The back-end process: Step 3 - Die attach step by step
03/01/2001
By Beat Mueller
In the past, most integrated circuit (IC) packages used wirebonding as the interconnect technology between chip and leadframe. Typically, several heavy wirebonds were used for power applications to give an appropriate connection for the higher currents (Figure 1). These heavy wirebonds have been increasingly replaced by clip (bridge) technology, whereby all of the heavy wires are replaced by one clip that connects the leadframe to the source of the chip.
 Figure 1. Traditional interconnect examples for small-outline packages. |
With the introduction of new interconnect technologies, companies have begun to produce devices on metal leadframes with flip chip interconnect technology instead of wirebond connections. Figure 2 shows an SO-6 package using flip chip connections for all inputs/outputs (I/Os). Each I/O has a solder ball and is directly bonded onto the appropriate leadframe. Figure 3 shows a micro leadframe package (MLP) (also called leadless plastic chip carrier [LPCC], quad flatpack no leads [QFN], micro leadframe [MLF], etc.) design. Such a design uses solder balls for all of the lead connections and heat dissipation requirements; additionally, the solder balls maintain the electrical properties of the ground plate through the middle metal pad.
 Figure 2. SO-6 package. |
The sketches in Figures 2 and 3 are only a snapshot of the range of potential flip chip packaging solutions on metal leadframes. Other designs being considered are combinations of flip chip interconnect with clip/bridge attach technology to further improve the electrical and thermal properties of performance-driven applications. Clip/bridge attach technology was introduced a number of years ago and replaces several heavy wirebonds with a simple metal clip.
Technical Advantages
The advantages of a flip chip package design for small-outline (SO) devices come in the forms of electrical and thermal performance. In terms of thermal performance, the dissipation of heat is much better than when compared with a wirebonded device because of the large cross-section and good conductivity provided by the solder balls (Figure 4). Heat is conducted directly from the active side of the die through the metal leads and onto the printed circuit board (PCB). There is also heat dissipation through the back of the die to the plastic packaging. This allows higher currents with the same package size and design.
 Figure 3. MLP package. |
Electrical performance also is enhanced when wirebonds are replaced by solder balls, because solder balls have a lower resistance and can carry a higher current. This is especially important for die used in portable and battery-driven devices, where the package needs to be small and consume less power. Flip chip connected components have advantages when running at higher frequencies compared to their wirebonded counterparts. This is particularly important with the ever-increasing switching speeds of today's communication and processing components.
Cost Advantages
Using flip chip technology enables an engineer to pack larger die into the same package area compared with conventional wire bonded packaging (Figure 5). The space that is required for the wirebond on the leadframe and the epoxy bleed-out around the chip can then be used for a larger die size. This offers several opportunities for a package designer, such as adding more functions in the same device without needing to change the package type or size. This translates into cost savings in two ways: There are more functions in the same package, and using the same package means no changes and the same surface area on the PCB side.
 Figure 4. Cross-section of SO package with flip chip interconnect. |
It is also possible to move a device from a large package into a smaller and cheaper one - for example, from a small-outline integrated circuit (SOIC) to a small-outline transistor (SOT). This saves packaging costs and gives the possibility of reducing the required footprint on the PCB. Additionally, the machine concept is flexible and can still be used for a traditional epoxy die attach process.
Flip Chip Connection Technologies
The industry uses a variety of interconnect technologies for flip chips. Some use solder balls or paste, while others use conductive epoxy to connect the bond pad on the chip to the metal leadframe. Figure 6 gives an overview of some interconnect designs used for flip chip, including eutectic solder ball, high melting solder ball with low melting solder paste, conductive epoxy without bumps, solder bump with conductive epoxy, and stud bump with conductive epoxy.
 Figure 5. Size comparison of flip chip vs. wirebond. |
All of these methods have one thing in common: The processing equipment needs to have the capability to dispense very small and exact amounts of flux, solder paste or conductive epoxy (depending on the process) to achieve a reliable result and a high yield.
Flip Chip Die Attach
The key issues for a competitive flip chip on metal leadframe process are: integrated die attach equipment (including solder paste application and reflow oven), high units per hour (UPH) (to reduce the cost per produced device), and a small footprint achieved by a specially designed integrated reflow oven for leadframes.
 Figure 6. Examples of different flip chip processes. |
A fully integrated die attach and reflow platform (Figure 7) performs the following process steps inline without manual handling or interfacing among the individual steps:
- unloading the leadframe from stack
- dispensing small solder paste dots
- die pick, flip and attach
- inline reflow
- unloading of leadframe into magazine.
Solder Paste Dispense
 Figure 7. A proprietary integrated solution for flip chip assembly. |
Because of a small bump pitch, it is necessary to dispense very small solder paste dots onto the leadframe. The dot diameter should be very small with the shape closely controlled and repeatable (Figure 8). Having already integrated the stencil print onto the die attach platform, a stencil printing step before die attach is not necessary, so this can reduce the overall investment for the whole flip chip line (Figure 9). It is also possible to apply solder paste dots onto bond pads that are lower than the leadframe surface.
 Figure 8. Solder paste process for small dots. |
The existing proprietary volumetric dispensing system had to be modified and is now able to apply solder paste onto the leadframe immediately before die attach, even for small bump pitches. It is important to use an appropriate solder paste of the proper quality and consistency to achieve the best results (Figure 10).
Flip Chip Die Attach
 Figure 9. Process comparison - screen print vs. dispense. |
The module for flip chip die attach is an enhanced bond head for normal chip attach with an integrated chip flipping unit. This module allows picking of the die from the wafer, turning the die face down and attaching it onto the leadframe without any loss of throughput. It also handles the chip gently so as not to damage the die surface, edges or bumps.
Integrated Inline Reflow Oven
 Figure 10. Cross-section of bump to leadframe connection inside an SO package. |
To avoid any displacement of the attached die before the reflowing process, it is necessary to keep handling between die attach and package reflow to a minimum. Today, bonded substrates are often put into a magazine and then manually transferred to a batch oven. The substrates are also transferred over a long inline track to a horizontal reflow oven. The use of this integrated system of an inline reflow oven that is directly attached to the die place machine enables substrate handling to be reduced to an absolute minimum.
High Throughput
With a unique bond head integrated flipping mechanism, it is possible to achieve a UPH of 2,500 to 3,000 for most flip chip on metal leadframe applications. This is important because SO packages are primarily used for low-cost devices where the cost per die attach is significant and has a major influence on the resulting total package price.
BEAT MUELLER, product manager for flip chip die bonding, can be contacted at Alphasem AG, Andhauserstrasse 64, CH-8572 Berg, Switzerland; +41-71-637 63 63; Fax: +41-71-637 63 64; E-mail: [email protected].
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