Orientación a Largo o a Corto Plazos (LTO)

Ball grid array (BGA) is a kind of high-density surface mount package.^[4]At the bottom of the package, pins are all ball-like and arranged in rows of a grid similar to a lattice pattern;

hence the name BGA. Figure 5.9 shows a typical BGA package and pads on a board.

98 Chapter 5 Device-level Packaging

Figure 5.9 Typical BGA package

The BGA package is a kind of ball array package with round or column solder joints formed by its I/O pins distributed in an array below the package. The large pitch and short lead length may avoid the coplanarity and warp caused by wires in fine-pitch devices. The structure of a typical BGA package is shown in Figure 5.10.

Plastic Chip Metal line

Solder ball Via Substrate Figure 5.10 Typical structure of BGA package

BGA is one of the most popular ways to interconnect between IC and PCB. The most remarkable feature of BGA is that it is still applicable in the current SMT process for BGA devices with I/O numbers over 200. SMT’s basic process is reflowing, which has proved suitable for assembling BGA devices. Although the time and temperature curve of BGA assembly corresponds with the standard curve of the SMT process, the special characteristics of this type of package should be kept in mind for this application. This is particularly important since solder joints of BGA, as opposed to the traditional devices, are usually below the device, between the device and the PCB. Therefore, the influence on the internal structure of materials is more significant than most traditional package forms. For this reason, it is necessary to decide reflow parameters with the measured value of BGA solder joint temperature as the reference value.

Features of the BGA package:

(1) Although the number of I/O pins increases, the pitch is wider than QFP, thus im-proving the assembly yield.

(2) The thickness is half that of QFP, while the weight is cut by more than three-fourths.

(3) Decreasing the parasitic parameters, such as signal transmission delay, greatly im-proves the frequency.

(4) Coplanar assembly soldering with high reliability is available.

(5) Like QFP and PGA, the occupied area of BGA is still oversized.

(6) Higher I/O numbers with larger pitch eliminates the problems of production cost and reliability for QFPs with high I/O numbers.

The BGA package family has a lot of members. They are not only different in size and I/O number, but also different in physical structure and package material.

5.5 Typical Examples of Device-level Packaging 99

1. Plastic BGA (PBGA)

PBGA is the most popular package in production. Figure 5.10 shows a typical plastic BGA package device. Its main features are

(1) Glass fiber and bis-maleimide-triazine (BT) resin substrate, with thickness about 0.4 mm.

(2) The chip is directly bonded onto the substrate.

(3) The chip is connected to the substrate by wire bonding.

(4) Molded plastic could encapsulate the chip, the interconnection wires, and the majority of the substrate surface.

(5) Solder balls (usually eutectic material) are soldered on the pads at the bottom of the substrate.

However, the area coverage of the substrate by plastic in this package should be considered carefully. In some cases of PBGA packaging, molding plastic will cover almost the whole substrate, while for others, the coverage is limited to a central area. This will affect the heat exposure of solder joints.

2. Ceramic BGA (CBGA)

The multilayer substrate with a metal interconnecting pattern is the most basic, material for CBGA. Its substrate is made of ceramic, and the package cover is made of aluminum.

The sealing quality of this kind of package is the most influential factor to the thermal conductivity that can pass through the package. A variety of materials are used for the package “cover.” An unfilled space may block the heat exposure of under-package solder joints. Though the power consumption is increased in this way, BGA can use controlled collapsed chip connection (C4) to improve its electrical thermal performance.

3. Enhanced BGA

The word “enhance” in enhanced BGA means enhancement of performance by adding some material to the structure. In general, the material added is metal that can improve IC thermal dissipation during operation. This is quite important because one of BGA’s advantages is that it can provide a large number of I/Os for IC. Heat dissipation must be designed in detail for this kind of package, because chips packaged in this way usually generate much heat in a small space.

Special enhanced BGA, called super BGA (SBGA), features a structure with an inverted copper cavity attached to the top of the package to improve the thermal dissipation to the surroundings, as shown in Figure 5.11. On the bottom of the copper piece is a soft and thin substrate, which is used as the pads attached with solder balls along the side. The inner wires connect the substrate with the chip pads. The chip is plastic packaged from the bottom. Obviously, this structure will be greatly helpful for the thermal dissipation of the chip.

Chip

Metal line Plastic Substrate

Cooper ring Solder ball Copper heat sink

Figure 5.11 Cross section of SBGA

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4. Tiny BGA Package

Tiny ball grid array (BGA) provides a ratio of chip: package area no less than 1:1.14. The application of this technology can increase the memory capacity by 2 to 3 times without changing the size of all DRAM memory in computers. Tiny BGA has replaced traditional thin small outline packaging (TSOP, which is the representative of second-generation mem-ory package technology that appeared in 1980s and features making pins around the packaged chip) with smaller volume and better thermal dissipation and electrical performance.

Like the technology for packaging the microprocessor, TSOP was no longer applicable for the next generation memory with high frequency and high speed, along with the development of packaging technology. New packaging technology, with tiny BGA and bottom leaded plastic (BLP) as representatives, gradually developed. Memory products with tiny BGA packages reduce the volume to a third that of TSOP packages while keeping the same capacity. In tiny BGA packaged memory products, I/O leads are drawn from the center of the chip and can effectively reduce the distance of signal transmission and signal attenuation.

Meanwhile, this type of package significantly improves not only the performance of anti-interference and anti-noise, but also the electrical properties of chip; tiny BGA packaging is very thin in size, with a height less than 0.8 mm, and the effective route from metal substrate to thermal sink is only 0.36 mm. Therefore tiny BGA memory has better thermal conductivity and is applicable for long-term operation systems with excellent stability. In the bottom leaded plastic (BLP) package, its ratio of chip:package area is 1:1.1. Not only are the height and area reduced, the electrical characteristics are also improved. In addition, production costs are also affordable. Hence BLP technology is becoming widely used.

5. Future Trends

Although BGA packaging is more advanced than QFP, its ratio of chip area to package area is still not ideal. Tessera had made some improvement based on BGA and developed another package technology namedμBGA, whose chip: package area ratio is close to 1:1 with 0.5 mm pitch, really a big step from basic type of BGA. The typical structure of Tessera’s μBGA is shown in Figure 5.12.

Chip

Outer frame

Wire

Polyimide Solder ball Elastic substrate Figure 5.12 Tessera’sμBGA

In September 1994, Mitsubishi Electric of Japan developed a new package structure with a chip: package area ratio to 1:1.1. Its package size is only slightly bigger than that of a bare chip. That means a package size is similar to the size of a single IC chip. This new package is called a chip scale package (CSP).