BGA is short for Ball Grid Array Structure. It is one of the most common ways to interconnect integrated circuits (ICs) and printed boards using an organic carrier board packaging method.
BGA package technology is improved from the pin grid array, is a surface to a grid arrangement of the way covered (or partially covered) with pins of the packaging method, in the operation of the electronic signals from the IC can be transmitted to the printed circuit board where it is located (printed circuit board, hereinafter referred to as PCB).
BGA packages offer more pins than other packages such as dual in-line packages or four side pin flat packages. The entire bottom surface of the device can be used as pins instead of just the periphery, and has a shorter average wire length than the peripheral limited package types for better high speed performance. Soldering BGA packages requires precise control and is usually done by factory equipment with automated processes; BGA packages are not suitable for slot mounting.
BGA Classification
BGA packages come in a variety of forms, forming a family of packages that differ not only in size and number of IOs, but also in physical structure and package materials. A specific form of BGA can have a range of sizes, but should use the same physical construction and the same materials. The following will focus on analysing three specific BGA packages, each with a different form of construction.
1, plastic BGA : Plastic Ball Grid Array Package (PBGA) is the most common form of BGA package in production today. Its attractive advantages are:
* glass fibre and BT resin substrate, about 0.4mm thick
*Chips are soldered directly to the substrate
*The chip is connected to the substrate by wires.
*Plastic moulding encapsulates most of the chip, wire connection and substrate surface.
Solder balls (usually eutectic material) are soldered on the bottom pad of the substrate. However, one parameter that is not universal is the area coverage of the moulding relative to the total area of the substrate. For some plastisols, the moulded plastic almost completely covers the entire substrate, in contrast, some are strictly limited to a small area in the centre. This will also have an impact on the heat applied to the solder joints.
2, ceramic BGA (CBGA): For any ceramic IC package, the most basic material in the ceramic BGA is a multilayer substrate of precious metal interconnect circuit. This type of package sealing for heat conduction through the package has the greatest impact. The package "lid" can be made of a variety of materials, and there is usually an unfilled space underneath the "lid". This gap will hinder the package body lower solder joints heat.
3, "Enhanced" BGA: "Enhanced" BGA is a relatively new term has not yet been accurately defined. Usually the word "enhancement" means the addition of a material in the structure to enhance its performance. In most cases, the material added is metallic and serves to improve the heat dissipation of the IC during normal operation. This is important because one of the advantages of BGAs is that they provide a large number of IOs for the IC, and since this type of chip usually generates a significant amount of heat in a very small area, the package needs to be designed to dissipate the heat. A special enhanced package, referred to in this paper as a "Super BGA" (SBGA), is constructed with an inverted copper cavity at the top of the package to enhance heat dissipation to the surrounding environment. A thin, flexible substrate is soldered to the bottom surface of the copper chip as a pad for attaching several rows of solder balls along the periphery (i.e., there is no distribution of solder balls in the centre, refer to JEDEC). Internal wires connect the substrate to the chip, and the chip is moulded from the bottom. Table 1 lists the physical parameters of the BGA package. PLCC84 is included in the table as a reference for characteristics and performance. It is interesting to note that except for the IO metric, all other metrics for PLCC are intermediate values.
BGA Advantages and Disadvantages.
1,BGA Advantages
High Density
BGA packaging technology is a solution to the challenge of having to reduce the size of the package when producing integrated circuits with hundreds of pins. The production of surface mount (small-outline integrated circuit; SOIC) packages in pinned grid arrays and dual in-line packages has led to difficulties in the soldering process as more and more pins have to be added and the gap between them has to be reduced. This leads to difficulties in the soldering process. As the package pins get closer to each other, the risk of accidental bridging to neighbouring pins during soldering increases, which is not a problem with BGA packaging technology in factory-implemented soldering.
Thermal Conductivity
Another advantage of BGA packaging technology over other packaging technologies with separate pins (e.g., pin-containing packaging technology) is the low thermal impedance between the package and the PCB. This allows the heat generated by the ICs inside the package to be more easily transferred to the PCB, preventing the chip from overheating.
