Reflow significance on package on package (PoP) assembly
Reflow significance on
package on package (PoP)
assembly
by S. Manian Ramkumar, Ph.D., Center for Electronics Manufacturing and Assembly at the Rochester Institute
of the Technology, Rochester, NY, and Brian O’Leary, KIC, San Diego, CA
Package on package (PoP) is a rapidly evolv- are becoming more critical as geometries
PoP assembly presents many
ing 3D packaging technique that meets the become smaller and substrates become
technical challenges, especially
demand for lighter, faster and miniaturized thinner. Compounding this issue is the use
electronics. PoP is the stacking of one pe- of lead-free solder, which melts at higher
during the reflow soldering
ripheral array BGA package on another full, temperatures, increasing the coefficient of
process, including the risk
peripheral or custom array BGA package. thermal expansion (CTE) mismatch and,
of solder balls dislodging or
One of the key concerns with PoP assembly hence, the package warpage.
migrating off the pad and de-
is the final assembly yield, affected by pack- A critical aspect of PoP assembly is the
fects attributable to package age warpage. The widely adopted process for type and amount of package warpage that
warpage, particularly when PoP assembly is single-pass reflow of the top can occur during the reflow process. Experi-
lead-free solders, with their
and bottom packages after being assembled mental studies show that the root cause for
higher melting temperatures,
on the PCB. The solder balls of the top more than 90 percent of the PoP assembly
are used.
package are either dipped in flux or solder defects is package warpage. Minimizing pack-
This article discusses
paste before being placed on the top of age warpage is possible by controlling reflow
common PoP assembly defects
the bottom BGA package during assembly. time, temperature, packaging materials and
The flux or paste dip, unlike solder paste selecting appropriate package design. From
and their root causes.
print, will be unable to accommodate for the PCB assembly perspective, the first two
the solder ball co-planarity issues and pack- parameters gain importance. Arriving at an
age warpage between the top and bottom acceptable reflow profile that falls within the
packages. This makes PoP assembly more specifications can control these parameters.
Keywords: Package on Pack-
challenging.
age (PoP) Assembly, Reflow
causes of package warpage
Soldering, Defects
Technical challenges Package warpage is caused due to the mis-
Many technical challenges are present in PoP match in CTE between the substrate, mold
assembly, especially during the reflow solder- compound (package overmold) and the die.
ing process. These include the solder balls Typically, higher glass transition temperature
dislodging or migrating off the pad with (Tg) mold compounds are used to balance
improper reflow profiles and excessive pack- the CTE mismatch between the die and the
age warpage, leading to solder ball bridging, substrate. The effect of the mold compound
solder slumping, head and pillow defects, or becomes negligible at reflow temperatures,
open joints. Substrate warpage characteriza- as they tend to become soft. The periphery
tion for the component packages and PCBs of the bottom package that accommodates
Figure 1. Sliding of top package prior to reflow Figure 2. Bridging of solder balls (usually the result Figure 3. Tombstone failure. [Photo courtesy Amkor
results in shifted columns. [Photo courtesy Amkor & of a major sliding of the top package). [Photo cour- & Panasonic Factory Solutions.]
Panasonic Factory Solutions.] tesy Amkor & Panasonic Factory Solutions.]
28 – Global SMT & Packaging – December 2008 www.globalsmt.net
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