Land-Pattern Planning for Chip-Scale BGA

By Vern Solberg Tessera Inc., San Jose, Calif.

A recent industry survey conducted in Japan indicated that 285 different chip-scale or chip-size packages are being planned, in development or already in production.

In the United States and Europe, a majority of the leading IC suppliers are either in the process of ramping-up chip-scale packaging for high volume applications or are already producing devices for the merchant market.

To date, most of the CSPs use the ball grid array style of contact, but because of the unique differences in packaging methods and materials used to manufacture the chip-scale devices, not all companies have come to an agreement on the same contact diameter for all CSP applications.

The most common pitch for flashmemory devices, for example, is 0.75 mm while 0.50 mm and 0.80 mm pitch CSPs are also gaining acceptance. The sphere diameter issues, however, remain unresolved.

All companies have agreed to a 0.30 mm diameter for CSPs with 0.50 mm contact pitch, and the same size sphere has been successfully adapted to 0.75 mm and 0.80 mm pitch CSPs as well. However, a few companies hope to compensate for the TCE mismatch between the package and circuit structure, and perhaps improve their product's reliability to a degree, by increasing the ball diameter as the pitch widens.

Contact Variations
These ball pitch and ball size variations cause serious concerns to those testing and using the devices. Of course, the socket manufacturers (many have already tooled for the 0.30 mm diameter spheres) and designers of the circuit board recommend a common ball diameter. Standardizing on a common contact pattern for all pitch variations will allow the PC board designer to furnish uniform land pattern geometry and plan better for efficient circuit routing schemes.

The socket suppliers, by engineering and manufacturing a single-sized contact, have better control of their development costs and can provide better service to their customers. With this logic in mind, and as a matter of simplification, this application note for chip-scale BGA land-pattern development will assume a single (0.30 mm) contact size for all pitch variations.

Land Patterns for Chip-Scale BGA

To date, most of the industry users of BGA devices have attached the package to either a round or square-shaped land pattern. Land-pattern geometry may be nearly the same size as the contact sphere. However, some companies have adapted an attachment site that is slightly smaller than the sphere's diameter. A smaller attachment site tends to prevent the full collapse of the ball during the soldering process, and board designers gain a slightly wider path between land patterns for circuit routing.

The contact sphere, which is typically a eutectic tin-lead or other alloy composition, is formulated to be compatible with conventional SMT reflow solder processes. This technique has provided a generally acceptable and reliable attachment method for most of the array-type devices.

The surface tension created as the alloys liquefy and merge actually improves the positional accuracy of the device over the land pattern sites. The solder joint developed between the sphere and attachment site, as well as peripheral length defined by the circumference of the land pattern, has proven to be strong as well as reliable for most applications.

Enhanced CSP Land-Pattern Geometry
Other land-pattern geometries, beyond the round shape, may be considered as well. For several years, the industry has enhanced the basic, round-shape pad (or land) with multiple narrow features or spokes extending from the attachment site center to an area well outside the pad.

These spoke-like features are generally adapted for two specific benefits: In one typical application, the extensions connect to larger planes on the circuit structure (for example, ground or power planes). The narrow extension has traditionally provided a thermalisolation to promote uniform heating and cooling of the solder during the assembly process.

Extensions Added
Another example where spoke extensions are added to the basic land pattern improves surface strength to the attachment site, typical of those illustrated in IPC-D-350, Printed Board Description in Digital Form and IPC-D275, Design Standardsfor Rigid Printed Boards and Printed Board Assemblies.

Originally published in 1991, the 275 document, widely accepted by the electronics industry, replaces many established military standards (see Figure 1).

Small-diameter land patterns that rely exclusively on the copper foil adhesive for mechanical strength are subject to damage at various stages of the assembly process. When the board is heated during reflow soldering„and even more sign)ficant„during rework or repair, the adhesives often become very soft and land patterns may shift or even lift off of the surface of the circuit structure.

For years, flexible circuit fabricators have recommended entrapment of the extended features by the cover layer, typically a dielectric film applied over the exposed copper circuit pattern.

Companies that experience damaged or lifted pads on rigid circuits have found the entrapment techniques work equally as well when the extensions are coated by the solder-mask layer, a polymer coating generally applied to the board's outer surface.

Land-pattern geometries for finepitch CSP array devices are comparatively smaller than geometries for the coarser pitch plastic BGA package. Extensions from the land pattern, typical of the example shown, may not be practical.

The land-pattern geometry for CSPs is considerably smaller, and when adding the extension features to the landpattern the attachment site may, for example, resemble an "X". Companies that adapt the square geometry may consider solder-mask coverage at the corners to reduce pad lift.

By combining the square geometry with the "X" illustrated in Figure 2, two distinctive advantages occur: First, the corners of the land pattern can be entrapped by the solder mask (or coverlayer) material, and the internally penetrating radii will provide even more perimeter exposure when attaching the contact sphere to the land pattern site.

Further study and testing of landpattern configurations will continue by the author and his colleagues. To share experiences with the industry, readers are encouraged to submit recommendations of alternative geometry for future publication here.

Please contact Chip Scale Review or Vern Solberg at Tessera by e-mail, vern@tessera.com or phone 408.383.3614.