Die stacking and miniaturising with Die attach films

With the advancements on small packages and mobile devices, 2.5D and 3D packages have become more and more popular. As a result we are seeing more stacked devices, thinner wafers and thinner dies than ever.

The package trends are towards tight D/P ratios, thinner packages that are less than 0.30mm and wafers that are thinner than 75um. These devices are meant for high density and SiP packages that are leading the way for greater efficiency in low voltage applications.

The die attach market has a lot of different competing technologies. Whether it is die attach pastes, die attach films, solders, silver glass or (semi)sintering pastes, they all fight the same turf war with some advancing more in some fronts than others.

Die attach materials

Die attach pastes are mainly used for discretes, ICs and MEMS. We use this tried and proven technology for low or medium power dissipation on small/medium die sizes. This slow, self filleting solution requires dispensing and great accuracy when placing the die, in order to avoid tilt.

Die attach films are for medium and large die sizes with the same, medium amount of power dissipation. These products are mainly destined for memory die stacking, MEMS and thin dies. They come in conductive and non conductive versions that we laminate on the wafer level.

Finally semi-sintering, the up and coming “trending” technology is used for high power dissipation discretes and power modules.

Honorable mention to solder (normal and eutectic) that is in principle used for discrete applications and the niche silver glass options that are used for high power military and aerospace applications regardless of die size.

All of these options have their pros and cons. Suitable for different die sizes, warpage tolerances, adhesion to various substrates, conductivity (or lack thereof) and controlling the flow.

Why do we want to control the flow?

Miniaturisation, our subject matter, is the leading factor for controlling the flow. With smaller and tighter devices it’s easier to contaminate adjacent wirebond pads if the adhesive bleeds or overflows on the board. This same issue can happen while die stacking and can overflow and cover the chip’s bonding pads. Warpage, bond line thickness and of course die tilt can also negatively affect die stacking.

Controlled flow plays an important role when working with thin devices. There might be issues with the fillet height, the die coverage and suck off induced warpage can potentially interfere with any stacking attempts. Generally, inconsistent fillets reduce the reliability for all the aforementioned reasons.

In practice, controlling the flow means minimal fillet size. Or in more simple terms, zero adhesive overflow on the side of the die. In the end, this has three major advantages:

Enables the use of thin die,
Allows us to use the same die with a smaller footprint,
We can use a larger die in the same footprint as before.

It generally saves space that we utilise however we deem necessary. More silicon, more flexibility and space for other packages.

Die attach films

Die attach films are the primary solution to control the uniformity of the bondline thickness and to essentially eliminate the fillets. They use solid resins and exhibit minimal squeeze out and minimal outgassing upon cure. The BLT remains stable before and after cure and they adhere great across multiple surfaces, making them a great solution to the dispensing challenges.

Classic solutions all have issues with advanced packaging since controlling the flow is almost impossible. That’s why films are practically the best way to stack dies and to form packages such as memories.Those conductive and non conductive films are applied on the wafer and ensure perfect fillets and uniformity.

The main application steps are similar for both dicing die attach and conductive (also dicing) die attach films:

Laminate the die attach film on the wafer
Dice the wafer
Remove the dicing tape with either UV or PSA peel
Pick up the dies
Check the dies
Place the dies
Cure
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Profit

Those films come in either 8″ or 12″ circles that perfectly align with the standard wafer sizes and can be cured or skip cured.

The typical adhesive thickness starts from 10um but it can also come in thicker versions. Alternatively multiple films can be stacked on top of each other to achieve your desired BLT. There’s no dispensing, no pressure and generally, from a process point of view, it is a much cleaner process. Finally, the dicing tape comes with either a UV release or just a pressure sensitive adhesive.

How can we help you?

We offer a wide range of Conductive and non conductive die attach films such as the industry leading, conductive, low warpage, universal film CDF625P that covers a wide die size range up to 10x10mm. In addition we offer, among others the non conductive dicing die attach film ATB120U that is used for chip on chip processes or even glass attach and absorbs the CTE mismatch between substrates.

As mentioned before, controlling the flow, attaching the die and optimising a package is not limited to one product category. We offer die attach pastes, semi sintering pastes, wafer backside coatings and B-stageable adhesives that can all be used for the same purpose.