Soldermask Registration Considerations for Fine-Pitch Area Array Package Assembly

Estimated reading time: 8 minutes

Assemblies with fine-pitch ball grid array (BGA) or chip scale package (CSP) components can develop perplexing, undetectable shorts. Research indicates that adding relatively simple designs and procedures in the early stages of printed circuit board (PCB) design and fabrication can be a solution.

By Hien Ly

SMT is moving toward smaller, higher I/O components for new electronics applications, including portable and handheld products. This drives a need for reduced size and higher density interconnect for new area array packages. The assembly process for BGA and CSPs has stabilized. However, products using fine-pitch BGA/CSP components at 0.8 mm pitch or below still pose many assembly challenges, including solder short prevention and detection.

When implementing fine-pitch BGA/ CSP components into production, some process issues can be caused by soldermask misregistration that occurred at PCB fabrication.

Figure 1a. Exposed trace/via.

Misregistration of a soldermask can commonly approach ±0.003" from datum point; however, it typically does not create problems in standard SMT processes. As the industry continues to use more array components with ball pitches of 0.8 mm and less, the chance for solder short defects related to soldermask design and registration increases. Soldermask misregistration can cause issues with exposed adjacent traces and/or vias underneath array packages, increasing the opportunity for shorts. Figures 1a and 1b show a drawing and an example of the exposed trace and/or via due to soldermask misregistration.

Figure 1b. Example of exposed trace.

Consequently, soldermask quality is a critical concern for high-density boards containing fine-pitch BGA/CSP devices, and impacts the assembly process. The acceptable soldermask registration criteria must be more stringent than standard PCB soldermask fabrication requirements. This article will identify the possible results of soldermask misregistration issues when assembling fine-pitch BGA/CSP components and provide unique design methodology, recommendations and considerations for controlling soldermask registration and design.

Figure 2a. X-ray detection issue of whisker short.

When an adjacent trace is exposed due to soldermask misregistration, solder bridging may have formed during the solder paste screen printing process. Misalignment of the solder paste printing process could aggravate the bridging issue. Placement of the fine-pitch component would further spread the solder paste into the soldermask cavity. During reflow, the hot-slump phenomenon spreads the molten solder, causing solder bridging. When the solder joint solidifies after reflow, a minute solder bridge is formed between the pad and the exposed trace or via. Typically, this minute solder bridging of pad to the exposed adjacent conductor or via is about 1 to 2 mils thick, and is referred to as a "whisker" short. Figures 2a and 2b show a cross-sectional view of a solder joint with whisker short and X-ray examinations. The whisker short could not be detected using 2-D X-ray because the image density of the solder ball diameter would overshadow the short. Laminography X-ray (5DX) also could not reveal the short. Therefore, as whisker shorts to adjacent traces or vias are difficult to detect following fine-pitch BGA/CSP assembly, the only solution would be to remove the components and inspect the fine-pitch BGA/CSP site. This solder short defect may require many hours of work to diagnose, and was observed only after the fine-pitch BGA had been removed.

Figure 2b. Enlarged view of 'whisker short.'

To resolve soldermask misregistration concerns during PCB fabrication, specifications must be created in the fabrication notes to explicitly indicate soldermask registration re-quirements. Critical areas such as fine-pitch BGA/CSP sites also must be identified to quality inspectors of the PCB fabrication vendors and assembly pro-viders for a thorough in-spection of those areas. The IPC-A-600 Revision F [Section 2.9.1, Page 37] specification can be used to supplement company specifications. If a defective board with the soldermask misregistration escapes incoming inspection, the problem can become difficult to detect after fine-pitch BGA/CSP components are assembled onto the board. Therefore, a soldermask registration method is recommended to aid PCB fabrication vendors and the assembly provider's receiving inspection operation in developing a verification process. Additionally, this soldermask registration feature can be used as a quick verification of soldermask registration if an assembly has an electrical short at the fine-pitch BGA/CSP component location not de-tected by X-ray processes.

Soldermask Registration Control

A special soldermask registration feature called a "chevron," as shown in Figure 3a, has since been implemented on designs with 0.8-mm area array devices to assist with soldermask registration verification. This registration feature consists of two 0.010" copper traces and a 0.010" soldermask opening slot. The soldermask slot is placed between the copper traces, and all three features are laid out in an L shape. A 0.010" space is designed between the soldermask slot and each copper trace. The recommended overall dimension for chevron design is 0.140" square. However, the dimensions can be adjusted to any reasonable size, as long as the L feature is observable and the spacing is greater than 0.005" between the copper trace and soldermask slot. This allows for standard copper etch or soldermask process variations so measurements still can be made if the copper trace or soldermask opening is over- or under-etched.

Figure 3a. Chevron soldermask registration design feature.

