FATIGUE FAILURE

BACKGROUND

[0001] Embodiments of the invention relate to Ball Grid Arrays (BGAs) in high- reliability-electronics apphcations and Printed-Circuit-Board Assemblies (PCBAs) with high-reliability-solder joints.

[0002] A BGA is a type of surface-mount packaging (a chip carrier) used for integrated circuits. BGA packages are used to permanently mount electronic devices, such as microprocessors, onto printed circuit boards. A BGA can provide more interconnection pins than can be put on a dual inline or flat package. The whole bottom surface of an electronic device, instead of just the perimeter, can be used for electrically connecting the electronic device to a printed circuit board. 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.

[0003] BGA underfill provides a strong mechanical bond between a BGA component and its connection to a circuit board, thereby protecting the solder joints from physical stress.

[0004] Improved ways of meeting high thermal-cycle count requirements for BGAs, for example in the automotive industry (-40 to 85°C, 1000 cycles), without using underfill or any other arrangements that add cost, would advance the state of the art.

BRIEF SUMMARY

[0005] Embodiments of the invention are directed to a ball-grid-array assembly (BGA assembly) that includes: a printed circuit board having: outer NonSolder -Mask-Defined (NSMD) pads that are positioned close to edges, including close to a plurality of corners, of the BGA footprint on the printed circuit board; a plurahty of inner Solder-Mask-Defined (SMD) pads that are positioned within an interior region of the BGA footprint on the printed circuit board such that the plurahty of inner SMD pads are surrounded by the plurality of outer NSMD pads; and a plurahty of solder balls that mount, and electrically connect, an electrical component to the printed circuit board through the outer NSMD pads and the inner SMD pads. A height of the plurahty of solder balls is a number-weighted average of a solder-ball height associated with using only the outer NSMD pads and a solder-ball height associated with using only the inner SMD pads, such that the increased height of the plurality of solder balls at the outer NSMD pads postpones thermal-fatigue failure as compared to using only NSMD pads to mount, and electrically connect, the electrical component to the PCB.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006] Fig. 1 depicts a hybrid-BGA-PCB-footprint -land-pattern example showing a center area with SMD pads and outer rows with NSMD pads.

[0007] Fig. 2 is a table depicting different solder joint geometries for various combinations of joint types and land patterns.

[0008] Fig. 3 depicts an example component NSMD joint showing an expected solder-height increase due to the hybrid land pattern of inner SMD joints and outer NSMD joints in accordance with embodiments of the invention.

[0009] Fig. 4 depicts a cross-section of a prototype build showing a solder joint height of 181 pm of the hybrid design, well matching a predicted height of approximately 182 gm. DETAILED DESCRIPTION

[0010] Thermal fatigue life of a BGA is influenced by the environmental profile, coefficients of thermal expansion of the component and circuit board, the solder joint geometry, and solder alloy. “Thermal fatigue life” as used herein refers to a duration, which may be measured according to thermal cycles, during which solder joints connecting an electrical component to a PCB, via a BGA, remain intact without experiencing thermal fatigue failure. “Thermal fatigue” as used herein refers to a fatigue failure of one or more solder joints with macroscopic cracks resulting from cyclic thermal stresses and strains due to temperature changes, spatial temperature gradients, and high temperatures under constrained thermal deformation.

[0011] A hybrid BGA footprint, in accordance with embodiments of the invention, targets optimization of the solder joint geometry by modifying PCB landpattern design. The important geometrical factors for BGA-joint geometry are joint height and diameter symmetry at the component and PCB side. Diameter symmetry refers to the solder diameter at the component pad being equal to the solder diameter at the PCB pad.

[0012] A solder mask (also referred to as a solder stop mask, or a solder resist) is a thin lacquer-like layer of polymer that is usually applied to the copper traces of a printed circuit board (PCB) for protection against oxidation and to prevent solder bridges from forming between closely spaced solder pads. A solder bridge is an unintended electrical connection between two conductors by means of a small blob of solder. PCBs use solder masks to prevent this from happening. Solder masks are typically used for mass- produced boards that are soldered automatically using reflow or wave soldering techniques. Once applied, openings are made, typically via photolithography, in the solder mask where components are soldered. [0013] Non-solder mask defined (NSMD) land patterns can provide longer fatigue life compared with solder mask defined (SMD) pads for BGAs. But SMD pads provide a taller joint height. A hybrid of approximately 50% or greater SMD central pads with the remaining percentage of outer NSMD pads increases height of the corner and edge NSMD joints and uses the optimal pad type (NSMD) for thermal fatigue on the corner and edge joints, which are under the highest strain.

