Download III. PCB thermophysical properties assessment

PCBs Thermophysical Properties in Lead-Free
Assembling Process Assessment
e-mail: [email protected]
I. Plotog, M. Branzei, P. Svasta, M. Miculescu, T.
Cucu
A. Thumm
“Politehnica” University of Bucharest, Center for
Technological Electronics and Interconnection Techniques
UPB-CETTI
Bucharest, Romania
Abstract— As elements of interconnection structures at level two
in electronic packaging, the pads could be considered as a
particular solution into a virtual space defined by the materials,
geometry, surface finishes and substrate [1, 2]. The assembling
processes emphasize the substrate contribution over thermal
mass and heat transfer [3]. Each of the pad particular solution
defined in the design stage will have unique values of
thermophysical properties (THP) [2]. In the paper it will be
presented the results of the THP measurements for different
types of PCBs having particular solutions defined by the pad
finishes, geometry and substrate materials using ANTER
equipments (FlashLine 3000 thermal diffusivity system and
Unitherm1161V dilatometer type). Finally, scientific and
practical conclusions shall be drawn in order to improve the
quality, reliability and increase the energetic efficiency of the
reflow soldering process.
IBL-Loettechnik GmbH,
Koenigsbrunn, Germany
are determined in principal by the thermophysical properties
(THP) of substrate materials for the same soldering conditions.
II. CONSEQUECES OF THE PCBS SUBSTRATE
THERMOPHYSICAL PROPERTIES IN PRACTICE OF THE
ASSEMBLING PROCESES
The practice of the assembling processes, emphases the
difficulties met when the PWBs substrates have as core
materials glass (Fig. 1a) or aluminum (Fig. 1b) [4, 5].
Especially in case of glass core, on convection line was very
difficult to obtain good results. After assembling process the
Glass Circuit Boards (GCBs) present delaminations (Fig. 1a)
and zones where the solder paste was melted only on electronic
components terminals (Fig. 1c) being practical drayed on pads
(Fig. 1d).
Keywords- PCB substrate, pad, thermal diffusivity
f
I.
INTRODUCTION
In the last decade new type of printed wire boards (PWBs)
having different core materials were promoted on the market.
Requirements for improved heat of the power electronic
components (Power LEDs applications, as example) led to a
new printed circuit boards (PCBs) technology, realized by
using as interconnection structures support PWBs having
copper or aluminum substrates [4]. Requirements for a very
clean or with high level of humidity environment led to create
PCBs with glass substrate [5]. Typical applications are for
medical instruments, products and devices. Addressing only to
SMT issues, the new materials are a challenge for conventional
assembling technologies. High thermal mass of the substrate
makes difficult to be soldered these PCBs type using infraredconvection reflow oven in Surface Mounted Technology
(SMT) assembling line, especially since the lead-free
technology raised the process temperature [4, 5]. In these cases,
Vapor Phase Soldering (VPS) technology seems to be the most
appropriate for obtaining the best assembling results [4, 5, 6].
The above mentioned PWBs substrate technology present
radically different thermal behavior compared to the classical
types (FR4, FR2, CEM). The results of the practical
experiments emphasize these differences especially regarding
comparison between the imposed thermal profile (TP) of the
oven and the real one’s at the PCB surface. These differences
e
a
a
c
d
b
Figure 1. The assembling process of PWB with glass & aluminum core
experience.
Delaminations were not totally eliminated even was used a
very low speed for conveyor (0.4m/min) according with TP P2
(Fig. 1e, f). The experiments demonstrated that the best
assembling solution for PCBs having glass or metal core
substrate was VPS [4, 5].
In order to point out the differences between the studied
PCBs, as function of substrate material, in the first step of the
experiments, all bare PCBs were simultaneously submitted to
the same TP and VPS process, in the lead-free technology
conditions. Their TP responses were investigated by adding
temperature sensors on each board (Fig. 2). The testing profile
was one normally used for a common FR4 board in the
assembling line of an EMS company. It was used the same
solder paste, SAC305 type3, OM338T Cookson. The results
showed that the aluminum board needs more heat quantity, so
it reaches reflow zone with some delay compared to FR4, and
also, GCB increases its temperature very slowly and had a too
short Time Above Liquidus (TAL) period in order to make a
good solder joint (Fig. 3). As conclusion, the high thermal
mass of metal and GCBs requires more heat and controlled
heat transfer conditions in order to accomplish the
requirements for a good soldering. The results of the previous
experiments are determined by the rate at which heat is
transferred from the working vapor to the PCBs characterized
by the differences between the THP as function of their
structure.
and substrate core material. Each of the particular solution
defined will have unique values for the THP with
consequences over TP. The heat conduction in PCB substrate,
the speed of heat transfer and heating inertia are influenced by
the THP (conductivity & diffusivity) of 4P Soldering Model
KPV, having as consequence the differences between specific
TPs presented in figure 3.
