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The
Importance of Preheating and Post-Cooling™ |
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[TECH 1] [TECH 2]
[TECH 3]
[TECH 4]
[TECH 5]
[TECH 6] |
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TWO
CRITICAL BENCHTOP
PROCESSES |
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Copyright © 1996, 2001, 2007, 2008,
2009, 2010, 2011, 2012 by
David Jacks |
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Pre-heating
a PCB Assembly is Illustrated Above Where a Bottom-Side Application of
Temp-Controlled Warm Air is Made Prior to Soldering or De-Soldering.
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Before the the introduction of
bottom-side
preheating stations in the early 1990's,
two
of the most critical processes which are instrumental to successful SMT
& BGA tasks at the bench were previously the two most commonly
neglected: 1) Properly preheating the printed circuit board (PCB) before
attempting reflow, and 2) Initiating a quick "cool down" of the solder
joints after reflow. This applies to all bench work from design,
prototyping and low volume production runs to rework and repair of the PCB
assembly (PCBA).
Still, today because the two fundamental processes of
pre-heating and post-cooling are often ignored by benchtop technicians,
many problems arise. Worse, with rework tasks, costly PCB's which
are sometimes considered "repaired" are, in fact, worse off after the
rework than they were before. While some "rework" damage can
sometimes be detected by a qualified, post-operation inspector, in many
cases the damage is not always visible or even immediately manifested in
a circuit test.
While I will mostly address the disciplines of rework and
repair in this paper, keep in mind everything here applies equally with
PCB prototypes, design tasks and low volume
PCB runs. |
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1.1
PRE-HEATING IS REQUIRED FOR SUCCESSFUL PCB PROCESSING |
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Certainly, the application of elevated thermal ranges (600°- 800°F or
315°C - 425°C) to the PCB for extended periods of time can present the
potential for many problems. Thermal damage such as lifted pads and
traces, substrate delamination, measling or bubbling, discoloration,
board warping and burning is usually noticeable to a trained inspector.
However, just because one hasn't "burned the board," doesn't mean that
it is not damaged.
Typically,
the "unseen" harm done to PCB's by high temperatures can be even worse
than the many problems listed above. Decades of countless testing has
repeatedly shown that PCB's and their components can "pass" post-rework
inspection and testing only to later fail at a higher than normal rate
due to the degradation of the circuit and components experienced during
high temperature "rework."
Such
"invisible" problems as the internal fracture of the substrate and/or
the degradation of its electronic components result from the rapid and
unequal expansion of dissimilar materials. Ominously, these problems may
not reveal themselves visually or even be detectable in an initial
circuit test, yet still latently lurk within the PCB assembly.
Although the "rework" looked good -- like the old saying, "the operation
was a success, but unfortunately, the patient died."
Imagine
the tremendous thermal stress which occurs when a PCB, which has been
stable at an ambient or room temperature of 70°F (21°C), is suddenly
subjected to a localized application of 700°F (371°C) of heat from
either a soldering iron, desoldering tool, or a hot air jet. There is an
immediate delta temperature change of 630°F (332°C) to the board and its
components. No wonder the term "pop-corning" has recently entered our
vocabulary. Pop-corning refers to the actual degradation to an IC
or SMD when moisture within the device is rapidly heated during rework
processes and a mini-explosion or rupture occurs. |
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In the late 1980's
the
term "pop-corning" entered our technical vocabulary. Pop-corning refers to the actual degradation to an IC or SMD when
moisture within the device is rapidly heated during rework processes
and a mini-explosion or rupture occurs.
For this very reason,
voices from within both the semi-conductor industry and those
involved in board manufacturing have been urging those who do the
electronic rework to "ramp up" to the reflow temperature with the
addition of a short preheating cycle.
After all, the simple
fact of the matter is most every single production process for
solder reflow in printed circuit board assembly work already
includes a preheating stage before reflow. Whether an
assembler uses wave soldering, IR Vapor Phase, or convection reflow,
each method is typically prefaced with a preheating or "soaking" of
the board at temperatures generally between 212°F to 302°F (100°C to
150°C).