As the soldermask shifts relative to the copper artwork, a measurable offset can be identified in both X and Y directions by taking the difference between the measurements and dividing by two (Figure 3b). The positive or negative sign of the calculation indicates the direction of soldermask misregistration. For example, if X2 > X1, then the soldermask has shifted toward the positive X direction of the Cartesian coordinate system, and vice versa. The same principle applies for Y. If Y2 > Y1, then the soldermask has shifted toward the positive Y direction.

Figure 3b. Chevron soldermask registration calculations.

To verify soldermask registration, a minimum of three "chevron" soldermask registration features are recommended at the corners of the PCB to assist in soldermask registration verification (Figure 4). If the soldermask registration features are located on the break-off tabs of a PCB panel, these tabs could be removed prior to test process, and the soldermask registration features would not be available for failure analysis. In this case, the soldermask misregistration issue cannot be identified unless the fine-pitch BGA/CSP component is removed from the PCB. Therefore, the preference would be to have the soldermask registration features on each PCB module. The soldermask registration features could be located close to the global fiducials while observing fiducial clearance requirements. It also is helpful to have a component reference designator, such as CV1, next to each soldermask registration feature for inspection and measurement reference.

Additional Considerations

To further reduce the possibility of a soldermask registration problem, the customer's initial PCB design needs to be reviewed for design for manufacturing (DFM) violations. The updated PCB de-sign should then be reviewed with a PCB supplier to minimize soldermask misregistration issues. Different PCB vendors will use different soldermask openings, so they all must be alerted to the special requirements for fine-pitch BGA/CSP. Minimizing soldermask clearances around array pads can reduce this problem; however, it also may reduce the selection of capable PCB vendors.

Figure 4. Chevron soldermask registration feature location.

null

PCB vendors should also include a capability study (Cpk) on soldermask registration and have a history of producing "fine-feature" PCBs. PCB vendors should be able to perform up-front DFM review of the PCB layer design and provide details on soldermask opening and alignment tolerances for critical fine-pitch BGA/CSP applications. PCB vendors should use an electronic DFM tool that provides "design rule checks" (DRC), and the DRC should be reviewed by customers and PCB vendors before beginning board fabrication.

From an assembly provider's perspective, receiving inspection must be equipped with adequate microscope measurement capability for inspection of fine-pitch BGA/CSP or high-density areas. The recommended magnification is 10X minimum for 0.8-mm pitch, 15X minimum for 0.65-mm pitch, and 20X minimum for 0.5-mm pitch components. A heightened sampling plan also should be established to obtain measurements of the chevron registration marks, and should be performed on a predetermined basis.

Conclusion

A completed PCB assembly with solder shorts under fine-pitch BGA/CSP components caused by soldermask misregistration can be difficult to detect with standard X-ray processes. To better control soldermask design and registration, a set of chevron soldermask registration features can be designed on the board to aid the receiving inspection of PCB vendors and assembly providers to help obtain verification of soldermask registration. If a defective PCB assembly is suspected of solder shorts under the fine-pitch BGA/CSP component that has not been detected by X-ray, the soldermask registration features can be used as a debugging tool without having to remove the fine-pitch BGA/CSP component. This ultimately will reduce time and cost for investigation of the solder short defect.

With additional considerations to reduce the possibility of a soldermask registration issue, PCB vendors' capabilities on soldermask registration tolerance should be well understood, and soldermask registration tolerance and requirements should be included in the design fabrication drawing. For the assembly provider, a critical area such as fine-pitch BGA/CSP sites should be identified and conveyed to receiving inspection personnel. The inspection criteria may require sampling of the fine features with proper equipment, such as mi-croscopes with measurement systems and adequate magnification.

With the registration feature design and proper board fabrication considerations, soldermask registration issues can be minimized and possibly eliminated. The time invested on the PCB design review and verification of soldermask registration will result in better assembly process and higher yields.

Acknowledgements

The author would like to thank the following people at Jabil Technology Services for providing comments and support: Tom Cipielewski, AMT manager; Paul Neathway, AME manager; Elizabeth Shea, JTS technical assistant; and the Jabil AME technical staff.

References

IPC Committee, IPC-A-600F Acceptability of Printed Boards, Illinois, IPC, 1999, Section 2.9.1 - 2.9.3.IPC Committee, IPC-7095 Design and Assembly Process Implementation for BGAs, Illinois, IPC, 2000.Ken Gilleo, "Area Array Packaging Handbook," New York, McGraw-Hill, 2002, pp. 18.4-18.11.

Hien Ly, senior advanced manufacturing technology engineer, may be contacted at Jabil Circuit Inc., 30 Great Oaks Blvd., San Jose, CA 95119; (408) 361-3377; Fax: (408) 361-3320; E-mail: hien_ly@jabil.com.