1. PoP (package on package) technology stacks memory components to increase density and functionality in smaller electronic products. Vapor phase reflow is well-suited for PoP assembly as it provides controlled, even heating without thermal gradients. 2. A study by EPIC Technologies found that vapor phase reflow of PoP assemblies using Kester flux and Henkel solder paste resulted in excellent solder joint formation that met IPC standards. X-ray inspection found minimal voiding and electrical testing showed 100% yield. 3. Vapor phase reflow offers advantages over convection for PoP assembly by providing better thermal transfer to reduce component cracking, as well as eliminating the need for nitrogen atmosphere during

1. www.globalsmt.net
The Global Assembly Journal for SMT and Volume 9 Number 9 September 2009
Advanced Packaging Professionals
ISSN 1474 - 0893
PoP (Package on Package) and vaPor
Phase technology Mike Nelson
Interview Inside
a baseline study of stencil and screen
Print Processes for wafer backside
coating NEW PRODUCTS
fatigue & creeP wearout in INDUSTRY NEWS
electronics: a historical
retrosPective INTERNATIONAL DIARY
sPecial Packaging feature:
imbedded component/die Plus:
ic packaging and
technology (ic/dt®): interconnection
is it ready for mainstream technologies’ 4th
design applications? dimension challenge

2. PoP (package on package) and vapor phase technology
PoP (package on package)
and vapor phase technology
by Dan Coada, EPIC Technologies LLC, Norwalk, Ohio, USA
Increased use of PoP technology is driven technology are not significantly different
The trend toward smaller, by continued product miniaturization. from individual BGAs. Following screen
more densely populated Smaller products require increased printed printing, the first component is placed.
electronic assemblies is circuit board assembly (PCBA) density. Then the second component is flux dipped
driving increased use of Stacking memory is one way to achieve and placed. In some cases, up to two or
both goals for enhanced functionality and more BGAs may be stacked. The only
package-on-package (PoP) greater packaging density. significant line modification is purchasing
technology ,and electronics PoP technology is not without manufac- a flux dip module for placement machines.
manufacturing services (EMS) turing challenges. While the ball grid array Thermal profiling during reflow is the
providers are developing viable (BGA) packages used are well understood, same as boards of similar thermal mass.
manufacturing solutions. a poor quality process carries a far higher Quality needs to be carefully moni-
cost in terms of rework or scrap, since two tored. Any distortion in the bottom com-
or more BGAs are affected. ponent will be reflected in the components
Placement requirements and process above it.
Keywords: Package on Package,
PoP, Vapor Phase Reflow, Solder
Joint Formation
Figure 1. A 2-D x-ray image of primary and secondary stacked BGAs pre-reflow indicated excellent placement
accuracy.
10 – Global SMT & Packaging – September 2009 www.globalsmt.net

3. 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

4. PoP (package on package) and vapor phase technology
the smallest ΔT at short dwell times of the As a surface
board in the condensing vapor. Thermal finish, ENIG
transfer is independent of form, color, offered good solder-
mass and mass distribution of the PCBA. ability, increasingly
Detailed x-ray inspections were significant strength
performed on each BGA group, looking and a considerably
for evidence of solder bridges, voids and improved board Affordable, intelligently designed x-ray
open connections. Voids were measured finish.
using RINCON measuring software and Visual inspec-
inspection systems since 1983.
found compliant with IPC-7095 where tion under 40, 150,
the accepted voiding area is less than 25% 600 and 1200X Patented real-time x-ray imaging
(the percentage of joint cross-sectional area magnifications re- technology that creates high resolution,
occupied by the void). Voiding was mainly sulted in 100% first compact systems.
caused by flux out-gassing within molten pass yield (FPY) for
solder joints. (These bubbles form and pop all BGAs that used
open when they either grow too large or Multicore LF318M
migrate to the edge of a joint; upon solidi- DAP solder paste
fication, the bubbles become voids.) While under VP reflow.
cross-sectioning was not perfectly centered All five BGAs
on the BGA balls, the solder joints exhib- (U1, U5, U8, U11,
ited in the analysis met all requirements and U15) were suc-
of IPC-610 Revision D for component cessfully tested for
alignment, voiding, solder ball spacing and continuity.
connection (Figure 1). All criteria for
Cross sections of the components that this process were
used Multicore LF318M DAP solder paste satisfied with excel-
together with VP reflow indicate adequate lent test results.
solder volume and uniformity (Figures 2
and 3). The calculated solder volume used
is within 5% of the solder volume that
should normally occur on a production
surface mount line.
To learn more, go to:
www.GlenbrookTech.com
references:
1. Munroe, C., “Beating
the RoHS Heat,” Circuits
Assembly, March 2008.
2. Wooten, R., “Vapor
Phase vs. Convection Re-
flow Technology” SMT,
July 2009
Dan Coada is a process/
manufacturing engineer
with EPIC Technologies. He
can be reached at coaddc@
epictech.com.
Figure 4. Vapor Phase profile that has been used to reflow the boards.
www.globalsmt.net Global SMT & Packaging – September 2009 – 13