Many of the problems
experienced in rework could be eliminated with the simple
introduction of a short preheat cycle to the PCB before attempting
solder reflow. It has certainly worked well in the initial
production reflow assembly for years.
The benefits from
preheating are multiple and compounding. Additionally, preheating
the board will allow a lower reflow temperature. In fact, this is
the primary reason that wave soldering, IR/Vapor Phase, and
convection reflow are all done at temperatures below 500° F (260°C).
These low reflow temps can be achieved at the bench with preheating. |
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1.2 THE BENEFITS OF
PRE-HEATING ARE MULTIPLE AND COMPOUNDING |
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First,
preheating or "soaking" the assembly
before
initiating soldering activates the flux removing oxides and/or surface films metal
surfaces to be soldered along with extraneous
volatiles from the flux itself. This
cleansing from the activation of the flux just prior
to reflow enhances the wetting process producing
better joints.
Preheat also raises
the entire assembly to a temperature slightly below the melting point of solder
(reflow
point). This substantially reduces the delta
between your assembly temperature and the
final reflow temperature application minimizing any
thermal shock risk.
Thermal shock occurs when rapid
heating increase temperatures inside an
assembly at different rates. The resulting thermal
discrepancies create thermo-mechanical stresses
including embrittling, fracturing and cracking the
materials with lower thermal expansion rates.
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SMT Chip resistors
and capacitors are especially prone to such damage
from thermal shock. Additionally, if the
entire assembly is preheated, both a reduced
temperature and a shorter duration of higher
temperature application at the final reflow stage is
possible.
This becomes very evident in cases of
PCB's with heavy ground planes and/or dense
component population where the heat sinking load of
the PCB makes rework unduly slow. Without
preheating, the only solution is either a further
elevated temperature application and/or a longer
dwell time at the reflow stage...neither of which is
recommended and should be avoided.
Many engineers and techs
are pleased to discover a preheated PCB permits the use
of
solder paste
and
lead-free solder paste
along with pin-pointed, precision hot air reflow by
a
hot air pencil.
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The
ZT-2-MIL AirPencil Shown With A Zephyrtronics AirBath™
Bottom-Side Preheater Producing the Optimum Solder Reflow
Profile.
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1.3 BENCH SOLDERING
TEMPS SHOULD MIMIC ORIGINAL PRODUCTION PROFILES |
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As a helpful reference point, all initial production solder reflow
processes typically have the following thermal ranges: A.) most wave
soldering operations occurring at temperatures between 464°F to
500°F (240°C to 260°C); B.) vapor phase soldering at temperatures
generally around 419°F (215°C); and C.) convection oven soldering at
approximately 464°F (240°C).
In fairness, there exists
one very real limiting factor that prevents rework temperatures
ever achieving the same low temperatures as are possible with
initial production reflow. While one can approach the same low
temperatures, one can never get down to the exact temperature level.
This is because rework requires the localized application of
soldering temperature to a targeted component
while production reflow requires a full application of
reflow temperature to the entire PCBA
whether it done with wave soldering, convection ovens, and/or with
IR/Vapor Phase Reflow.
Equally limiting the lowering of
the reflow temps in rework is the industry standard requirement that
the adjacent components to that of the targeted rework must never be
subjected to over 338°F (170°C). Therefore, the only time that
reflow temps in rework can be identical to those of initial
production reflow would be when the PCB assembly itself is roughly
the same size as the targeted component for reflow and with little
or no other components. |
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| Rework &
Prototyping is Local: The rework process is
localized and typically targets a single chip at a time. This prohibits
heating up both the top and bottom of the entire PCB as it
is done in production to elevated soldering temperatures.
In other words, the very nature of rework which is localized
necessitates and therein, somewhat dictates a higher initial thermal range
than those seen in production process in order to offset the load of
the entire PCBA which can only be elevated to 338°F
(170°C) max. Still... |
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Bench rework
temps should be close to the lower, safer temps used in production processes
to achieve the recommended temperatures of the semiconductor manufacturers..
