a defect at some point.
That’s where the packaging helps. It protects the chip in transit and acts as an interposer to help prevent latent failure over the lifecycle of the product. Chip on board may be used to “skip the line” when it comes to product development and the WLCSP would be cut in for the production cycle. As simple as they are, the packaging costs money and takes time to procure.
The slight increase in size from the die itself is to allow tolerance for the cap that goes over the substrate. Coming at this from a phone chip company, the difference between the size of the die and the final package is about 20% increase per side.
Why Not Just Use A Normal BGA Package?
Traditional BGA packages, whether flip chip or wirebond style, are considerably larger. For one thing, a BGA or PGA can have more than one piece of silicon to create a multi-chip module. Further, the wire bond cage or flipped die is only the beginning.
From there, a traditional substrate has actual routing that spreads out the pitch of the pins to something that enables plated through-hole vias. This is still considered a leg up when it comes to high reliability for harsh environments such as those found in the automotive or aerospace industries.
On the other hand, the WLCSP package passes the signals more or less straight through from the die pads to the bottom of the device. The term “redistribution layer” implies that the bumps on the die will not necessarily align with the package. The fact that smaller is better in terms of signal integrity and power consumption make this a compelling choice for both the marketing team and the SI/PI engineers.
Figure 2. Image Credit: Renesas - A cross section of a WLCSP device showing polyimide in green and an optional under-bump-metalization layer.
Chip companies that want to be included in mobile devices as well as challenging environments often package the same chip in WLCSP, BGA and QFN packages to get into every possible market. Once you have a working die, the development costs for the different applications are pretty reasonable in comparison.
This idea started way back when the only parts available were through-hole DIP packages using TTL or CMOS technology. They’re still around. You can buy a hex inverter in a DIP-14 package that is either plastic or ceramic depending on your temperature requirements. It makes little difference to the PCB Designer.
Fanout and Routing of a WLCSP
This is where the fun starts. The job of spreading out the high circuit density of the device now falls on the layout person. Microvias are essential for anything with nine or more pins. There are plenty of four-pin devices with a 0.4 mm pitch. The old-school PCB technology can be still used for regulators and sensors that are found in those miniscule packages.
As the pin-count increases, so does the layer count required to get the job done. Approaching 100 or more contacts puts us on full HDI boards with stacked microvias throughout the board. You only need one such device to drive the PCB cost and lead time to that level. Precise placement requirements call for a pick and place machine rather than a hands-on approach.
Each year, it seems to get harder to find CODECs and WiFi modules or other popular circuits in the larger package types. Those chip foundries only have so much bandwidth so they need to know that there is a demand for something before they try to fill the niche. The wafer level chip scale package is often the first one out of the chute even if the others are on the roadmap. Plan accordingly and think small, my friends.