IPC-7095C-2013.pdf - 第97页
should be maintained between 230-245°C for lead-free. The time above liquidus (T AL) should be 60 to 90 seconds. Extended duration above the solder melting point or T AL will damage temperature sensitive components. It a…
7.1.4.5 Thermocouple Attachment Figure 7-8 shows
recommended locations of thermocouples on a board. It is
important that thermocouples be attached onto small and
large components at the solder joints. For BGAs, it is also
important to attach a thermocouple at the top of the pack-
age.
In developing any profile, it is very important to use the
right thermocouple. Type k thermocouples with wire gauge
of 36 AWG should be used. Thicker thermocouple wires
add too much heat sink. Thermocouple wire length should
not exceed three feet for good accuracy. To ensure accu-
racy, thermocouple junctions must be welded. No twisting,
crimping or soldering should be used.
Care should be exercised when using high temperature
tapes such as polyimide (kapton®) or aluminum tapes.
Tapes tend to come loose during reflow and the system
measures the temperature of the air in the oven and not the
temperature of the solder joints. It is important to make
sure that there is good contact of the tape, otherwise, a high
temperature solder or thermally conductive adhesive
should be used to attach thermocouples to the solder joints.
One benefit with using tape is that the thermocouples can
be reused repeatedly without damaging them.
In the case of BGAs, drill holes in the center and corner
balls of the BGA from the bottom of the board and push
the thermocouples to the top to correctly measure the tem-
perature of BGA balls. It is important that the difference in
temperature of the center and corner balls of the same
BGA are within 2°C of each other. There are also some
thermocouples that can be inserted under the BGA, elimi-
nating the hole drilling process; however, in this instance,
the thermocouples may only be measuring the temperature
under the device.
Four to six thermocouples should be attached at various
component locations to represent the lowest to highest ther-
mal mass areas including at least two thermocouples for
BGAs. Figure 7-9 shows locations of thermocouples on a
BGA.
7.1.4.6 Soak Zone The soak zone is intended to bring
the temperature of the entire board up to a uniform
temperature. The ramp rate in this zone is very slow,
almost flat when raising the temperature from 75-220°C.
The soak zone also acts as the flux activation zone for sol-
der paste. The consequences of having too high a tempera-
ture in the soak zone are solder balls, solder splatter due to
excessive oxidation of paste, and spent flux activation
capability. The purpose of long soak zone is to minimize
voids, especially in BGAs. It is also common practice not
to use soak zone but to steadily ramp the temperature from
preheat zone to peak reflow. However, the likelihood of
voids may be increased when ramping steadily to peak
reflow temperature.
7.1.4.7 Reflow Zone The peak temperature in the reflow
zone should be high enough to obtain good wetting, and
create a strong metallurgical bond. However, it should not
be so high as to cause component or board damage or dis-
coloration or, in worst case, charring of the board. If the
temperature is too low, cold and grainy solder joints, non
melted solder, or poor inter-metallic bonding may result.
As shown in Table 7-3, the peak temperature in this zone
Figure 7-7 Examples of Lead-Free Profiles with Soak (Top)
and Ramp to Peak (Bottom) with Multiple Thermocouples.
The Profiles with Soak Tend to Reduce Voids in BGAs.
Figure 7-8 Locations of Thermocouples on a Board with
Large and Small Components
IPC-7095c-7-9
Figure 7-9 Recommended Locations of Thermocouples
on a BGA
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should be maintained between 230-245°C for lead-free.
The time above liquidus (TAL) should be 60 to 90 seconds.
Extended duration above the solder melting point or TAL
will damage temperature sensitive components. It also
results in excessive intermetallic growth which makes the
solder joint brittle and reduces solder joint fatigue resis-
tance.
7.1.4.8 Cooling Zone The typical cooling rate for most
assemblies has been 4-6°C per second, driven primarily by
throughput and SnPb intermetallic thickness concerns.
With the transition to Pb-free solders, the pad cratering
defect has become more common due to the increased stiff-
ness of the SAC solders and the reduction in the flexure
resistance of laminates. Pad cratering has been identified
directly after the reflow process leading to several experi-
ments designed to understand the impact of cooling rate.