Certainly, the
punishing temperatures of 650°F to 800°F (343°C to 426°C) currently
found in rework with soldering irons, desoldering tools, and hot air
jets are wrong. Introducing a brief pre-heating
stage into the rework process corrects this problem. |
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1.4 FOUR METHODS OF PRE-HEATING PCBA's AT THE BENCHTOP BEFORE OR
DURING REFLOW |
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1.) IR Preheaters: There are many
drawbacks to IR which is why it really never completely caught on. Some of drawbacks
which have been enumerated in articles in SMT Magazine, Circuits
Assembly and in white papers at electronic conventions are the
difficulty in ramping temps (some are better than others); shadowing
caused by high profile components on PCB's; and if the IR preheat
grid is very large it can make working on small PCB's very
uncomfortable for the technician (a very common complaint).
Still another great disadvantage to IR preheaters is that they can
never truly be "temperature controlled" without the technician
having to pre-assemble an external thermocouple and tape it to every
board before working on it. And that's a continuous hassle and
headache replete with quality pitfalls and problems with
inconsistent results. There are more, but these are some of the key
setbacks.
NASA & JPL: 'IR
Preheaters Are Risky':
Addressing the critical preheating rework process for plastic and
heat sensitive components in their 49-page published report, two JPL
engineers, Dr. Rajenshuni Ramesham and Dr. R. David Gerke, wrote "Hot
plates and infrared preheaters are not recommended for this type of
rework. The reason that they should not be used is that the
thermal reaction times, energy transfer rates and efficiency are
never consistent. However, they can be used for large metal and
ground-plane boards in limited applications, e.g., where the size of
the board matches that of the preheater in area."
Citing the late William Scheu, the authors point to the preheating
superiority of bottom-side forced convection. Moreover, the NASA
Survey by JPL spotlighted more IR preheat
deficiencies:
"They have little capacity to ramp and soak to perform properly
engineered repair scenarios or to support the creation and
application of complicated thermal profiles. They also are limited
in their ability to preheat beyond the physical dimensions of the
heating surface. Hot-air preheating can be ramped, soaked and, on
some systems, synchronized with the reflow process, permitting
duplication of the actual profile used in manufacturing the
assemblies. BGA's and photoelectrical parts are sensitive to higher
temperatures and any attempt
to preheat with marginally controllable sources is risky." --
NASA & JPL Survey of Rework Methods & Equipment for Various
Packaging Technologies."
-- August 2005.
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2.) The Oven: Using ovens to
pre-heat PCB's before soldering or de-soldering chips can yield
uniform temperature profiles as it warms both the PCB's top and
bottom. But one can't crawl inside an oven and simultaneously
perform soldering tasks.
Still, one can preheat a PCB in an oven and
then race with it in high-temp gloved hands back to the bench, but
it's hardly a solution either. Also, (and this is
important) quickly cooling for solder joint strength is nearly
impossible with ovens.
3.) The Hot Plate:
The obvious limitation to hot plate
is not all PCB's are single sided. In today's world of hybrid and
mixed technologies, PCB's that are entirely flat or plane on one
side are rare. PCB's typically have heat sinks, connectors, jumpers and transformers on both sides
of the substrate. These uneven surfaces on the board present an
indirect path of conduction from the plate to the PCB.
Another disadvantage
is once soldering is done, hot plates continue applying conductive
heat to the PCB and chips--- even if turned off because they have residual stored heat
in the plate. This
continued heating impedes cooling down the new solder joint. Delayed
cooling of
the joint can induce an unwanted lead pool formation making
weaker, inferior solder joints.
4.)
Forced Convection Preheat, The AirBath™: The
market has long spoken with regards to the distinct advantages and
superiority of a
warm air bath
in the pre-heating
process. Forced convection completely disregards the topography (or
bottomography) of the PCB, allowing immediate, direct access of the
warm air into all of the nooks and crannies of the PCB assembly. As
with the new popular forced convection commercial ovens,
circulating warm air is far more effective than static warm air.
