PoP (package on package) and vapor phase technology
!
Excessive heat in convection reflow
processes when lead-free components are
used can be a concern because the top
component is flux dipped; there is no addi-
tional solder volume. The use of VP reflow
is the only one that actually helps PoP
technology by controlled peak temperature,
no ΔT, and SVP (soft vapor phase) technol-
ogy that gives an accurate control on ramp
rates and reflow time.
VP reflow technology is not a new
process to the EMS industry. It is simply
an alternative process for SMT reflow
that has been in existence since the early
1970’s. Today’s VP reflow process makes
use of the heat produced by an environ-
mentally friendly boiling fluorinated fluid.
!"!#$%&'#()*%#
The vapor blanket is a uniform tempera-
ture zone in which the PCBA solder is
reflowed. Heat is transferred to the PCBA
through condensation as it is immersed
into the vapor area until the PCBA reaches
temperature equilibrium with the boiling
point of the fluid.
The primary soldering benefits of VP,
!"!#+%,'%-# in comparison to infrared (IR) or convec-
tion, include an oxygen-free (inert) environ-
ment without the need for nitrogen, fixed
highest temperature exposure, and superior
heat transfer on thermally challenging
PCBAs.
Vapor phase reflow offers two main
advantages for PoP assembly:
!"!#.)/0'#1)*%#
• Better thermal transfer, which
reduces the potential of “potato
Figure 2. A micro-section image showed excellent ball collapse on primary and secondary packages after reflow.
chipping” or cracking of the
components that can occur
if the proper thermal mass
temperature is not achieved.
• Reduced cost by avoiding a ni-
trogen requirement for reflow.
In our Reliability Laboratory at EPIC,
we performed analysis on PoP assemblies
reflowed using VP technology. The aim
was to verify that solder joint formation
between PoP BGAs and the PoP assembly
and board pads conformed to IPC stan-
dards. The test vehicle used a BGA305 and
BGA128 assembly on a PCB200-12mmcad
Practical Components testing board. Kester
Tacky Flux RF743 was used for the flux dip
operation and Henkel no-clean lead-free
Multicore LF318 DAP88.5 for the solder
paste. Five PoP BGA sets were placed and
reflowed at 240˚C. Electrical testing, x-ray
and cross sectioning were used to assess
solder joint quality.
In vapor phase reflow, heat is trans-
ferred when the hot, saturated vapor
condenses on the PCBA surface and gives
up its latent heat of vaporization. The fluid
boiling point is the governing factor in
Figure 3. A darkened image of micro-section indicated excellent solderability, wetting, and ball collapse on
peak temperature. The vapor encapsulates
primary and secondary BGAs. Shown under a stereo (top) and metallurgical (bottom) microscope.
the entire surface of the board, resulting in
12 – Global SMT & Packaging – September 2009
www.globalsmt.net
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