During the cooling phase, the various materials in the
y-axis of a solder joint will cool at different rates. Typically
the BGA package will cool faster than the solder joint and
much faster than the PCB. This differential cooling can
create mechanical strain on the weakest spot in the
interconnect, the laminate below the BGA pad. By slowing
the cooling rate significantly, to as low as 1.5°C per sec-
ond, all of the materials in the y-axis will cool more slowly
and will reduce the strain placed on the laminate. Experi-
mental work performed by consortia has shown that this
slowed cooling does not appear to negatively impact the
solder joint intermetallics nor the grain structure of the
joint. If pad cratering is found immediately after reflow or
if the assembly is determined to be at risk for cratering,
then the cooling rate of the PCA should be slowed to
reduce strain.
7.1.4.9 Thermal Profile for Backward Compatibility
Developing a reflow profile is made difficult when dealing
with backward compatibility issues where some lead-free
components are used on a primarily tin/lead board. Back-
ward compatibility is a scenario where some components
are available only with lead-free surface finishes. Such a
scenario arises since it may not be economical for many
component suppliers to supply both tin/lead and lead-free
versions of the same component. It is not an issue when
using leaded components such as small outline integrated
circuit (SOIC), plastic leaded chip carrier (PLCC) or fine
pitch with lead-free surface finishes. Most tin/lead compo-
nents primarily have 85% tin surface finish with about 15%
lead. When component manufacturers eliminated the lead
from parts they plated pure tin; however, the solderability
suffered. To correct this condition, up to 5% bismuth was
added to the plating alloy to improve the wetting or solder-
ability to the part.
The real problem arises when using lead-free BGAs on a
primarily tin/lead board. If the tin/lead profile with maxi-
mum peak temperature of 220°C is used, the lead-free
BGA balls will not reflow at all or will partially reflow,
creating a serious solder joint reliability problem.
If tin/lead components are soldered along with some lead-
free BGAs in the same oven (since tin/lead versions were
not available), a peak temperature must be used that is not
damaging to all the tin/lead components, but is also suffi-
cient to reflow the lead-free BGAs. Using tin/lead solder
paste is appropriate, since most of the components on the
board are tin/lead. As shown in Table 7-3, peak temperature
of 210-220°C will be fine for tin/lead but inadequate for
lead-free BGA balls with a melting point of 217-221°C.
But a peak temperature of 228-232°C with 60 to 90 sec-
onds time above liquidus or TAL will be sufficient to
reflow lead-free BGAs without seriously damaging all the
tin/lead components on the same board.
If the tight reflow temperature band of 228-232°C is diffi-
cult to achieve, in order to solder both tin lead and lead-
free BGAs in backward compatibility scenario, consider
selective laser soldering to solder lead-free BGAs after
other tin lead components have been soldered in a convec-
tion reflow oven, or find an alternative source for BGAs
with tin/lead balls.
7.1.4.10 Unique Profile for Each Printed Board Assem-
bly
A reflow program is not the same as a reflow profile.
A program is the combination of machine settings and con-
veyor speed, while a profile is the visual representation of
the time and temperature that a thermocouple sees during a
PCAs travel through the reflow oven. Every unique PCA
needs to be profiled to show that all locations on the board
meet the various requirements for creating acceptable sol-
der joints. A single program will produce very different
profiles for different, unique PCAs. There is some misun-
derstanding that one oven profile will work for all boards
and hence there is no need for developing unique profiles
for each board. This is simply not true because each board
has a unique thermal mass or different loading patterns
(distance between boards as they are loaded in the oven).
A double-sided board, depending upon component place-
ment and distribution of copper planes, will require a dif-
ferent profile for each side. Profiles may look the same for
many PCAs but will generally require different machine
programs to create these similar profiles. It is common to
have a small number of standard machine programs, but it
must be shown that a specific program produces an accept-
able profile.
Once the program has been optimized to create the desired
profile, it is recommended that an actual production board
with solder paste and components for reflow be created.
After reflow, inspect the quality of the solder joints to
verify that the solder joints across all of the various com-
ponents meet the requirements of the IPC-A-610 and any
customer specific requirements. A random problem only in
a certain section of the board may be related to solderabil-
ity; a consistent problem in a given section may be related
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to the solder profile due to nonuniform heating (wide band
width). Note too that consistent problems may also be
related to paste quality and land pattern design.
Once the program is found to give the desired results
(assuming design and other material variables have been
optimized), document the program. After this point, no
changes should be allowed in the program and the result-
ing profile.
