CSP - Chip Scale Package

Chip Scale Package (CSP)

Chip Scale Package, or CSP, based on IPC/JEDEC J-STD-012 definition, is a single-die, direct surface mountable package with an area of no more than 1.2 X the original die area. The acronym 'CSP' used to stand for 'Chip Size Package,' but very few packages are in fact the size of the chip, hence the wider definition released by IPC/JEDEC.

The IPC/JEDEC definition likewise doesn't define how a chip scale package is to be constructed, so any package that meets the surface mountability and dimensional requirements of the definition is a CSP, regardless of structure. For this reason, CSP's come in many forms - flip-chip, non-flip-chip, wire-bonded, ball grid array, leaded, etc.

Because of this variety of chip scale packages developed in the industry, one can not make any generalized assumptions on the manufacturability or reliability of the CSP as a homogeneous package group. It is often necessary to determine what the structure of the CSP is before any conclusion on its robustness or manufacturability can be made.

Figure 1. Small size is the main advantage of CSP's. Note how Xilinx and Philips used a pencil (left) and a cell phone key pad (right), respectively, to illustrate this.

In an effort to systematically characterize the CSP as a package group, some quarters have come up with four (4) classifications or types for the CSP. These are: 1) the flex circuit interposer type; 2) the rigid substrate interposer type; 3) the custom leadframe type; and 4) the wafer-level assembly type.

The advantages offered by chip scale packages include smaller size (reduced footprint and thickness), lesser weight, relatively easier assembly process, lower over-all production costs, and improvement in electrical performance. CSP's are also tolerant of die size changes, since a reduced die size can still be accommodated by the interposer design without changing the CSP's footprint.

Chip scale packaging can combine the strengths of various packaging technologies, such as the size and performance advantage of bare die assembly and the reliability of encapsulated devices. The significant size and weight reduction offered by the CSP makes it ideal for use in mobile devices like cell phones, laptops, palmtops, and digital cameras.

Figure 2. Example of a Wafer-Level CSP from Maxim;

note the bumps on the die

CSP's are generally built using a lead frame, wherein many devices can be contained on the same substrate, allowing the assembly of many packages in bulk. Doing so maximizes the use of interposer area.

A typical chip scale packaging process starts with the mounting of the die on the interposer using epoxy, usually of non-conductive type (although conductive epoxy is also used when the die backside needs to be connected to the circuit). The die is then wirebonded to the interposer using gold or aluminum wires. Wirebond profiles must be as low and as close to the die as possible in order to minimize package size.

Plastic encapsulation to protect the die and wires then follows, usually by transfer molding. After encapsulation, solder balls are attached to the bottom side of the interposer, after which the package is marked. Finally, the parts are singulated from the leadframe.

Figure 3. Cross-section of a Wirebonded CSP

Figure 4. Cross-section of a Flip-chip CSP

The chip scale package is relatively new, so industry standards for producing CSP's have not yet been completely developed. Nonetheless, the Institute for Interconnecting and Packaging Electronic Circuits (IPC) has already released J-STD-012, "Implementation of Flip Chip and Chip Scale Technology." This document discusses technology overview and design considerations, as well as material, processing, mounting, interconnection, reliability, and standardization aspects of CSP manufacturing.

The industry is moving towards the development of more chip scale packaging standards. Some of the new standards being developed as of this writing are, in fact, defined by J-STD-012. These standards are shown in Table 1.

Table 1. Flip Chip and CSP Standards Currently Undergoing Development

Std No. 102	Mechanical outline Standard for Flip Chip or Chip Scale Configurations
Std No. 103	Performance Standard for Flip Chip/Chip Scale Bumps
Std No. 104	Test Methods for Flip Chip or Chip Scale Performance
Std No. 105	Flip Chip/Chip Scale Carrier Tray Standard
Std No. 106	Bare Dice as Flip Chip or Chip Scale Configuration Management Standard
Std No. 107	Design Standard for Flip Chip and Chip Scale Mounting Structures
Std No. 111	Design Standard for Flip Chip/Chip Scale Assembly Configuration
Std No. 112	Standard for Flip Chip/Chip Scale Assembly Performance Requirements
Std No. 113	Test Methods for Qualification and Evaluation of Flip Chip/Chip Scale Assemblies
Std No. 114	Standard for Flip Chip/Chip Scale Assembly Rework and Repair Techniques
Std No. 115	Flip Chip/Chip Scale Assembly Reliability Standard
Std No. 120	Qualification and Performance Standard for Flux used in Flip Chip Assembly

Wafer-level Packaging; Flip Chip Assembly; BGA; IC Manufacturing; Assembly Equipment

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