Thermal mass (THM) is a concept in electronic packaging
which describes how the mass of the ensemble PCBs
(substrate, metallic interconnection structures with pads having
different finishes, solder paste deposits, components leads and
body) provides "inertia" against temperature fluctuations
determined by the TP in reflow soldering process. THM
represent the ability of the ensemble PCBs to store the heat
transmitted in the soldering process. Physically thermal mass is
equivalent to thermal capacitance or heat capacity (Cth, (J K-1)):
Cth = m c
(1)
Where: m = mass of PCB structure (kg)
c = specific heat capacity (J kg-1 K-1)
Thermal inertia is a measure of the thermal mass and the
velocity of the thermal wave which controls the gradient
materials temperature. Physically, inertia is equivalent to
thermal effusivity (e, (W s1/2 K-1 m-2)):
e = (λ ρ c)1/2
(1)
Where: λ = thermal conductivity (W m ·K )
ρ= m/V, density (kg m-³)
ρ c = volumetric heat capacity (J m-³·K-1)
V = volume of PCB structure (m³)
-1
Figure 2. GCB, Al and FR4 PCBs on carrier in VPS machine
Volumetric heat capacity (VHC) describes the ability of
PCB in the soldering process to store the heat as internal
energy while undergoing a given temperature change according
with TP, but without undergoing a phase change. For a given
specific heat value of the material, one can convert it to the
VHC multiplying the specific heat by the material density. In
the soldering process, a higher value of the VHC, that means a
longer time for the PCB ensemble to achieve temperature
according with imposed TP for specific soldering process.
The speed of heat diffusion is characterized by the thermal
diffusivity (α) of PCB ensemble (PCB substrate, metallic
interconnection structures, pads having specific finishes, solder
paste deposits, components leads and cases), which govern the
heat flow at the PCB surface and from the surface into their
interior with the SI unit ( m²/s):
250
[°C]
FR4
200
Al
Glass
150
Carrier
100
50
0
0
60
120
180
240
300
-1
[s] 360
Figure 3. VPS thermal profile as function of substrate type
As consequence, the PCBs can be considered a complex
element in the Lead-Free assembling process completing the
4P Soldering Model key process variables (KPV): Pad-PastePin-Process [2]. Defined as final product of assembling
process, the solder joints could be considered result of KPV
synergistically interactions into reflow soldering process.
Consequently the heat transfer according to specific reflow
soldering process TP depend and is strongly influenced by the
THP of the KPV, each of which being complex function
defined in the Design for Manufacturing (DFM) stage. The pad
variable is function of the different finishes types, geometry
α=λ/ρc
(2)
Materials with high thermal diffusivity rapidly adjust their
temperature to that of their surroundings, because the heat
transfer is quickly in compare to their VHC (ρ c).
In the soldering process the heat transfer must be controlled
in order to realize TP on PCB assemble mass taking into
consideration process time constant [7]. The heat transfer time
constant τ can be defined as function of τcv. specific for the heat
transfer by convection to the PCB and another, τth specific for
the internal heat transfer by conduction into the PCB structure:
τcv = ρ c V / h Acv =m c / hAcv = Cth Rcv
(3)
τth = Cth Rth = Cth x / λ Ath = m x / α ρ c Ath = x2 / α
(4)
Where: Acv = convective heat transfer surface areas (m2)
Ath = PCB conductive heat transfer surface areas (m2)
x = PCB structure thickness (m) = V / Ath
h = heat transfer coefficient (W / m2 K)
R = 1/h Acv, convective thermal resistance (K W-1)
R = x/ λ Ath, conductive thermal resistance (K W-1)
As general conclusion, τcv is proportional with masses ρV
and larger heat capacities Cth lead to slower changes in
temperature, while larger heat transfer surface areas Acv and
better heat transfer h lead to faster temperature changes (3).
Thermal diffusivity has influence in reducing τth in the heat
transfer of soldering process (4), which is strongly dependent
by ensemble PCB thickness. The practical problems are
generated by the complex geometry of ensemble PCB, being
difficult to estimate heat transfer time constant. In
consequence result the necessity of extended THP
measurements over the PCB assembly having different
substrate materials.
III.
Thermal conductivity is calculated from (2) using value for
diffusivity, specific heat and sample density.
Figure 4. Schematic of flash diffusivity measurement and temperature rise
curve.
PCB THERMOPHYSICAL PROPERTIES ASSESSMENT
A. Theoretical support
Conclusions of the experiments at this stage emphasizes the
influence of materials THP not only in assembling area but also
in the development, specification, and quality control of
materials used in electronics packaging and thermal
management. The THP are measured for pure substances and
intrinsic materials. In the electronic packaging the necessity of
THP measurements not only for intrinsic materials but also for
PCBs ensemble becomes strongly required. This data can be
critical to a successful DFM of an electronic product,
especially with the rapidly increasing cooling requirements in
the assembling process that result from the packaging of higher
performance devices. A variety of methods, involving both
steady state and transient techniques, are available for
measuring thermal diffusivity, specific heat and thermal
conductivity. In order to define the soldering conditions for
PCBs with different core materials, the THP of PCBs, GCB,
CEM and FR4 types, were measured using ANTER
equipments (FlashLine 3000 thermal diffusivity system and
Unitherm11611V dilatometer type) specialized on the flash
diffusivity method (ASTM E1461).
