Minapad 2014, May 21 – 22th, Grenoble; France

Solutions to Controlling Flow of Conductive Die Attach Adhesives

Anthony Winster

Technical Advisor Henkel Ltd

Wood Lane End Hemel Hempstead

Hertfordshire HP2 4RQ United Kingdom

Phone +44 7747 633737 e-mail tony.winster@henkel.com

Mina Chow-Taing Tech Service Engineer

Henkel Electronic Materials LLC 14000 Jamboree Road

Irvine CA 92606

USA Phone +1 626 297 7633

e-mailmina.chow-taing@henkel.com

Abstract

There is a clear trend in the semiconductor packaging industry towards the use of thinner die, and reduced size packages, driven mainly by higher functionality of hand-held and portable devices. This trend causes some challenges to the use of traditional die attach paste adhesives, as the paste spreads to form a fillet, and also risks the adhesive flowing into the top of the die. At the same time, demands for electrical and thermal performance together withreliability are increasing. What’s more, manufacturing yield continues to be crucial to control costs.

This paper compares several formats of conductive die attach adhesive - improved pastes, wafer applied pastes and wafer applied films , and then focuses on the option of silver-filled film adhesives, provided in formats suitable for wafer application.

The authors illustrate how the physical properties of the adhesive, such as adhesion and modulus, can be optimised for a range of die-sizes, leadframe materials and package types. Integration of an assembly process using conductive film can be integrated into a high volume packaging plant, using pre-cut films and dicing tapes optimised for application to wafers, and using readily available assembly tools will also be discussed.Data comparing electrical, thermal and reliability results from packages assembled with film and traditional paste adhesives will be presented

Finally, the authors will share a roadmap indicating the growing applications for conductive films, and how those films might enable future developments in semiconductor packaging and other electronics applications.

Key words: Conductive Adhesive, Film, Die Attach

Introduction

There is a well-recognized trend in electronics to incorporate increased functionality into a reduced package size (Fig. 1). This also applies to semiconductor packages, where the desire is to reduce the package footprint as much as possible, so that it approaches the size of the die inside.

Also, die are getting thinner. This reduces the package height, and can also improve performance by reducing the electrical resistance.

Typically, die are attached onto leadframes within the package using an electrically conductive paste adhesive. This die attach adhesive spreads out from the edge of the die, taking up valuable space within the package outline, and also flows up the edge of the die, to form a fillet. The fillet acts as a “reservoir” to accommodate variations in dispense volumes and paste location.

As die get thinner, and designers attempt to

reduce the fillet width, it becomes increasingly

difficult to control the adhesive dispense volume. In extreme cases, the adhesive could flow beyond the edge of the leadframe pad, or up onto the die surface.

Figure 1: Roadmap & Trends for Packaging

Potential Solutions

One obvious solution is to reduce variability in the die attach paste. This has been an ongoing effort over the past years, as manufacturers have tightened tolerances on raw materials specifications (for example particle size distribution) and developed improved mixing and dispersion methods. This has enabled reductions in variation of viscosity and consequently allowed improvements in dispense reproducibility. Unfortunately, further improvements become more difficult. For example adhesive viscosity varies with temperature (Fig. 2), so any variations in clean room temperature control will induce changes in dispense quality. Also, die attach paste is typically based on thermo-setting resins, so they have a limited storage and working life. Viscosity inevitably changes during storage and worklife.

Figure 2: Rheology of Paste

A second potential solution is to use a “self

filleting” paste. This is a die attach formulated to flow to the edge of the chip, and then stop, leaving a minimal fillet (Fig. 3). Although attractive in principle, in practice any variation in location of the

dispense pattern or the location of the die can lead to die tilt (Fig. 4).

