Pad cratering is a mechanically induced fracture in the resin between copper foil and outermost layer of

fiberglass Fiberglass (American English) or fibreglass (Commonwealth English) is a common type of fiber-reinforced plastic using glass fiber. The fibers may be randomly arranged, flattened into a sheet called a chopped strand mat, or woven into glass cloth ...

of a

printed circuit board A printed circuit board (PCB; also printed wiring board or PWB) is a medium used in Electrical engineering, electrical and electronic engineering to connect electronic components to one another in a controlled manner. It takes the form of a L ...

(PCB). It may be within the resin or at the resin to fiberglass interface. The pad remains connected to the component (usually a

Ball Grid Array A ball grid array (BGA) is a type of surface-mount packaging (a chip carrier) used for integrated circuits. BGA packages are used to permanently mount devices such as microprocessors. A BGA can provide more interconnection pins than can be p ...

, BGA) and leaves a "crater" on the surface of the printed circuit board.

Overview

Pad cratering most often occurs during dynamic mechanical events such as

mechanical shock A mechanical or physical shock is a sudden acceleration caused, for example, by impact, drop, kick, earthquake, or explosion. Shock is a transient physical excitation. Shock describes matter subject to extreme rates of force with respect to ti ...

or board flexure due to

In-circuit test In-circuit testing (ICT) is an example of white box testing where an electrical probe tests a populated printed circuit board (PCB), checking for shorts, opens, resistance, capacitance, and other basic quantities which will show whether the assemb ...

(ICT), board depaneling, or connector insertion.http://www.dfrsolutions.com/hubfs/Resources/services/Preventing-Pad-Cratering-During-ICT-Using-Sherlock.pdf?hsCtaTracking=95bec082-e4c1-40d3-a379-dfe6d7a5727a%7Ce96e5f51-abc5-4c7a-9a2e-28a78cb24e8e However, pad cratering has also been known to occur during

thermal shock Thermal shock is a type of rapidly transient mechanical load. By definition, it is a mechanical load caused by a rapid change of temperature of a certain point. It can be also extended to the case of a thermal gradient, which makes different pa ...

or even thermal cycling. Susceptibility to pad cratering can be impacted by several factors such as: PCB thickness, PCB laminate material properties, component size and stiffness, component location, and

solder Solder (; NA: ) is a fusible metal alloy used to create a permanent bond between metal workpieces. Solder is melted in order to wet the parts of the joint, where it adheres to and connects the pieces after cooling. Metals or alloys suitable ...

alloy An alloy is a mixture of chemical elements of which at least one is a metal. Unlike chemical compounds with metallic bases, an alloy will retain all the properties of a metal in the resulting material, such as electrical conductivity, ductility, ...

selection among other factors.

Testing

IPC-9708 provides three test methods to characterize the pad cratering of a component and PCBA: pin pull, ball pull, and ball shear testing. In the pin pull test a pin is soldered to pads and pulled until fracture. It is a useful test for all pad geometries and is sensitive to board design and materials. The ball pull test is specifically design for BGA components and has a large sensitivity to the solder alloy and joint formation. The ball shear test is also specified for BGA components and involves shearing the solder balls of the BGA. This test is typically the most convenient but is less sensitive to the design and material as compared to the ball pull test.D. Xie, D. Shangguan and H. Kroener, "Pad Cratering Evaluation of PCB", APEX 2010, Las Vegas, NA. Although IPC-9708 specifies procedures for each test type, the challenge is that no standard pass/fail criteria are defined. This is viewed as application-specific and must be defined by the user based on their design, environment, and reliability requirements. Another applicable test method is IPC/JEDEC-9702, which is a monotonic bend test method used to characterize board level interconnects. This can be relevant for pad cratering resulting from board flexure, however this test method is broader and does not specifically focus on pad cratering failure modes. Board level reliability testing is a common approach to assessing product reliability. Performing temperature cycling, mechanical drop/shock, and vibration testing is a good way to evaluate pad cratering. However, similar to IPC/JEDEC-9702, this can be cost and time intensive and does not specifically focus on pad cratering failure modes.

