A PRINTED CIRCUIT BOARD ASSEMBLY WITH IMPROVED BYPASS DECOUPLING FOR BGA PACKAGES

Abstract: A printed circuit board assembly with improved bypass decoupling for BGA packages. In one embodiment, a capacitor (103) may be interposed between a BGA package (110) and a PCB (101) within a perimeter of the contact pads (105) that form a BGA footprint (102). The capacitor (103) may have physical dimensions which allow a BGA package (110) to be mounted such that there is no physical contact between the capacitor (103) and the BGA (110).

Description: TITLE: A PRINTED CIRCUIT BOARD ASSEMBLY WITH IMPROVED BYPASS DECOUPLING FOR BGA PACKAGES BACKGROUND OF THE INVENTION 1 Technical Field This invention relates to printed circuit board assemblies, and more particularly, to the use of thin decouplmg capacitors in conjunction with ball-gπd array (BGA) packages 2 Background Art Decouplmg capacitors are used m electronic circuits to filter out transients in their associated power distribution systems Transients such as voltage spikes and momentary voltage drops may have adverse affects on various circuits, and may possibly cause erroneous operation Transients may often times be filtered out of a power distribution system by the use of capacitors (this is often referred to as decouplmg) When electrically connected between a power line and an electrical ground, a capacitor will tend to filter out many transients, as the voltage across a capacitor cannot change instantaneously The amount of capacitance necessary for effective elimination of power system transients can vary based on several different factors One such factor is the frequency of operation of the circuits for which decouplmg is to be provided Circuits which operate at higher frequencies, such as some radio circuits and high-speed computer systems, may produce more transients than low frequency or DC circuits Furthermore, some high frequency circuits may produce a number of harmonics or sub-harmonics, which may result m transients at different frequencies Another factor which may affect the amount of capacitance required for effective decouplmg is the distance between the circuits and the decoupling capacitors Various factors may affect the placement of decouplmg capacitors on a printed circuit board (PCB) One such factor is the packaging used for mtegrated circuits (IC's) mounted on the PCB Some types of packaging, in conjunction with other PCB layout constraints, may result in decoupling capacitors being placed a significant distance from their associated circuits As the distance between the decouplmg capacitors and associated circuits increase, inductance from the circuit lines connecting the capacitors to the packaging (known as Equivalent Series Inductance, or ESL) may become a problem ESL provides an inductive element to the impedance between a power line and electrical ground, and may thus reduce the effect of the capacitive impedance Thus, capacitors of greater value (or a greater number of capacitors) may be required to overcome the effects of ESL The use of ball-grid array (BGA) integrated circuit packages may add further complications to the problem of providmg decouplmg capacitance Figures 1A and IB illustrate two possible BGA configurations Each drawing illustrates a view of the BGA from the bottom side (l e the side that mounts to the PCB) In Figure 1A, the BGA shown includes a plurality of electrical contacts in a "ring" arrangement near the peripheries of the package A plurality of electrical contacts is also located in the center of the package, with an open space in between Typically, electrical contacts for conveymg power m this type of BGA are located on the inner portion of the ring, with ground contacts located in the center Figure IB illustrates a similar BGA arrangement, minus the central group of electrical contacts In this arrangement, power and ground contacts are typically located on the inner portion of the ring Both types of BGA's may be mounted to corresponding contact pads on a PCB The plurality of contact pads to which a BGA is mounted is sometimes referred to as a "footprint" Typically, decouplmg capacitance is provided for BGA's through the use of one or more surface mounted capacitors If the decouplmg capacitors are mounted on the same side of the PCB as the BGA, they will typicall} be placed some distance away from the power and ground pms of the BGA package In such cases, a greater number of capacitors may be required to overcome the effects of ESL As an alternative, ESL may be minimized by mounting the decouplmg capacitors nearer the pow er and ground contacts of