[0014] Reflow soldering is a process in which a solder paste (a sticky mixture of powdered solder and flux) is used to temporarily attach electrical components to their contact pads, after which the entire assembly is subjected to controlled heat. The solder paste reflows in a molten state, creating permanent solder joints. Heating may be accomphshed, among other ways, by passing the assembly through a reflow oven.

[0015] During reflow, molten joints will minimize surface tension by creating a truncated spherical shape, whose height depends on the pad types, sizes, and solder volume. In the hybrid design, the ideal truncated spherical shape of the NSMD joints will be elongated, while the SMD joints will be compressed, resulting in an equal solder joint height between the copper PCB pads and component pads across the joints of the entire component. The resulting solder joint height will be a number count weighted average of the expected heights of the of SMD and NSMD types.

[0016] NSMD joints within a hybrid footprint on the corner and outer edge are under the highest strain, and are most susceptible to cracking, during thermal cycling. NSMD joints within the hybrid footprint will have an increased height and, therefore, fatigue life compared to a BGA with a conventional footprint of all NSMD joints. [0017] Fig. 1 depicts a hybrid-BGA-PCB-footprint -land-pattern example showing a center area with SMD pads and outer rows with NSMD pads. The percentage of SMD pads is at least approximately 50%, ideally greater.

[0018] In Fig. 1, a dashed-line rectangle has been added to depict a separation between the NSMD pads, which are located outside the dashed-line rectangle, and the SMD pads, which are located inside the dashed-line rectangle.

[0019] Fig. 2 is a table depicting different solder joint geometries for various combinations of joint types and land patterns. The conventional column shows expected solder joint heights for footprints consisting of all NSMD (top) or all SMD (bottom). When the two pad types are placed on the same footprint as in the hybrid design, the resulting height is the same for both types of pads and is a number weighted average of the expected height of each type. So, for example, if a hybrid design includes 30 NSMD joints, with an expected conventional height of 174 gm and 42 SMD joints along the edges, including the corners, with an expected conventional height 190 gm, then a number weighted average would be 183.3, which is calculated by, for each type of pad, multiplying the number of joints by the expected conventional height (30 x 174 = 5,220; and 42 x 190 = 7,980), summing those results (5,220 + 7,980 = 13,200), and dividing that sum by the total number of joints (13,200 72 = 183.3 gm).

[0020] Ansys Sherlock is a reliabihty physics-based electronics design tool that provides fast and accurate life predictions for electronic hardware at the component, board, and system levels in early-stage design.

[0021] Fig. 3 depicts an example component NSMD joint showing an expected solder-height increase due to the hybrid land pattern of inner SMD joints and outer NSMD joints in accordance with embodiments of the invention. The example component NSMD joint shows an expected solder-height increase of 12%, which results in an improvement of 45% in an Ansys Sherlock cycles to failure fatigue hfe prediction as a result of using a hybrid design, in accordance with embodiments of the invention, as compared to a conventional land pattern. Growth in height is a result of the center SMD pads lifting the component relative to an all NSMD height combined with an optimized NSMD pad size. Compared to the conventional footprint, the optimized NSMD pad size is reduced to achieve a solder diameter at the component pad size equal to the solder diameter at the top of the PCB pad. The heights and diameters of the solder joint depend on the component and PCB pad type, diameter, thickness of copper or solder mask and solder volume. For example, decreasing the diameter of the SMD pads will increase the expected joint height, however the diameter should not be decreased below the component pad size so much that the inner joints become a fatigue risk.

[0022] Fig. 4 depicts a cross-section of a prototype build showing a solder joint height of 181 gm of the hybrid design, well matching a predicted height of approximately 182 gm. Note the NSMD joints on the two outer edges and SMD joints in the central pins and that, for the component depicted in Fig. 4, there are 49% SMD pads, which results in a predicted height of 182 gm.

[0023] As set forth above, high thermal-cycle count requirements for BGAs, for example in the automotive industry (-40 to 85°C, 1000 cycles), may be achieved, in accordance with embodiments of the invention, without using underfill or any other arrangements that add cost.

[0024] While the present invention has been illustrated by a description of various embodiments and while these embodiments have been described in considerable detail, it is not the intention of the applicants to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. The invention in its broader aspects is therefore not limited to the specific details, representative apparatus and method, and illustrative example shown and described. Accordingly, departures may be made from such details without departing from the spirit or scope of applicant’s general inventive concept. For example, approximate numerical values and approximate ranges of numerical values include the stated numerical values and stated approximate ranges plus or minus a 10% variance.