Detection and Failure Analysis

Pad cratering can be difficult to detect during functional testing. This is especially the case with small or partial cracking that can escape testing and cause latent field failures.http://www.dfrsolutions.com/hubfs/Webinar%20Slides%20for%20YouTube/Avoiding-Pad-Cratering-and-Cracked-Capacitor-Webinar.pdf Even if a component failure is identified, diagnosing the failure mode as pad cratering can be difficult. Conventional nondestructive testing and failure analysis techniques such as visual inspection and

X-Ray microscopy An X-ray microscope uses electromagnetic radiation in the soft X-ray band to produce magnified images of objects. Since X-rays penetrate most objects, there is no need to specially prepare them for X-ray microscopy observations. Unlike visible li ...

may not detect the issue. Electrical characterization is an example of a nondestructive technique that can be useful, however this may not detect an anomaly if there is only partial cracking. Typically, pad cratering is detected or confirmed via

destructive testing In destructive testing (or destructive physical analysis, DPA) tests are carried out to the specimen's failure, in order to understand a specimen's performance or material behavior under different loads. These tests are generally much easier to car ...

and failure analysis such as dye and pry, acoustic emissions, cross sectioning, and Scanning Electron Microscopy.

Mitigation

There are several mitigation techniques that can used to reduce the risk of pad cratering. The appropriate method(s) is often driven by design and resource constraints. Limiting Board Flexure: If cratering is due to mechanical overstress then limiting board flexure is typically the best mitigation technique.https://www.smta.org/chapters/files/uppermidwest_padcratering.pdf Simulation: Modeling and simulation can help proactively avoid pad cratering failures. Relevant examples include ICT failures or products with potential for large shock events (i.e. portable electronics).

Finite Element Analysis The finite element method (FEM) is a popular method for numerically solving differential equations arising in engineering and mathematical modeling. Typical problem areas of interest include the traditional fields of structural analysis, heat ...

can be done using a physics of failure approach to determine risk of overstress and pad cratering. This proactive approach can rapidly evaluate multiple designs early on, potentially avoiding expensive design changes or warranty costs later on. Underfill, Edge Bonding, and Corner Staking: Epoxies and underfill materials can be added to provide mechanical support and reduce board and solder strain during flexing. This is more common in cases where the component selection and PCBA design are fixed. There are differences between each technique which makes proper understanding of the environment and application important. Solder Alloy: Solder alloy selection can impact susceptibility to pad cratering. Typically, pad cratering is considered a high strain rate event with minimal creep, however there is still potential for plasticity in the solder. More compliant solders or those with lower yield points will reduce pad cratering potential by providing additional load sharing. Board Thickness and Laminate Material: Board thickness and laminate material properties such as

Young’s modulus Young's modulus E, the Young modulus, or the modulus of elasticity in tension or compression (i.e., negative tension), is a mechanical property that measures the tensile or compressive stiffness of a solid material when the force is applied len ...

and

Coefficient of Thermal Expansion Thermal expansion is the tendency of matter to change its shape, area, volume, and density in response to a change in temperature, usually not including phase transitions. Temperature is a monotonic function of the average molecular kineti ...

(CTE) will impact susceptibility to pad cratering. Board Redesign: If pad cratering persists then a redesign may be required. This could include changing component location or adjusting between solder mask defined (SMD) and non-solder mask defined (NSMD) pads.

Pad Cratering Images

File:Pad Cratering 04.jpg, BGA pad and

solder ball In integrated circuit packaging, a solder ball, also a solder bump (ofter referred to simply as "ball" or "bumps") is a ball of solder that provides the contact between the chip package and the printed circuit board, as well as between stacked pa ...

exhibiting pad cratering. File:Pad Cratering 02.jpg, Magnified view of cross section of BGA pad and solder ball. Dielectric has cracked and the pad has started to lift, eventually creating pad cratering. File:Pad Cratering 03.jpg, Pad crater left on printed circuit board after copper pad from a BGA connection has been pulled away.

External links

Additional information on pad cratering in printed circuit boards can be found in the following links: *http://www.smtnet.com/Forums/index.cfm?fuseaction=view_thread&Thread_ID=13953

*http://www.ipc.org/de/ContentPage.aspx?pageid=IPC-ehrt-Best-Papers-an-der-IPC-APEX-EXPO *http://integral-hdi.com Integral Technology *http://integral-hdi.com/news/2010/11/next-generation-electronic-materials- Integral Technology pad cratering blog.

References

{{Reflist, 30em Printed circuit board manufacturing Soldering defects