Figure 3: Self-Filleting Mechanism

Figure 4: Self-Filleting – Misalignment Causing Die Tilt

The third solution, which is popular for

selected package types (such as “chip-on-lead”) is to print a layer of adhesive onto the backside of the silicon wafer. The paste is then dried (“B-staged”) and the wafer can then be diced. Each die can be picked together with its die attach adhesive, and placed on a leadframe using heat and pressure. Because the adhesive is now in solid form, the flow during attachment is minimal – typically less than 50 micron. This “wafer backside coating” (WBC) process (Fig. 5) works well if the wafers are relatively thick and robust. However, wafers thinner than about 100 micron are difficult to handle on a screen or stencil printer.

Figure 5: Wafer Backside Coating Process

The fourth solution is to fabricate the die

attach adhesive as a dry film which can be laminated onto the wafer backside. After lamination, the adhesive behaves similarly to WBC adhesive, and can be diced together with the wafer. Film adhesives are already extensively used in assembly of memory die. However, these films have no electrical conductivity, and are not suitable for use on metallised wafers.The following information relates

to relatively new die attach films which are electrically conductive.

Conductive Die Attach films

Figure 6: Format of Pre-Cut Film

Two pre-requisites for die attach film are that

the film should be supplied “bundled” together with a suitable dicing tape (Fig. 6), and pre-cut into circles appropriate to the wafer diameter (Fig. 7). This gives compatibility with existing industrial equipment, and minimizes additional processing.

Figure 7: Concept of Pre-Cut Film

In considering the formulation of a film,

properties should be optimized for certain applications and performance criteria

1. Dicing Tape Small die (typically less than 2mm x 2mm) need a high tack dicing tape, to reduce the risk of die fly during dicing. However, larger die need a lower tack tape, to enable easy die pick-up. 2. Flow (uncured state). Film flow needs to be minimal, but some flow is essential to allow good wet-out to the substrate. Organic substrates tend to be much rougher than metal leadframes, so the film needs to flow to fill in the roughness. 3. Modulus (after final cure) Small die need a high modulus die attach to enable fast wirebonding. Larger die, however need a lower

modulus to absorb stress caused by dissimilar expansion of die and substrate. 4. Conductivity Some die need a die attach adhesive with good electrical and thermal conductivity – but for others this is not critical. Therefore, it is impractical to have one single film formulation which will suit all die and package types.

Project A1 – Precut Film as Alternate to Paste

The following data compares a pre-cut conductive film to a conventional silver-filled paste. The A1 film was formulated and manufactured with a 15 micron dried thickness. Bulk properties of the film were compared to a standard die attach paste – and showed broadly equivalent results. However, it is worth noting that the hot die shear and the modulus at high temperature are significantly higher for the A1 film (Fig. 8).

Figure 8:Table of A1 material properties

A1 cDAF film was then laminated onto the

wafer backside using a standard laminator at a temperature controlled between65°C to 70°C. Then the wafer was diced, and die attach carried out onto metal leadframes. 7.0mm x 7.0mm QFN packages were assembled, using three types of leadframe (bare Cu, Ag/Cu and PPF/Cu), and two die sizes (2.0mm x 2.0mm and 5.0mm x 5.0mm). These were subjected to MSL testing. The 2.0mm x 2.0mm diesize easily achieved Level 1. The 5.0mm x5.0mm showed some failures at Level 1, but achieved Level 2. (Fig. 9). This is broadly equivalent to the authors experience with a QFN built with paste die attach adhesives.

Figure 9:A1 MSL results

Therefore, Project A1 provides a film with performance at least as good as a paste, but with the benefits of controlled flow.

Project A2 – Film with Higher Conductivity

The film produced in Project A1 has thermal and electrical properties roughly equivalent to paste die attach. The next stage of the work was to attempt to improve these properties, so as to enable the assembly of higher power devices. This was achieved by reformulating the film to allow the use of a more conductive filler system, designated “Project A2”

Past experience suggests that results from measurements of bulk properties of die attach do not always correlate with results of “in-package” tests. Usually, this is because the effect of interfacial resistances cannot be estimated from bulk measurements. Therefore, packages were assembled containing test die, and were then measured for RdsON and thermal resistivity. The thermal resistance and RdsON of packages built with A2 film was significantly improved compared to A1 film; and equivalent to packages built with the best paste and WBC adhesives. (Fig. 10 and Fig. 11).