Low Inductance Pins
Shorter conductors also mean less unwanted inductance, a characteristic that can lead to unwanted signal distortion in high-speed electronic circuits.BGA package technology, with a very short distance between the package and the PCB, has low inductance pins, which provide superior electronic characteristics compared to pin devices.
BGA Disadvantages
Non-extensible contact
One of the disadvantages of BGA packages is that the solder balls are not able to stretch like long pins, so they do not have material stiffness in their physical properties. All surface mounted devices can break solder joints due to bending (thermal stress) or stretching and vibration (mechanical stress) due to the difference in the coefficient of thermal expansion between the PCB substrate and the BGA package.
The problem of thermal expansion can be solved by matching the similar thermal characteristics of the PCB and the package, and usually plasticised BGA devices can more closely match the thermal characteristics of the PCB than ceramic BGA devices.
The widespread adoption of RoHS-compliant lead-free solder alloys in production lines further demonstrates the challenges that BGA packages need to face, such as the "Head-in-Pillow" and "pad-cratering" problems during the reflow process, as compared to the "pad-cratering" problems that BGA packages face. Compared to BGA packages with leaded solder, some BGA packages are less reliable in extreme environments such as high temperatures, high thermal shock, and high G-force due to the low ductility of RoHS-compliant solders.
Mechanical stress issues can be addressed by bonding the device to the board through a process called "Underfilling", where an epoxy mixture is injected underneath the device after it has been soldered to the PCB, effectively bonding the BGA device to the PCB. Several underfill materials are available to provide different properties for different applications and thermal transfer requirements. Another benefit of underfill is that it limits the growth of tin whiskers.
Another solution to address non-ductile joints is to place a "ductile coating" inside the package that allows the solder balls on the bottom to move to their actual position relative to the package. This technique has become one of the standards for BGA packaged DRAMs.
Other techniques used to increase package reliability at the PCB level include low ductility PCBs for ceramic BGAs (CBGAs), intermediary boards (interposers) introduced between the package and the PCB, or repackaging devices.
Inspection Difficulty
When the package is soldered into position, it becomes difficult to find defects in the soldering. In order to detect the bottom of the soldered package, the industry has developed X-ray machines, industrial computer tomography scanners, special microscopes, and endoscopes and other equipment to overcome this problem. If a piece of BGA package is found to paste the soldering failure, can be in the "rework station" (commonly known as rework station) to remove it, which is equipped with infrared lamps (or hot air blower) fixtures, as well as the availability of thermocouples and vacuum devices in order to suck packages. "de-tinning" (or reballing) and reinstalled on the board.
Because of the expense of visual X-ray BGA inspection, circuit test methods are often used instead. A common boundary-scan test method can be performed via the IEEE 1149.1 JTAG interface connection.
Difficulties in Developing Circuits
It is impractical to solder the BGA device to a fixed point during the development phase. Instead, a slot is usually used first, although this is more unstable. There are two common types of slots: the more reliable type has spring loaded pins that fit underneath the solder balls, but it is not permissible to use a BGA device where the solder balls have been removed, as the spring loaded pins may not be long enough.
The unreliable type is a type called a "ZIF slot" (Zero Insertion Force), which has a spring-loaded clamp to hold the solder ball. But this is not easy to succeed, especially if the balls are too small.
Equipment Costs
To reliably solder BGA devices, expensive equipment is required. Manual soldering of BGA devices is very difficult and unreliable, and is usually only used for a small number of small devices. However, as more and more ICs are available only in lead-free (e.g. quad-flat no-leads package) or BGA packages, various DIY methods (heavy duty) have been developed that can be used with inexpensive heating sources, such as heat guns, household ovens, and flat-bottomed hotplates.
BGA disassembly
1, do a good job of component protection
2, in the IC to be disassembled above the appropriate amount of flux, try to blow into the bottom of the IC, which will help the chip under the solder joints uniform melting
3, adjust the wind gun temperature and wind, do not blow the middle of the IC, the heating time can not be too long
4, bga chip de-encounter, chip pads and boards have a surplus of tin should be sufficient amount of solder paste to remove it.
