AN-9055 Datasheet, PDF (1/5 Pages) Fairchild Semiconductor – Assembly Guidelines for MicroFET 2x2 Dual Packaging

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AN-9055
Assembly Guidelines for MicroFET 2x2 Dual Packaging
Summary
The Fairchild MicroFET 2x2 dual is a 2mm x 2mm package
based on Molded Leadless Packaging (MLP) technology.
This technology is often used for power-related products
due to its low package height and excellent thermal
performance. Large thermal pads in the center of the
package solder directly to the Printed Wiring Board (PWB).
Modularity in package design, single- and multi-die
packages, is within MLP capability.
The MicroFET 2x2 has two large die-attach pads, allowing
direct soldering to the PWB for best thermal and electrical
performance. The MicroFET 2x2 is designed to be used
with Fairchild discrete MOSFET products. This application
note focuses on the soldering and back-end processing of
the MicroFET 2x2.
Board Mounting
The solder joint and pad design are the most important
factors in creating a reliable assembly. The pad dimensions
must allow for tolerances in PWB fabrication and pick and
place, which are necessary for proper solder fillet formation.
MLP packages, when the pre-plated lead-frame is sawn,
show bare copper on the end of the exposed edge leads. This
is normal and is addressed by IPC JEDEC J-STD-001C
“Bottom Only Termination” standard. In practice, optimized
PWB pad design and a robust solder process often creates
solder fillets to the ends of the lead due to the cleaning
action of the flux in the solder paste.
PWB Design Considerations
Any land pad pattern must take into account the various
PWB and board assembly tolerances for successful
soldering of the MLP to the PWB. These factors are
considered for the recommended footprint in the datasheet;
follow this footprint to assure best assembly yield, thermal
performance, and overall system performance.
Figure 2. Exposed Copper on Package Edge, Solder
Wetting after Reflow, from Singulation Process
Pad Finish
The MicroFET 2x2 is sold with a lead-free lead finish.
Immersion silver, immersion nickel gold, and Organic
Surface Protectant (OSP), are the pad finishes of choice for
lead-free processing. Hot-air solder-level pad finish does not
have chemistry compatibility issues, but is not
recommended due to inconsistency in the thickness of the
finish. Each finish has useful properties and each has
challenges. It is beyond the scope of this paper to debate
each system’s merits. No one finish is right for all
applications, but the most common in large-scale consumer
electronics, largely due to cost, is OSP. A high-quality OSP,
formulated for the rigors of lead-free reflow, like Enthone^®
Entek^® Plus HT, is recommended.
Figure 1. MicroFET 2x2 Dual (Bottom)
© 2011 Fairchild Semiconductor Corporation
Rev. 1.0.1 • 4/8/11
www.fairchildsemi.com