QFN-28 5×5 mm Footprint: DFM, Layout, and Assembly Guide

QFN-28 5×5 mm is a no lead package used for surface mount assembly, also seen labeled 28-QFN, MLF-28, 0.5 mm-pitch QFN. A dependable footprint follows the exact orderable-device drawing rather than the family name: nominal body 5.0 × 5.0 mm nominal, overall span 5.0 × 5.0 mm body, seated height Often 0.75–1.0 mm, pitch 0.5 mm, pin count 28 perimeter terminals plus optional exposed pad, and exposed pad Common and often thermally important.

Use the orderable part's terminal, corner, and exposed-pad tolerances; pin count and 5 mm body are insufficient identifiers.

Typical uses include CAN transceivers, radio ICs, embedded controllers. QFN-28 5 × 5 mm packages commonly use 0.5 mm pitch, but exposed pad and terminal construction remain MPN-specific.

• Seven leads per side form a long hidden joint row. A larger body gives more center-pad area but also more opportunity for voiding and paste imbalance.
• A no-lead package is defined by its complete manufacturer drawing: nominal body and pitch do not fix terminal length, pullback, exposed-pad size, or corner geometry.

Use the extra perimeter for orderly interface fan-out, maintain reference-plane continuity, and connect the center pad with an assembly-compatible thermal-via grid.

• Escape outer terminals without narrowing below the fab profile, keep copper out from under the package unless intended, and connect any exposed pad to its documented net.

Hand assembly is rated expert-only. Stencil reflow with profiled center-pad apertures. Watch for package float, hidden opens, and unsafely placed thermal vias.

• Set an acceptable void criterion for the device's actual thermal load and use several paste windows rather than one large central deposit.
• Use a windowed or otherwise reduced paste pattern on a large exposed pad so the body is not floated above its perimeter joints.
• Agree mask-defined versus non-mask-defined geometry, via fill, and inspection method with the assembler before release.

Inspection focus:

• X-ray the center pad and perimeter alignment on early builds; thermal-load test verifies whether the land and via structure meet the assumed junction path.
• Perimeter wetting may be only partly visible and the center pad is hidden. Use X-ray or validated process evidence when voiding and opens matter.

Common mistakes:

• Treating exposed-pad void percentage as cosmetic can hide a thermal bottleneck on a regulator, driver, or radio power amplifier.
• Do not copy a generic QFN, DFN, or SON footprint with the right pin count but a different exposed pad or terminal pullback.

1. Before approving QFN-28 5×5 mm, compare the exact orderable-device drawing with the library item: body range (5.0 × 5.0 mm nominal), terminal or lead span (5.0 × 5.0 mm body), pitch (0.5 mm), pin count (28 perimeter terminals plus optional exposed pad), height (Often 0.75–1.0 mm), and exposed-pad definition (Common and often thermally important). Record the source drawing revision and every intentional courtyard, toe, heel, side, mask, or paste adjustment.
2. Treat the expert-only hand-solder rating as a prototype-planning input, not proof of production yield. Review package float, hidden opens, and unsafely placed thermal vias with the assembler, confirm that stencil reflow with profiled center-pad apertures is compatible with the build, and require the S1 connectivity gate plus relevant S2 geometry checks to pass against the released footprint and selected fabrication profile.

Manufacturing gate checks:

1. S1Pad count, numbering, and schematic parity. The gate should require the center-pad net, via construction, solder-mask clearance, and all twenty-eight pin assignments to be explicit.
2. S1Exposed-pad connectivity and paste segmentation. A missing or wrongly netted center pad can break electrical, thermal, and mechanical performance while remaining visually hidden.
3. S2Courtyard and body clearance. The body, leads, placement tolerance, rework access, and nearby height limits all belong in the manufacturing review.