Processing and Reliability of CSPs with Underfill
Processing and Reliability of CSPs with Underfill
Jing Liu & R. Wayne Johnson
Laboratory for Electronics Assembly &
Packaging – Auburn University
162 Broun Hall, ECE Dept.
Auburn, AL 36489 USA
334-844-1880
johnson@eng.auburn.edu
Erin Yaeger, Mark Konarski
& Larry Crane
Loctite Corporation
1001 Trout Brook Crossing
Rocky Hill, CT 06067 USA
860-571-2599
Larry_Crane@loctite.com
Abstract
The use of CSPs is rapidly expanding, particularly in portable electronic products. Many CSP
designs will meet the thermal cycle or thermal shock requirements for these applications. However,
mechanical shock and bending requirements often necessitate the use of underfills to increase the
mechanical strength of the CSP-to-board connection. This paper examines the assembly process with
capillary and fluxing underfills. Issues of solder paste versus flux only, solder flux residue cleaning and
reworkability are investigated with the capillary flow underfills. Fluxing underfills eliminate the issues of
flux-underfill compatibility, but require placement into a predispensed underfill. Voiding during placement
is discussed.
To evaluate the relative performance of the underfills, a drop test was performed and the results
are presented. All of the underfills significantly (5-6x) improved the reliability in the drop test compared to
non-underfilled parts. Test vehicles were also subjected to liquid-to-liquid thermal shock testing. The use of
underfill improved the thermal shock performance by >5x.
Key Words: Chip scale packages, Underfill, Drop Test
Introduction
As the pitch of CSPs continues to decrease and the corresponding solder volume and pad size
decreases, the susceptibility of CSPs to fail in mechanical drop tests increases. The two options to address
product failure when dropped are to improve the mechanical design or use underfills. Improving the
mechanical design of the final product to minimize the magnitude of the shock on the CSP/PWB and the
flexing that occurs will improve reliability in a drop situation. This type of design is complex and time
consuming. In portable products, where time-to-market is a critical parameter for profitability, underfill is
often chosen to mechanically couple the CSP to the PWB. By coupling the CSP to the PWB, the assembly
can withstand larger mechanical shock, vibration and bending forces.
Capillary flow underfills provide one underfill option [1-3]. Capillary underfills are dispensed and
cured after the completion of the reflow process. In a no-clean assembly, flux-underfill interactions must be
considered due to the relatively large volume of flux residue from the printed solder paste. One of the
advantages of CSPs over flip chip-on-laminate assembly is the ability to rework if a defective component is
placed. Conventional underfills do not permit rework. In this investigation, a thermally reworkable
underfill was studied. Details of the assembly process with capillary underfills including rework are
presented along with drop test results.
A second option for CSP underfill is fluxing or no-flow underfills. In this process fluxing underfill
is dispensed at the CSP site prior to CSP placement. No solder paste is printed at the site. The CSP is
placed and reflowed in a standard reflow cycle. The underfill provides the fluxing action for good solder
wetting. The underfill may cure during the reflow cycle or a post reflow cure may be required. Fluxing
underfills eliminate the issue of flux residue-underfill compatibility. In addition, as the electronic density in
portable products increases, the spacing between components decreases, making underfill dispense after
CSP placement and reflow more difficult. Fluxing underfill reduces the component spacing limitations.
Assembly issues with fluxing underfills include CSP placement, CSP floating, solder wetting, and reflow
profile. Experiments have been performed to optimize the assembly process.
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