the BGA, but on the opposite side of the PCB While this alternative may reduce the effects of ESL. it may add complexity to the assembly of the finished product By placing capacitors on the opposite side of the PCB, extra soldering operations may be required Furthermore, glumg operations to secure the capacitors to the PCB prior to soldering may be required as well The extra manufacturing operations may result in a significant increase in the cost of the finished product, and increase the likelihood of defects during the assembly process DISCLOSURE OF INVENTION The problems outlined above may in large part be solved by a printed circuit board assembly with improved bypass decouplmg foi BGA packages, m accordance with the present invention In one embodiment, a capacitor may be interposed between a BGA package and a PCB within a perimeter of the contact pads that form a BGA footprint The capacitor may have physical dimensions which allow a BGA package to be mounted such that there is no physical contact between the capacitor and the BGA In one embodiment, the capacitor used is an thin capacitor, with a thickness of no more than 0 5 millimeters The small thickness of the capacitor may allow it to be easily interposed between a BGA and a PCB One embodiment of the thm capacitor has no leads Terminals located on the capacitor package may be soldeied directly to the appropriate contact pads on the PCB Since the package is leadless, equivalent series inductance (ESL) may be minimized In some cases, the minimization of ESL may result in the need for fewer capacitors to effectively decouple a BGA package In various embodiments, multiple capacitors may be embodied within a common package For example, m one embodiment an ultra-thm capacitor package includes two capacitors which differ in capacitance value by a factor of 10 By using packages with multiple capacitors, power system transients occurring at different frequencies may be effectively filtered out Thus, m various embodiments, improved bypass decoupling for BGA packages may be accomplished by interposing thin capacitors (or capacitor packages) between a BGA and a PCB By interposing capacitors between a BGA and the PCB, some manufacturing operations may be eliminated which may result m cost reductions to the finished product The use of leadless capacitor packages may help minimize ESL and thus allow the use of fewer capacitors to effectively decouple a BGA The use of capacitor packages with multiple capacitors may allow the effective filtering of transients occurring at different frequencies The use of open space under a BGA may allow for more efficient utilization of circuit board area for other components BRIEF DESCRIPTION OF DRAWINGS Other objects and advantages of the invention will become apparent upon readmg the following detailed description and upon reference to the accompanymg drawings in which Figure 1A (Prior art) is a view of the underside of one embodiment of a ball-grid array (BGA) package, Figure IB (Prior art) is a view of the underside of another embodiment of a BGA package, Figure 2 is a drawing illustrating a view of the placement of one embodiment of a mm capacitor withm a BGA footprint, Figure 3 is a drawing of a printed circuit board (PCB) illustrating the mountmg of capacitors under a BGA in one embodiment, Figure 4 is a side view of one embodiment of an ultra-thm capacitor mounted upon a PCB and interposed between a BGA and the PCB upon which it is mounted, Figure 5A is a top view of one embodiment of a thm capacitor package which may be interposed between a PCB and a BGA, Figure 5B is a bottom view of the thm capacitor package of Figure 5 A, and Figure 5C is a side view of a capacitor such as those contained in the capacitor package illustrated m Figures 5A and 5B While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example m the drawmgs and will herein be described in detail It should be understood, however, that the drawmgs and description thereto are not intended to limit the invention to the particular form disclosed, but, on the contrary, the invention is to cover all modifications, equivalents, and alternatives falling with the spirit and scope of the present invention as defined be the appended claims MODE(S) FOR CARRYING OUT THE INVENTION Turning now to Figure 2, a drawing illustrating a view of the placement of one embodiment of a thm capacitor withm a BGA footprint is shown Printed circuit assembly (PCA) 100 includes printed circuit board (PCB) 101 PCB 101 includes two BGA footprints 102, each of which is formed by a plurality of contact pads 105 A plurality of