7.1.5 Material Issues Flux has two key attributes. First,
it must remove contamination and, second, it must protect
the solderable surfaces after contamination removal. A
common mistake is to use a time/temperature profile that
consumes the flux before the solder melts. Ideally, the flux
would be consumed just as the solder begins to melt. Acti-
vation time should range from 90 to 120 seconds. Flux
usually becomes active at around 130°C for tin lead solder
pastes. Typically, solder paste activation for lead-free sol-
der will be higher, in the 150°C range; however, it is rec-
ommended to work with your solder paste supplier for rec-
ommendation on that specific solder paste.
Components can be damaged by the incorrect application
of heat. All components have a heat exposure limit. Most
tin/lead surface mount components should tolerate a peak
temperature of 220°C for up to 60 seconds. Lead-free
BGAs will be rated to a higher temperature which is
approximately 240-260°C. Thermal shock, caused by the
rapid application of heat, can crack certain components.
However, since the peak temperature of reflow ovens var-
ies, the intent is to heat the solder in a controlled estab-
lished profile to a solder joint temperature of 210-220°C
for tin lead products and for 235-245°C for lead-free
products.
Component lead finish will affect solderability. There are a
number of lead finishes being used today, including tin/
lead, gold, tin and palladium. It is important to select a flux
and solder alloy that works well with the lead finish being
used.
7.1.6 Vapor Phase Vapor phase reflow can be operated
as a single fluid system or a two fluid system, utilizing a
primary and a secondary fluid. The process was developed
using the two fluid approach in batch equipment; but mod-
ern in-line systems are normally operated with only one
fluid. Whichever system is used, the maximum temperature
reached by assemblies in vapor phase (VP) reflow depends
on the choice of the primary fluid. Primary fluids are avail-
able in a number of temperature ranges, with 218-222°C
being common with tin lead products and 235-245°C for
lead-free products. While all the primary fluids can be
classed as perfluorocarbons, the basic structure (amine,
cyclic or ether) will determine the key properties of in-use
stability, solder paste chemicals solubility, and overall
process economics. The choice of a fluid is normally based
on the melting point of the solder alloy to be reflowed.
For the range cited, the lower temperatures are suitable for
the typical tin/lead or tin/lead-silver alloys used for stan-
dard attachment processes. The upper end of the range will
permit reflow of high lead alloys, which are used to attach
pins to PGA packages. Users faced with reflow of a spe-
cialty alloy have been successful in mixing two primary
fluids to tailor a vapor phase system for a specific stable
boiling point. Higher temperatures will permit shorter
times, which may be advantageous with some solder
pastes.
The primary vapor phase should be inert and not introduce
contaminants that must be removed later. Solder paste
chemicals that dissolve in the fluid are carried in the high
boiling vapor then deposited on the surface of the boards.
Such residues tend to be difficult to remove. Minimizing
solder paste residue in the primary fluid will maximize the
lifetime of the fluid, prevent boiling point elevation due to
dissolved paste ingredients, and simplify cleaning.
The secondary vapor blanket was originally CFC-113, a
lower boiling fluorinated material, which formed a low cost
sacrificial ‘‘lid’’ over the more costly primary fluid. The
constant exposure to the high boiling primary fluid at the
interface of the two fluids could cause the secondary fluid
to undergo thermal decomposition at the interface, generat-
ing HCl (hydrochloric) and HF (hydrofluoric) acid vapors.
These corrosive vapors often attacked the soldering equip-
ment over time. While in theory the vapors could be
absorbed in flux residues and cause problems for high reli-
ability products, this was rare in comparison to the attack
on the equipment. With the phase out of CFC-113, a low
boiling perfluorocarbon was introduced to replace it. This
second generation secondary blanket fluid was more stable
than CFC-113 for prolonged exposure to the high boiling
vapor phase fluids.
As surface mount technology grew, most users converted
to the higher throughput in-line machines, which used the
single fluid approach. Defluxing after vapor phase reflow
should be done with either a bipolar solvent formulation or
include an aqueous cleaning formulation that can ensure
removal of all the solder paste residues, with the choice of
cleaning process based on the composition of the solder
paste. Secondary factors influencing the decision would be
compatibility, and the component to PWB surface spacing.
In addition, most companies gave serious thought to con-
sidering the potential chemical loss from using this type of
equipment since many perfluoro compounds are very long-
lived global warming compounds.
7.1.7 Cleaning vs. No-Clean The selection of the solder
paste/flux technology has many implications. The two
basic types are materials where the residues require clean-
ing or can be left on the PCA without causing damage.
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Copyright IPC-Association Connecting Electronics Industries
Provided by IHS under license with IPC
Not for Resale
No reproduction or networking permitted without license from IHS
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