Figure 10: ProjectA2 Thermal results

Figure 11:Project A2 Electrical results

Project B – Film for Large Die

One of the limitations of cDAF from project A1 was that good MSL reliability could be achieved only with small die sizes. For automotive applications, larger die sizes are needed (up to around 8.0mm x 8.0mm), although high thermal and electrical conductivity are not of major concern. Therefore a cDAF formulation was developed with lower modulus (Fig. 12), to reduce stress with the package during solder reflow.

Figure 12:Project B Modulus data

Again, 7.0mm x 7.0mm QFN packages were assembled and subjected to MSL testing. Level 1 was achieved for die up to 5.0mm x 5.0mm (Fig. 13), which is a significant improvement overcDAF Project A1

Figure 13:Project B MSLdata

Project C – Film for Laminate Substrates

So far, all the work described was carried out with metal leadframes. However, there is also a need to use die attach with minimal flow on laminate substrates – typically because of space restrictions between die and wirebond pads. Usually, surface roughness of laminate substrates is significantly higher than a metal leadframe surface, so the cDAF needs to flow slightly more to give complete wetting on laminates.

Therefore, the resin composition of the cDAF

film was modified to reduce the melt viscosity (Fig. 14).Film thickness was increased to 25 microns.

Figure 14:Project C Melt viscosity

Next, 10.0mm x 10.0mm die were attached to

organic substrate for 15 x 15 SCSP packages (Fig. 15).

Figure 15: Laminate Substrate for 10mm x 10mm die

Die were attached at 130°C with a force of

1.5Kg and a time of 1.5 sec. The adhesive squeeze-out beyond the edge of the die was never greater than 40 micron.

After curing at 175°C, diewarpage was

around 60 micron. Industry standard paste adhesives give similar warpage (around 50 micron).

The parts showed good results in MSL Level 2 testing (Fig. 16).

Figure 16: TSAM of 10mm x 10mm die at MSL1

Ongoing Work

The authors plan to continue development work on next-generation films which will have “in-package” thermal resistances comparable to solder.

In addition, optimised parameters for laser

dicing, which will extend the use of films from silicon to other materials such as Gallium Arsenide, will also be studied. Preliminary results with laser dicing are encouraging (Fig 17)

Fig 17. Preliminary Results on Laser Dicing

Industrialisation & Cost of Ownership

In parallel with the development work on the

four cDAF formulations described, equipment was developed and commissioned to manufacture the films in a format compatible with industrial lamination equipment (Fig. 18). All formulations can be manufactured for 200mm and 300mm wafers sizes, and the technology is suitable for production of other customized shapes if required.

Fig 18. Configuration of Pre-Cut Rolls

All the test parts prepared for this paper were assembled on industrial die attach equipment, with readily available die pick-up and placement tooling. Examples shown in Fig. 19

Fig 19. Die attach Tooling

Summary

Conductive die attach films offer solutions to

many of the miniaturisationchallenges faced by semiconductor packaging specialists. These materials provide controlled bondline thickness, minimal die tilt and controlled flow. (Fig. 20) These features allow reduced package size and enhanced reliability.

Figure 20: Cross Sections with Paste & Film In addition, it has been demonstrated that

films can be formulated with a range of properties optimized for die-size, substrate type, and electrical and thermal conductivity. (Fig. 21)

Figure 21: Conductive Films Space

Films can be supplied bundled together with

dicing tape, and are compatible with assembly equipment designed specifically for semiconductor packaging.

Overall, the development of a selection of

conductive film adhesives allows this die attach technology to be used on a wide range of package types, thereby enabling a reduction in package sizes and / or an increase in device functionality. (Fig. 22)

Fig 22. Shrinkage of Packages or Increased Functionality