Ball grid array


Ball grid array
Intel Embedded Pentium MMX (bottom view)

A ball grid array (BGA) is a type of surface-mount packaging used for integrated circuits.

Contents

Description

BGA ICs assembled on a PCB

The BGA is descended from the pin grid array (PGA), which is a package with one face covered (or partly covered) with pins in a grid pattern. These pins conduct electrical signals from the integrated circuit to the printed circuit board (PCB) on which it is placed. In a BGA, the pins are replaced by balls of solder stuck to the bottom of the package. These solder spheres can be placed manually or with automated equipment. The solder spheres are held in place with a tacky flux until soldering occurs.[1] The device is placed on a PCB with copper pads in a pattern that matches the solder balls. The assembly is then heated, either in a reflow oven or by an infrared heater, causing the solder balls to melt. Surface tension causes the molten solder to hold the package in alignment with the circuit board, at the correct separation distance, while the solder cools and solidifies.

Advantages

High density

The BGA is a solution to the problem of producing a miniature package for an integrated circuit with many hundreds of pins. Pin grid arrays and dual-in-line surface mount (SOIC) packages were being produced with more and more pins, and with decreasing spacing between the pins, but this was causing difficulties for the soldering process. As package pins got closer together, the danger of accidentally bridging adjacent pins with solder grew. BGAs do not have this problem when the solder is factory-applied to the package.

Heat conduction

A further advantage of BGA packages over packages with discrete leads (i.e. packages with legs) is the lower thermal resistance between the package and the PCB. This allows heat generated by the integrated circuit inside the package to flow more easily to the PCB, preventing the chip from overheating.

Low-inductance leads

The shorter an electrical conductor, the lower its inductance, a property which causes unwanted distortion of signals in high-speed electronic circuits. BGAs, with their very short distance between the package and the PCB, have low inductances and therefore have far superior electrical performance to leaded devices.

Disadvantages

X-ray of BGA

Noncompliant leads

A disadvantage of BGAs, however, is that the solder balls cannot flex in the way that longer leads can, they are not compliant. As with all surface mount devices, bending, due to a difference in coefficient of thermal expansion between PCB substrate and BGA (thermal stress), or flexing and vibration (mechanical stress) can cause the solder joints to fracture.

Thermal expansion issues can be overcome by matching the mechanical and thermal characteristics of the PCB to those of the package. Typically, plastic BGA devices more closely match the PCB thermal characteristics than ceramic devices.

Mechanical stress issues can be overcome by bonding the devices to the board through a process called "under filling", which injects an epoxy mixture under the device after it is soldered to the PCB, effectively gluing the BGA device to the PCB. There are several types of under fill materials in use with differing properties relative to workability and thermal transfer. An additional advantage of under fill is that it limits tin whisker growth.

Another solution to non-compliant leads is to put a "compliant layer" in the package that allows the balls to physically move in relation to the package. This technique has become standard for packaging DRAMs in BGA packages.

Difficulty of inspection

Once the package is soldered down, it may be difficult to look for soldering faults. X-ray machines, Industrial CT Scanning machines[2] , and special microscopes have been developed to overcome this problem. If a BGA is found to be badly soldered, it can be removed in a rework station, which is a jig fitted with infrared lamp (or hot air), a thermocouple and a vacuum device for lifting the package. The BGA can be replaced with a new one, or the BGA can be refurbished (or reballed) and re-installed on the circuit board. Packets of tiny ready-made solder balls are sold for reballing BGA's.

Due to cost of X-ray BGA inspection, electrical testing is very often used. Very common is boundary scan testing using IEEE 1149.1 JTAG port.

Difficulties with BGAs during circuit development

During development it is not practical to solder BGAs into place, and sockets are used instead, but tend to be unreliable. There are two common types of socket: the more reliable type has spring pins that push up under the balls, although it does not allow using BGAs with the balls removed as the spring pins may be too short.

The less reliable type is a ZIF socket, with spring pinchers that grab the balls. This does not work well, especially if the balls are a bit small.

Cost of equipment

Expensive equipment is required to reliably solder BGA packages; hand-soldering BGA packages is very difficult and unreliable, usable only for the smallest packages in the smallest quantities.

Variants

  • CBGA and PBGA denote the Ceramic or Plastic substrate material to which the array is attached.
  • CABGA: Chip Array Ball Grid Array
  • CTBGA: Thin Chip Array Ball Grid Array
  • CVBGA: Very Thin Chip Array Ball Grid Array
  • DSBGA: Die-Size Ball Grid Array
  • FBGA or Fine Ball Grid Array based on ball grid array technology. It has thinner contacts and is mainly used in system-on-a-chip designs.
    • Known as FineLine BGA by Altera.
  • FCmBGA: Flip Chip Molded Ball Grid Array
  • LFBGA: Low-profile Fine-pitch Ball Grid Array
  • UFBGA and UBGA and Ultra Fine Ball Grid Array based on pitch ball grid array.
  • MBGA: Micro Ball Grid Array
  • PBGA: Plastic Ball Grid Array
  • MCM-PBGA: Multi-Chip Module Plastic Ball Grid Array
  • TEPBGA: Thermally Enhanced Plastic Ball Grid Array
  • SuperBGA (SBGA): Super Ball Grid Array
  • TABGA / TBGA: Tape Array BGA
  • TFBGA or Thin and Fine Ball Grid Array

See also

References

  1. ^ Soldering 101 - A Basic Overview
  2. ^ "CT Services - Overview." Jesse Garant & Associates. August 17, 2010. http://www.jgarantmc.com/ct-services.html

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