Ball grid array: Difference between revisions

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{{short description|Surface-mount packaging that uses an array of solder balls}}

{{More citations needed|date=September 2010}}

[[Image:Solder_ball_grid.jpg|thumb|300px|A grid array of solder balls on a printed circuit board after removal of an integrated circuit chip.]]

[[File:Bga und via IMGP4531 wp.jpg|thumb|300x300px|Cross-cut section of BGA mounted circuit]]

A '''ball grid array''' ('''BGA''') is a type of [[surface-mount]] packaging (a [[chip carrier]]) used for [[integrated circuit]]s. BGA packages are used to permanently mount devices such as [[microprocessor]]s. A BGA can provide more interconnection pins thatthan can be put on a [[Dual in-line package|dual in-line]] or [[Quad Flat Package|flat package]]. The whole bottom surface of the device can be used, instead of just the perimeter. The traces connecting the package's leads to the wires or balls which connect the die to package are also on average shorter than with a perimeter-only type, leading to better performance at high speeds.{{cn|date=January 2020}}

Soldering of BGA devices requires precise control and is usually done by automated processes such as in computer-controlled automatic [[reflow oven]]s.

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== Description ==

[[Image:BGA RAM.jpg|thumb|300px|BGA [[Integrated circuit|ICs]] assembled on a [[Random access memory|Memory module]]]]

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]] which, in operation, conduct electrical signals between the integrated circuit and the [[printed circuit board]] (PCB) on which it is placed. In a BGA, the pins are replaced by pads on the bottom of the package, each initially with a tiny [[solder ball]] stuck to it. These solder spheres can be placed manually or by automated equipment, and are held in place with a tacky flux.<ref name=indiumcorporation>{{Cite web |url=http://www.indium.com/_dynamo/download.php?docid=323 |title=Soldering 101 - A Basic Overview |access-date=2010-12-29 |archive-url=https://web.archive.org/web/20120303025834/http://www.indium.com/_dynamo/download.php?docid=323 |archive-date=2012-03-03 |url-status=dead }}</ref> The [[SMT placement equipment|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]], melting the balls. [[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, forming soldered connections between the device and the PCB.

In more advanced technologies, solder balls may be used on both the PCB and the package. Also, in stacked [[multi-chip module]]s, ([[package on package]]) solder balls are used to connect two packages in a "[[package on package]]" configuration.

== Advantages ==

=== High density ===

The BGA is a solution toadresses 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 ([[Small-outline integrated circuit|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 [[Solder bridging|bridging]] adjacent pins with solder grew.

=== 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 unwanted [[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 lead inductances, giving them superior electrical performance toover pinned devices.

== Disadvantages ==

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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 PCB thermal characteristics than ceramic devices.

The predominant use of [[RoHS compliant]] lead-free solder alloy assemblies has presented some further challenges to BGAs including "[[Head in pillow (metallurgy)|head in pillow]]"<ref>{{cite web |title=Reducing Head in Pillow Defects - Head in pillow defects: causes and potential solutions |author=Alpha |date=2010-03-15 |orig-year=September 2009 |version=3 |url=http://www.slideshare.net/Alpha1Cookson/head-on-pillow-v3-sept-09 |access-date=2018-06-18 |url-status = live|archive-url=https://web.archive.org/web/20131203002809/http://www.slideshare.net/Alpha1Cookson/head-on-pillow-v3-sept-09 |archive-date=2013-12-03}}</ref> soldering phenomenon, "[[pad cratering]]" problems as well as their decreased reliability versus lead-based solder BGAs in extreme operating conditions such as high temperature, high thermal shock and high gravitational force environments, in part due to lower [[ductility]] of RoHS-compliant solders.<ref>{{cite web |url=http://teerm.nasa.gov/NASA_DODLeadFreeElectronics_Proj2.htm |title=TEERM - TEERM Active Project - NASA-DOD Lead-Free Electronics (Project 2) |publisher=Teerm.nasa.gov |access-date=2014-03-21 |archive-url=https://web.archive.org/web/20141008003914/http://teerm.nasa.gov/nasa_dodleadfreeelectronics_proj2.htm |archive-date=2014-10-08 |url-status = dead}}</ref>

Mechanical stress issues can be overcome by bonding the devices to the board through a process called "underfilling",<ref name=underfill>[https://archive.today/20130719180947/http://www.electroiq.com/articles/ap/print/volume-10/issue-12/features/bga-underfills.html Solid State Technology: BGA underfills - Increasing board-level solder joint reliability, 12/01/2001]</ref> 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 underfill materials in use with differing properties relative to workability and thermal transfer. An additional advantage of underfill is that it limits [[Whisker (metallurgy)|tin whisker]] growth.

Another solution to non-compliant connections 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.

Other techniques for increasing the board-level reliability of packages include use of low-expansion PCBs for ceramic BGA (CBGA) packages, [[interposer]]s between the package and PCB, and re-packaging a device.<ref name=underfill/>

===Difficulty of inspection===

Once the package is soldered into place, it is difficult to find soldering faults. [[X-ray]] machines, [[industrial CT scanning]] machines,<ref>"CT Services - Overview." Jesse Garant & Associates. August 17, 2010. {{cite web|url=http://www.jgarantmc.com/ct-services.html |title=Industrial Computed Tomography Scanning Services – JG&A |access-date=2010-11-24 |url-status = dead|archive-url=https://web.archive.org/web/20100923150138/http://www.jgarantmc.com/ct-services.html |archive-date=2010-09-23 }}</ref> special microscopes, and endoscopes to look underneath the soldered package 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 it can be refurbished (or ''reballed'') and re-installed on the circuit board. Pre-configured solder balls matching the array pattern can be used to reball BGAs when only one or a few need to be reworked. For higher volume and repeated lab work, a stencil-configured vacuum-head pick-up and placement of loose spheres can be used.

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

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leaving the innermost square empty.

Intel used a package designated BGA1 for their [[Pentium II]] and early [[Celeron]] mobile processors. BGA2 is Intel's package for their [[Pentium III]] and some later Celeron mobile processors. BGA2 is also known as FCBGA-479. It replaced its predecessor, BGA1.

For example, the "micro-FCBGA" (flip chip ball grid array) is Intel's current{{When|date=May 2011}} BGA mounting method for mobile processors that use a [[flip chip]] binding technology. It was introduced with the ''Coppermine'' Mobile Celeron.{{Citation needed|date=April 2016}} Micro-FCBGA has 479 balls that are 0.78&nbsp;mm in diameter. The processor is affixed to the motherboard by soldering the balls to the motherboard. This is thinner than a pin grid array socket arrangement, but is not removable.

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== Procurement ==

Primary end-users of BGAs are [[original equipment manufacturer]]s (OEMs). There is also a market among electronic hobbyists [[Do it yourself|do it yourself (DIY)]] such as the increasingly popular [[maker culture|maker movement]].<ref>{{cite news|title=More than just digital quilting: The "maker" movement could change how science is taught and boost innovation. It may even herald a new industrial revolution|url=httphttps://www.economist.com/nodetechnology-quarterly/215403922011/12/03/more-than-just-digital-quilting|newspaper=The Economist|date=Dec 3, 2011}}</ref> While OEMs generally source their components from the manufacturer, or the manufacturer's distributor, the hobbyist will typically obtain BGAs on the aftermarket through electronic component brokers or [[:Category:Electronic component distributors|distributors]].

== See also ==