The flash method (Fig.4) consists in heating one surface of
a small disk of the material with a single pulse from a flash
energy source, and measuring the resulting temperature rise on
the opposite surface as a function of time [8, 9].
The thermal diffusivity value is obtained using formula:
α = 1.388 x2/ t50 (4)
Where: t50= the “half max rise time”, is the time for the
back face temperature to reach 50% of its maximum value,
and x is the thickness of the sample.
The specific heat of a material sample can be measured
with this method by comparing the sample temperature rise to
the ones of a reference sample of known specific heat
measured under the same conditions:
c PCB sample
= (m c ΔT)ref / (m c ΔT)PCB sample
(5)
The absorptive efficiency of the front surface of the
samples to the energy pulse and the radiative efficiency of the
back surface to the IR detector are controlled by coating the
calibration and test samples with the same graphite film.
B. Experiments
In the experiments the multilayer structure of ensemble
PCBs was take into consideration, an equivalent property
referring to each of experimental structure being practically
measured. The probes used (fig. 5) were made (for each PCB
type) from bulk material of substrate, completed with copper
foil, traces and assembled with components in order to
emphases the THP differences function of PCB structure.
Sample geometry is a disk with 31.5 mm in diameter. The
thickness depends on PCBs type manufacturing stage.
Figure 5. FR4 thermophysical properties measurements probes: a=FR4 base material,
b=FR4 with traces, c= FR4 assembled with resistors 1206 type.
C. Experiment result and disscutions
The measurements results for the back face temperatures
for FR4 type substrate material probes (Fig. 6) are used for
THP calculations. The results are presented in Tab. 1.
Could be notice the contribution of each layer; so,
compared to FR4 substrate the copper traces increase the
diffusivity and decrease the conductivity. Regarding to entire
structure, the THP emphasize the major contribution of
substrate in establishing of soldering process heat transfer
necessities, according with TP requirements for each phase
(Tab. 1). This conclusion offer practical solution for heat
transfer time constant calculation as function of thickness and
substrate material diffusivity with an acceptable error
estimation.
FR4 Substrate
Substrate &
Copper Traces
IV.
CONCLUSIONS
The THP of PCBs as complex term of the “4P” Soldering
Model emphasizes the major contribution of substrate as a
function variable, especially from soldering process heat
transfer necessities according with thermal profile
requirements for each phase.
Based on the experiments results and the THP
measurements over intrinsic materials and ensembled PCB
appear as consequence the necessity to transfer in the
soldering process more heat to substrate by increasing heating
rate on bottom side or preheat top side. The VPS using the
boiling process ensures these conditions as consequence of
physical principles and could be improved using infrared
preheats. The experiments will be extended in order to realize
a data base for printed circuit boards (PCB) having different
core material as substrate and different pads finishes.
ACKNOWLEDGMENT
The authors are very grateful to the leading staff of IBLLöttechnik GmbH for continuous support and collaboration to
fulfill experiments used in the presented paper.
Many
thanks
from
the
authors
to
FELA
Leiterplattentechnik GmbH, the company that supplied the
glass and metal core PCBs for experiments and to Cookson
Electronics which offered the solder pastes for experiments..
This paper represents the result of work done for promoting
innovative technologies in the frame of INOINDEX project
(CNMP: 91-002/2007).
Substrate,
Copper Traces
& Components
REFERENCES
Figure 6. Temperature rise curves for FR4 probes
TABLE I.
PCB THERMOPHYSICAL PROPERTIES
FR4
Probe Type
T
[°C]
FR4 Substrate
Substrate & Copper Traces
Substrate,
Copper Traces & Components
106
110
TABLE II.
[1]
105
(α)
(λ)
(c)
Thermal
Thermal Specific
Diffusivity Conductivity heat
[cm2/s]
[W/mK] [J/KgK]
0.0027
0.47
916
0.003
0.42
746
0.0021
0.38
936
COMPARATIVE MEASUREMCOMENTS OF THP FOR PCBS
HAVING FR4, CEM1 AND GLASS SUBSTRATES
(α)
(λ)
(c)
Substrate Thickness Density
Thermal
Thermal
Specific
T [ºC]
Type
[mm] [g/cm3]
Diffusivity Conductivity
heat
[cm2/s]
[W/mK]
[J/KgK]
FR4
0.71
1.943
CEM1
1.51
1.563
GLASS
3.94
2.527
106
119
157
221
107
118
157
220
108
159
0.0021
0.0021
0.0019
0.0015
0.0011
0.001
0.0009
0.0008
0.0051
0.0042
0.387
0.394
0.405
0.422
0.236
0.231
0.219
0.175
0.658
0.616
936
958
1082
1221
1403
1425
1494
1612
511
581
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