chips 107 is mounted upon PCB 101 Mounted within the periphery of each BGA footprint are capacitors 103 Each capacitor 103 is a thin capacitor that may be interposed between a BGA and PCB 101 Capacitors 103 may be physically and electrically connected to PCB 101 by soldering them to additional contact pads 104, which are located withm the periphery of the BGA footprint formed by contact pads 105 Capacitors 103 may be electrically coupled to a power distribution line at one terminal and a ground at the other terminal In the embodiment shown, capacitors 103 do not have leads, and thus are soldered to contact pads 104 via terminals located on the body of the capacitor packaging Capacitors 103 provide decouplmg for BGA packages that may be mounted to contact pads 105 of BGA footprints 102, and may be effective m filtering out power system transients Various alternate embodiments may use more than two capacitors per BGA (as shown in the drawing), while others may use only one Moving now to Figure 3, a drawing of a printed circuit board (PCB) illustrating the mountmg of capacitors under a BGA m one embodiment is shown Durmg assembly operations, capacitors 103 may be mounted to PCB 101 Terminals on capacitors 103 may be soldered to contact pads 104 which are located withm the periphery of BGA footprint 102 m this embodiment Contact pads 104 are also located on the same surface of PCB 101 as contact pads 105 BGA footprint 102 is formed b\ a plurality of contact pads 105 In some embodiments capacitors 103 may be secured to PCB 101 by an adhesive prior to soldering Following the placement of capacitors 103, BGA 110 may be placed on PCB 101 , aligning it with contact pads 105 of BGA footprint 102 When BGA 110 has been properly placed, a soldering operation may be used to physically and electrically connect BGA 110 to PCB 101 In some embodiments, a smgle soldering operation may be used to solder both BGA 1 10 and capacitors 103 to their respective contact pads while separate soldering operations may be used in other embodiments Figure 4 is a side \ lew of one embodiment of a th capacitor mounted upon a PCB and interposed between a BGA and the PCB upon which it is mounted Capacitor 103 may be mounted to PCB 101 as described with reference to Figure 2 and Figure 3 BGA 1 10 is also mounted to PCB 101 Electrical contacts 1 1 1 may be used to couple BGA 110 to PCB 101 In the embodiment shown, each electrical contact 111 include;, a pre-formed ball of solder that may be used to solder the BGA to contact pads such as those shown in Figure 2 Because of the small thickness of capacitor 103 may be mounted on the same surface as BGA 1 10, and thus interposed betw een BGA 1 10 and PCB 101 Typically, the thickness of the type of capacitois used in the embodiment shown are no more than 0 5 millimeters and may be sigmficantlv less Figures 5 A and 5B illustrate a top view and a bottom view , respectively, of one embodiment of a capacitor package 1030 that may be used m various embodiments Capacitor package 1030 includes a pluralm of contact terminals 1031 , which may include exposed metal suitable for soldering to a contact pad on a PCB Metal may be exposed on both sides of the terminals in some embodiments, which may allow for a more secure solder connection between the capacitor package and the contacts of the PCB In the embodiment shown m Figure 5 A, two of contact terminals 1031 are electrically connected to a capacitor plate 1033 by conductors 1032 Conductors 1032 provide an electrical connection between a contact terminal 1031 and capacitor plate 1033 Alternate embodiments of the capacitor package may be constructed such that a capacitor plate makes direct contact with a contact terminal, thus eliminating the need for a conductor such as that shown here Capacitor plates 1034 and 1035 are show n m Figure 5B, which is a bottom view of the capacitor package 1030 Each of these capacitor plates is electrically connected to a terminal 1031 by a conductor 1032 Since there are two separate capacitor plates present on the bottom side, capacitor package 1030 essentially includes two capacitors Each capacitor m this embodiment is a two-plate capacitor, with the capacitor plates show n in Figure 5B each forming a capacitor with capacitor plate 1033 shown in Figure 5 A The capacitor plates may be separated by a dielectric material, as w ill be explained m further detail below Some embodiments of capacitor package may include only a smgle capacitor, while others may include more than two capacitors Furthermore, capacitor packages may include a network of several capacitors of