Reflow significance on package on package (PoP) assembly
the balls of the top package is devoid of die, thinner encap-
any mold compound. Therefore, at reflow sulant materials
temperature, the exposed thin substrate etc. While all these
will expand more when compared to the design parameters
substrate under the mold compound, help achieve better
resulting in a smiling (concave) warpage of assembly yield, they
the bottom package. On the other hand, present additional
the top package that has the mold com- challenges in the
pound extending to its edges experiences a assembly, especially
crying (convex) warpage. the reflow process.
Reworking PoP
Defects and root causes after the reflow pro-
The defects that are caused during the cess also requires
reflow process and the possible root causes a greater level of
are identified in Table 1. operator skill, there-
fore requiring the
solder on pad (sop) pop technology first pass assembly
Solder on pad (SOP) PoP assembly technol- yield to be high.
ogy addresses several issues, like standoff
height, solder joint collapse size and the conclusion
ability to solder fine pitch components. In The extent of PoP
SOP technology, solder balls are present warpage during
on the topside of the bottom package. reflow dictates
Mission Possible.
These balls must be aligned with the balls the final package
Versatility that beats complexity.
of the top package during assembly. The stacking yield on
It’s not just a promise, it’s a fact!
ball-to-ball stacking provides a larger solder the PCB. The main
assembleon.com/facts
joint collapse compared to non-SOP based objective of a good
PoP. This large collapse helps to absorb the reflow profile must
package warpage. On the other hand, the be to control the
ball-to-ball stacking presents new chal- package warpage
lenges, particularly in the reflow process. throughout the pro-
The most common challenges are the balls cess. Since the use
of the top package sliding off from the of lead free solder
balls of bottom package and the inability demands a narrow process window in the
yields.
to create a good solder joint from the two reflow process, mor
2008-11-24 FA 87x124 Global SMT & Packaging 3.indd 1
e emphasis should be
11/5/2008 2:19:16 PM
contacting solder spheres. placed on arriving at an acceptable reflow
S. Manian Ramkumar, Ph.D., is professor and
profile. For extended reliability, different
director at the Center for Electronics Manufac-
Future challenges lead-free solder alloy combinations are also
turing and Assembly at the Rochester Institute
The technology trend towards package being considered for the top and bottom
of the Technology, Bldg. 78-Room 1518; 78
miniaturization necessitates thinner packages, making reflow soldering the
Lomb Memorial Dr., Rochester, NY 14623;
substrates, reduced standoff height, smaller most important process for achieving good
585-475-6081;
www.rit.edu/CAST/CEMA.
Brian O’Leary, North American sales manager
Defects Root Causes
for KIC, may be contacted at KIC, 15950
Bernardo Center Drive, #E, San Diego, CA
Bridging and slidingF Excessive package warpage
92127; 858-673-6050; Fax: 858-673-0085;
F High preheat rates
boleary@kicmail.com;
www.kicthermal.com.
F High peak temperatures
Open jointsF Irregular warpage between both top and bottom pack-
ages
F Flux evaporation in preheat leading to solder
oxidation
TombstoningF Waveform type warpage leading to one side of the
package getting soldered first resulting in lift up of the
other side
F Velocity of air or nitrogen may lift up the component
VoidingF Excessive preheat rate
F Out gassing of volatiles
Head and pillow defectsF Poor melting of the solder joint
F Package warpage
Table 1. Reflow defects and possible root causes
www.globalsmt.net Global SMT & Packaging – December 2008 – 29
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