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IPC-7351 Revs Forward
December 31, 1969 |Estimated reading time: 6 minutes
By John Perry, IPC
February 2005 marked the release of IPC-7351, Generic Requirements for Surface Mount Design and Land Pattern Standard, a successor to the IPC-SM-782 document for the printed board design community. IPC-7351 represented a major shift in how land patterns were defined and created, and provided the industry with multiple geometries for a given part. This article highlights benefits and features of this standard.
Figure 1. PQFN.
PC-7351 represented a major shift in how land patterns were defined and created, and provided the industry with multiple geometries for a given part, enabling designers to select a land pattern that best matched the printed board component density configuration.
Mathematical models used to develop the IPC-SM-782 remained, but took into account design complexity and density aspects. IPC-7351 was released two years ago. Since then, lead-free soldering processes have continued to be assimilated within the global electronics manufacturing industry. The proliferation of new surface mount component families has continued, and increased board densities have used techniques such as via-in-pad design. IPC recognized the need to respond with a new revision to IPC-7351, and released IPC-7351A in February 2007.
Lead-free Land Pattern Development
While it is true that all three land pattern geometry variations in IPC-7351A are considered compliant for lead-free soldering processes, and that derived land patterns are based on mathematical models that are transparent to those processes, the fact that some lead-free solder alloys react differently than traditional tin/lead solders makes determining the optimum land pattern critical. Ensuring proper wetting of lead free alloys means that, in some cases, IPC-7351A will recommend using oblong pads in lieu of traditional rectangular pads to provide for an adequate pull of solder onto the pad. In cases where components with thermal pads that dictate a reduction in copper area of the pad length are used, or with components that have a body standoff height less than the paste-mask stencil thickness (requiring a trimming of the heel portion of the pad), a rectangular pad is recommended.
IPC-7351A also discusses other factors affecting the reliability of lead-free solder joints, including component lead finish, land pattern surface finish and the reflow profile. Various component sections of the document have been updated to provide minimum time and temperature requirements for immersing components in various molten solder alloys, in addition to defining the package peak reflow profiles for tin/lead and lead-free solders (Tables 1 and 2).
New and Updated Component Families
IPC-7351A addresses unique variants to existing components. The original IPC-7351 introduced the quad flat no-lead (QFN) and small outline no-lead (SON) devices, neither of which have conventional perimeter lead terminations. IPC-7351A introduces the pullback lead variation of both packages (PQFN and PSON), consisting of pullback of the terminal metal pad from the edge of the plastic body (Figure 1). Like BGA packages, these components require a unique concept for developing a land pattern - there are no toe, heel, or side fillets for the solder joint - instead, the land periphery is similar throughout the entire pad termination (PQFN and PSON packages), or has a circular periphery (BGA packages). IPC-7351A also has expanded its coverage of BGA packages by providing land approximations for collapsible and non-collapsible solder balls.
The move to address these parts in IPC-7351A is important for the industry. “Lead-free technology is changing component lead technology, and the migration from gullwing leads to flat no-lead is changing component package technology rapidly,” said Tom Hausherr, CEO and director of technology for PCB Libraries Inc, a partner with IPC in the development of IPC-7351A Land Pattern Viewer and enhanced commercial versions of the software tool. “Terms such as ‘toe,’ ‘heel,’ and ‘side’ are being replaced with ‘periphery.’ As printed board speeds increase, and the need for smaller and cheaper electronic devices becomes greater, micro-miniature components are the obvious solution.”
A number of new component family packages also are discussed, including the chip-array component lead package style, primarily used for resistors, capacitors, and inductors containing multiple discrete parts. This family includes concave, convex, and flat chip-array variants, all of which require specialized solder joint engineering goals due to terminal leads that are embedded into the package. Figure 3 shows an example of a concave chip component.
Via structures within the surface mount component attachment land, or via-in-pad, have become important in the printed board design process as board component densities continue to increase. IPC-7351A provides discussion on appropriate via-in-pad designs, including the significance of voids in BGA solder joints and the need for proper via filling and capping. The latter is emphasized to avoid missing solder at the center of the solder ball, which can result from entrapped gas and solder paste volatiles in reflow soldering operations.
IPC-7351A also expands on its coverage of land pattern surface finishes. Recognizing that there is no single surface finish for all applications, the impact of commonly used surface finishes such as hot-air solder leveling (HASL), organic solderability preservative (OSP), immersion tin, immersion silver, and electroless nickel/immersion gold (ENIG) on various characteristics such as board shelf life, solder joint reliability, test-point probing, and surface finish thickness control are addressed.
IPC-7351A Land Pattern Viewer
Introduced with the original version, IPC-7351A Land Pattern Viewer is a shareware program that lets users view component and land pattern dimensional data in tabular form, as well as graphical images that illustrate how a component is attached to the land pattern on the board. The Viewer is provided on a CD-ROM with the IPC-7351A standard. An updated version of the Viewer, including dimensional data for new component families, is provided as a free download from www.ipc.org.
Figure 2. Slide concave chip component.
IPC-7351A Land Pattern Viewer is complemented by a shareware version of the IPC-7351A Land Pattern Calculator. Known as IPC-7351A Land Pattern Calculator Limited Edition, this tool allows for the calculation of CAD library parts for a limited number of component families that comply with IPC-7351A fabrication, assembly, and component-tolerance-based mathematical algorithms. The tool enables building IPC-7351A-compliant land patterns for:
- BGAs
- Ceramic Flat Packs (CFP)
- Ceramic Quad Flat Packs (CQFP)
- Leadless Chip Carriers (LCC)
- Metal Electrode Faces (MELF)
- Molded Bodies (MLD)
- Plastic Leaded Chip Carriers (PLCC)
- Quad Flat Packages (QFP)
- Small Outline ICs (SOIC)
- Small Outline J-Leads (SOJ)
- Small Outline Transistors (SOT223)
- Transistor Outlines (TO)
Conclusion
The process of amassing information needed for further revisions to the IPC-7351 standard has been made easier through IPC’s partnership with PCB Libraries Inc. Dieter Bergman, director of technology transfer for IPC, notes “Once all of the algorithms had been computerized, the entire revision process took on a new structure. At this point, users of the Viewer and associated software tools can place elements of IPC-7351A in front of printed board designers throughout the world. Input from the printed board design community on enhancements or modifications to the standard goes to the tool provider directly, and then is fed back to the IPC 1-13 Land Pattern Subcommittee for resolution. This enables a standard to be updated in real time.”
This virtual industry interaction is poised to yield dividends on an ongoing basis. New variations to periphery-style no-lead component families are entering into the industry such as dual flat no-lead (DFN) devices. As advances in lead-free processing and new solder finish applications emerge, so will the need to provide board designers with guidance in IPC-7351 Revision B.
John Perry, technical program manager, IPC, may be contacted at (847) 597-2818; johnperry@ipc.org. www.ipc.org.