Testing has shown
that PCB preheating with less than 10 cubic feet per minute (285
liters/minute) in volume of bottom-side heated air is ineffective
even with very small boards. If preheat volume is too low,
technicians are tempted to compensate by turning up the temp
settings of their top-side tools (soldering irons, hot air nozzles).
Quick cool-down of
the PCB, critical with BGA's, is also possible with an AirBath™. It is no wonder that bottom-side, temperature controlled, forced
convection is the industry standard for preheating PCB assemblies:
there are so many advantages. |
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1.5 POST-COOLING™ the
PCBA FOR ROBUST SOLDER JOINTS |
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Important Feature: The AirBath's™ Post-Cooling™ Mode Cools Down BGA's After
Soldering Within Seconds, Solidifying the Joints & Allowing Lifting Nozzles
Sooner Without Bridging. |
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As mentioned, the challenge of at the bench is that the rework
process should mimic that of initial production in both processes
and profiling. It is interesting to note that just as pre-heating
the PCB assembly prior to reflow has proven essential to successful
PCBA production, so has a quick cool down of the assembly
immediately after reflow. Yet both of these simple two processes
have traditionally been equally ignored within most rework
processes. However, the swift replacement of SMT over thru-hole
technology along with the miniaturization of delicate components
makes both preheating and post-cooling more necessary than ever
before.
Most high-volume production reflow equipment, such as conveyor
ovens, incorporate a final cooling stage after the reflow
stage. Fanning of ambient air across the PCB as the
board exits a reflow zone standard practice. Post-cooling™ -- a
key component in production -- is essential at the bench, too. |
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Quick
Cooling Prevents Quality Problems: Understand that a slow cool down of
a PCB assembly after solder reflow is not a good thing. It
lets unwanted lead-rich crystals pooling within the liquid solder.
Such lead-rich pool formation at the metallurgical interface
results in physically weaker and inferior solder joints.
Even with
lead-free solder alloys, a slow cool-down
of the PCB results with solder joints filled with pits,
voids and stretch marks. This is all so easily prevented by
simply an accelerated cooling. Quicker cooling yields tighter grain structure,
higher quality and more robust solder joints. |
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Quick Cool
Down to Prevent Solder Bridges & Moving Chips: A
quicker rate solder joint solidification by rapid cooling translates into
minimizing time for accidental movement or vibrations to the
PCB after solder reflow which can cause a host of quality troubles.
Some problems include solder bridging, especially with Ball
Grid Arrays (BGA's) or tomb-stoning chip capacitors and
resistors. For these very reasons high-volume production
conveyor ovens all have cooling fans precisely at the final
phase of the soldering process. Quickly cooling down your
PCB immediately after solder reflow is actually more
important at the soldering bench than it is in initial high
volume production. Ignore it at your own peril. |
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The benefits
from a proper pre-heating and a post-reflow cool
down of the PCB assembly are many and compounding. The time involved
to include these two simple procedures into a
technician's basic rework routine is negligible. In fact,
while the PCB is preheating, a technician can be
busy doing other prep work such as applying paste
and / or flux to the board.
Bottom-side,
convection preheat enhances all soldering and
desoldering processes at the bench whether one is
working with a traditional
through-hole soldering station
or a
soldering iron,
reflowing tiny chips and
SMD hot air pencil soldering,
desoldering
through-hole connectors,
desoldering with
low melting de-solder
that co-metalizes for SMD removal, or performing
BGA rework and repair.
In fact, even
BGA and CSP re-balling
requires preheat. Bottom-side,
effective pre-heat
is your best solution. And preheating is imperative with all lead-free rework and/or
soldering.
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Certainly, the assurance
of not having to replace lifted traces or lands, or
needing to re-troubleshoot a newly reworked board
because it will not pass a circuit test translates into genuine time savings. Further, it
goes without saying, that the cost savings realized
from not having to scrap PCB's thermally damaged in
"rework" must factor into any technician's equation.
