ADVANCED PACKAGES REQUIRE A GOOD REWORK PROCESSAuthor: Paul Wood
Company: Metcal Inc.
Date Published: 9/22/2002 Conference: SMTA International
These packages are called LLP (Leadless Leadframe Package manufactured by National Semiconductor) MLF, (Micro lead frame) manufactured by Amkor; LGA Land Grid Array is generic terminology (For all of the above). These packages contain multiple pad sizes on the same device and are soldered with all terminations on the underside of the package similar to BGA except no balls exist only pads.
THESE COMPONENTS LEND THEMSELVES TO NORMAL ASSEMBLY TECHNIQUES SUCH AS SCREEN-PRINTING PCB, PICK AND PLACE AUTOMATICALLY THEN CONVECTION REFLOW. The LLP design has the body of the component touching the PCB and often is soldered to the center of the LLP as a ground or power connection.
The center pads soldered act as a heat spreader to the PCB substrate, this helps keep the part cool and this controls expansion of the part to the substrate so that the TCE (Thermal coefficients of expansion) tolerances of the part to the PCB board material are minimized.
The solder thickness being used as the joint is the solder paste printing of 4 to 5 mil or (thousands of an inch) thick. Rework process requires all the pads to be soldered to the same height or opens and potential for small land connections could be an issue.
The volume used for the center pas and array pads have to be controlled to achieve good reliable joints.
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