The back-end process: Step 4 - Die attach
Today's challenges
BY PETER BÜHLMANN AND DOMENICO TRUNCELLITO

In back-end semiconductor manufacturing, the die attach process is a critical step. In simple terms, die attach is picking a chip from a wafer and placing it onto a substrate or metal lead frame. The way the chip is bonded defines the die attach process - epoxy, soft solder, eutectic and flip chip are the most widely used techniques. Die attach seems to be a simple process step in the semiconductor manufacturing chain. However, the continuously escalating requirements of today's applications create difficult challenges in die bonding.

Typical Die Attach Process Flow

In high-volume production, die attach is performed on fully automated assembly equipment. The basic die attach steps, some of which are performed simultaneously, are:

* A robotic loader picks up a lead frame from a stack and places it on the input area of the workholder.
* The lead frame is moved from the input position to the dispense position. Depending on the required placement accuracy, mechanical or optical alignment points are used to define the dispense position. Epoxy is dispensed in a pattern and volume appropriate for the chip size.
* A sophisticated vision system inspects the lead frame, dispensing pattern and bond pads before the substrate is transported to the bonding position.
* In the meantime, a pattern recognition system locates a good die on the sawn wafer.
* A vacuum pick-up tool mounted on a bond head grabs the aligned die from the wafer and places it on the programmed and pre-dispensed bond position on the substrate.
* Appropriate bonding time and bonding force result in a strong bond, according to the specified process requirements. An additional optical inspection is performed to ensure that placement position and epoxy bleed-out requirements are met.
* Each bond pad on the lead frame or substrate goes through this process before it is unloaded into an output magazine.

Pressed Ceramic Packages
Pressed ceramic packages are usually a three part construction:
base, lid and leadframe. The base and lid are manufactured
in the same manner by pressing ceramic powder into
the desired shape and then firing. Glass is then screened
onto the fired base and lid. The glass paste is then fired. During
package assembly, a separate leadframe is embedded
into the base glass. The hermetic seal is then formed by
melting the lid glass over the base and leadframe combination.
This seal method is referred to as a frit seal, and therefore
this package is often called a glass frit seal package.
The pressed ceramic packages are typically lower in cost
than the multilayer packages. However, the simple construction
does not allow for many electrical enhancements.
Metal Can Packages
Metal can packages consist of a metal base with leads exiting
through a glass seal. This glass seal can be a compression
seal or a matched seal. After device assembly in the
package, a metal lid (or can) is resistance welded to the
metal base forming the hermetic seal. The metal can packages
are usually low leadcount, less than 24 leads, and low
in cost. Certain outlines, such as the TO-3, have very low
thermal resistance. These packages are used in many linear
and hybrid applications.
Hermetic Package Configuration
National offers DIP configurations in the pressed ceramic
(Cerdip) package style as well as the multilayer ceramic
sidebrazed (SB) package. Other through hole package
styles are the metal can packages (TO) and the ceramic pin
grid array (CPGA) packages.
Many ceramic surface mount packages are offered by National.
Dual in line packages such as the pressed ceramic
Cerpack, and the multilayer ceramic flatpack are available in
the lower leadcounts. These packages typically have a lead
pitch of 50 mils. Applications requiring higher lead density
use quad packages such as the multilayer ceramic quad flatpack
(CQFP) and the pressed ceramic Cerquad. Lead pitch
of 25 or 20 mils (or the metric equivalent 0.65 mm and
0.50 mm) are most common for the quad packages.
The following table provides configuration and characteristic
data regarding each of the ceramic and metal can packages
offered by National.
August 1999
Hermetic Packages
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