An ounce of prevention is worth a pound of cure.
Preheating and post-cooling go a very long way in eliminating excessive scrap
(money and time) from substrate delamination, measling, bubbling,
warping, discoloration, and scorching or from
cracked ceramic chips, degraded semi-conductors,
bridged solder joints, weak solder joints with pits
and voids and more. Proper pre-heating and post-cooling
your PCB assembly are the two simplest and
yet, perhaps the most necessary benchtop processes
of all.
--- David Jacks, Los Angeles, California 1996. |
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Postscript from the Author:
How prescient
this article seems now some sixteen years later. Today,
our reality at the electronic benchtop has us
working with SMD packages so small they are
only visible under a microscope along with
micro-BGA's and the most-delicate of ceramic
capacitors and glass diodes, all of which are at
extreme risk of thermal damage from high
temperatures and rapid thermal expansion.
The advent of lead-free solders has
further exasperated the reflow process for SMD and
BGA prototyping, production and reworkr.
In all of these cases, it turns out that effective
preheating before reflow is the only solution, isn't
it?
BGA Rework
requires preheat.
Lead-Free rework
requires preheat.
Micro BGA rework
requires preheat.
SMT rework requires preheat.
And yes, if you really want quality solder joints,
even
Thru-hole rework
requires preheat.
My first SMT benchtop "machine" was the old Ungar®
4700 hot air soldering system introduced in 1982, some
thirty years ago! That simple machine was quite
revolutionary in its day because we had the
foresight to engineer bottom-side preheating right
into the machine. Yes, we were laughed at back in
the "old soldering iron days", but time has proven
us right. I am still kidded from time to time as
"Mr. Preheat" and I must say that I like
it. --- DJ, 2012.
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ABOUT THE AUTHOR:
David Jacks was
Director of Engineering at three Fortune 500
corporations along with the two largest
soldering equipment manufacturers on earth for
13 years before launching Zephyrtronics
in 1994 with fellow
engineer, Randy Walston.
David's professional design career
stretches from the early 1970's. His original products have been
spotlighted in feature articles in both Popular Science®
and Popular Mechanics®
magazines.
He has designed
products, tools and appliances marketed by Sears®,
Black & Decker®,
RadioShack®,
Motorola®,
Stanley Tools, Snap-On Tools®,
Rubbermaid®,
CooperTools®,
Weller®,
Hakko®,
Ungar®,
Farmer Brothers®
and Brewmatic®.
Any electronics catalog
of soldering equipment, tools and products today
reflects David's long and enduring influence on the
printed circuit board industry world-wide.
David holds many patents (both utility and design) in North
America, the European Union, Japan and around the world. His
patented inventions have been cited as prior art by
firms from IBM to Mitsubishi. He has authored technical articles for international journals,
and routinely speaks to electronic professional societies.
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©1996 - 2011, 2012, 2013 by Zephyrtronics®. All rights reserved.
The information, text, images, photographs, charts, graphs you
receive online from Zephyrtronics® are protected by the
copyright laws of the United States. The copyright laws prohibit
any copying, redistributing, retransmitting, or repurposing of
any copyright-protected material. Zephyrtronics is the
registered trademark property of JTI, Inc. "The Science of
Zephyrtronics" and "Simplicity Through Innovation" and "Zephlux"
and "ZeroLead" and "Zero Balling" and "Zero Residue" and "Post
Cooling" and "Post Cooler" and "AirBath" and "SolderGlide" and
"SolderMill" and "Just So Superior" are the protected trademark
property of JTI, Inc. "Zephyrtronics" and "Low Melt" and "Air
Fountain" and "Fountainhead" are the registered trademark
properties of JTI Inc. *The above names are the registered
property of their respective owners. |
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Click link
below to return to the top of the page |
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TWO
CRITICAL BENCHTOP
PROCESSES |
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SMD
Rework,
SMT Rework
AirBath Air Bath,
SMD Rework Stations,
Hot Air Pencil Soldering,
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Updated May 20, 2013 |
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