equal capacitance, or multiple capacitors with different \ alues of capacitance Since the area of capacitor plate 1034 is greater than that of capacitor plate 1035, its capacitance value will also be greater as well In one embodiment, the capacitance values of two capacitors m a package may differ by a factor of 10 By decouplmg in a "decade" fashion such as this, different frequencies may be filtered out when power system transients occur The capacitors shown m this embodiment have a tolerance value of ± 20%, although greater or lesser tolerance values may be present m other embodiments Movmg now to Figure 5C, a side view of a capacitor such as those contamed in the capacitor package illustrated in Figures 5A and 5B is shown It should be noted that the dimensions as shown m this drawing are exaggerated for clarity, and are not intended to represent the actual dimensions of the various embodiments Capacitor 103 is a capacitor in a standard two-plate configuration including capacitor plates 1033 and 1034 Each capacitor plate may be formed from an electrically conductive material Capacitor plates 1033 and 1034 are separated by a dielectric layer 1036 Dielectric layer 1036 may be formed from any suitable dielectric material While the present mvention has been described with reference to particular embodiments, it will be understood that the embodiments are illustrative and that the invention scope is not so limited Any variations, modifications, additions, and improvements to the embodiments described are possible These variations, modifications, additions, and improvements may fall withm the scope of the inventions as detailed withm the following claims INDUSTRIAL APPLICABILITY This invention is applicable to prmted circuit board assemblies, and more particularly, to the use of thm decouplmg capacitors m conjunction with ball-grid array (BGA) packages

Claims: WHAT IS CLAIMED IS: 1 A printed circuit board assembly (100) comprising a printed circuit board (PCB) ( 101), wherein said PCB (101) includes a plurality of contact pads ( 105) which forms a ball grid array (BGA) footprint (102), a BGA package ( 110) having a plurality of contacts ( 1 1 1) electrically connected to said plurality of contact pads (105), and a capacitor ( 103 ) interposed between said PCB (101 ) and said BGA package (110) 2 The prmted circuit board assembly (100) as recited in claim 1 , wherein said plurality of contact pads ( 105) are located on a first surface of said PCB ( 101), and wherein said capacitor (103) is connected to additional contact pads (104) located on said first surface of said PCB (101) 3 The prmted circuit board assembly (100) as recited in claim 2, wherein said additional contact pads ( 104) are located within an area defined by a perimeter of said BGA footprint ( 102) 4 The printed circuit board assembly ( 100) as recited in claim 3, wherein said capacitor ( 103) is a leadless capacitor 5 The prmted circuit board assembly ( 100) as recited in claim 4, wherein said capacitor ( 103) is contained withm a capacitor package ( 1030) 6 The printed circuit board assembly ( 100) as recited m claim 5, wherein said capacitor package ( 1030) includes a first capacitor and a second capacitor 7 The printed circuit board assembly ( 100) as recited in claim 6, wherein said first capacitor and said second capacitor have different capacitance values 8 The printed circuit board assembly ( 100) as recited m claim 7, wherein capacitance values of said first capacitor and said second capacitor differ by a factor of 10 9 The prmted circuit board assembly (100) as recited in claim 4, wherein said capacitor ( 103) is no more than 0 5 millimeters in thickness 10 A method for providing decouplmg capacitance for a ball-grid array (BGA) package O 10) including a plurality of electrical contacts (11 1 ), the method comprising mountmg a capacitor ( 103) on a printed circuit board (PCB) ( 101), wherein said PCB ( 101) includes a plurality of contact pads (105) which forms a ball grid array (BGA) footprint (102), and mountmg said BGA package ( 110) to said PCB (101), wherem said electrical contacts ( 111 ) are electrically connected to said plurality of contact pads (105), and wherem said capacitor (103) is interposed between said BGA package ( 1 10) and said PCB ( 111)

Inventors: BENNETT JOHN, DRAKE MICHAEL, GUERRERO EDWARD, SCHELLING GREG, TRESSLER CHRIS, BENNETT, JOHN, DRAKE, MICHAEL, GUERRERO, EDWARD, SCHELLING, GREG, TRESSLER, CHRIS
Applicants: ADVANCED MICRO DEVICES INC, ADVANCED MICRO DEVICES, INC.
Application year: 2000
Main classification: H05K1
IPC codes: H01L23, H01L25, H05K1