a PCB Assembly is Illustrated Above Where a Bottom-Side Application of
Temp-Controlled Warm Air is Made Prior to Soldering or De-Soldering.
Before the the introduction of bottom-side
preheating stations in the early 1990's,
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.
Poor Rework & Repair is
Expensive 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
1.1 PRE-HEATING IS REQUIRED FOR SUCCESSFUL PCB PROCESSING
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.
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."
"invisible" problems (internal substrate fracture and
component degradation) happen because of too rapid heating acceleration and
unequal expansion of dissimilar materials.
these problems may not be visible or even detectable
in an initial circuit test, yet still latently lurk within the PCB
the "rework" looked good -- like the old saying, "the operation was a
success, but unfortunately, the patient died."
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
"Pop-corning" refers to
internal IC or SMD degradation from moisture within the device is heated
too quickly mini- ruptures occur.
Hence, preheating is imperative at the
bench. This short video teaches the
4 Methods of PCB preheating.
They are also taught below in this technical paper.
Thermal Ramping at
the Bench. For these very reasons, as early as the 1980's I began
lecturing the semi-conductor industry and those involved in labs and
factories who were soldering or desoldering devices on PCB's to
first "ramp up" to reflow temperature with a short preheating cycle. Thermal profiling was as
important on a bench as on the production floor.
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 pre-heating stage just before your soldering or
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
Eliminates Problems! Most quality issues in PCB
prototyping and rework can be eliminated with 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
your printed circuit board assembly will permit you to use a lower,
final soldering 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).
Yes, these same low reflow temperatures
and shorter soldering dwell times can be achieved at right the bench with
only the addition of preheating.
1.2 THE BENEFITS OF PRE-HEATING ARE MULTIPLE AND COMPOUNDING
Your Fluxremoving oxides and/or surface films metal
surfaces to be soldered along with extraneous
cleansing from the activation of the flux just prior
to reflow enhances the wetting process for
the Entire Assembly to a temperature slightly below the melting point of solder
point). This substantially reduces the delta
between your assembly temp and the
final reflow temp application minimizing
thermal shock risk.
occurs when rapid heating increase temperatures
inside an assembly at different rates. The resulting
thermal discrepancies create thermo-mechanical
stresses that embrittle, fracture and crack
materials with lower thermal expansion rates.
SMT Chip Resistors
& Capacitors are especially prone to such damage
from thermal shock. But if the
PCB assembly is preheated both a reduced
temperature and a shorter final reflow stage application is
PCB's With Heavy
Ground Planes, high copper content or that are
densely populated create tough heat sinking loads
and makes rework unduly slow. Without
preheat, the only solution is either a further
elevated temperature application and/or a longer
dwell time at the reflow stage...both of which should be avoided.
Shown With A
Produces the Optimum
1.3 BENCH SOLDERING TEMPS SHOULD MIMIC ORIGINAL PRODUCTION PROFILES
High-volume production soldering
processes typically have these final thermal ranges: A.) wave
soldering operations at temps at 464°F -
500°F (240°C - 260°C); B.) vapor phase soldering at temps at 419°F (215°C); and C.) convection oven soldering at
approximately 464°F (240°C).
In fairness, while one can approach these
temps at the benchtop, one can never get down to the exact temperature level.
This is because rework requires localized application of
soldering temps to a single targeted component while production
reflow requires full application of reflow temps to
the entire PCBA. Further complicating the task of benchtop
soldering or desoldering is the industry requirement
that adjacent components to a targeted chip
must never be subjected to over 338°F (170°C).
So the only time benchtop
soldering temps in rework can be identical to those of high
volume production are when PCB's are roughly the same size
as the targeted component for soldering.
Prototyping is Localized and typically targets a single chip at a time
prohibiting applying solder reflow temps to both the top and
the bottom of entire PCB. So this localization
necessitates higher initial thermal ranges
than those seen in production process in order to offset the load of
the entire PCBA which can only be elevated to 338°F
that fact does not negate the need to make benchtop
soldering as close as possible to those lower, safer temps
of production processes. What's critical in high volume
production is still equally critical at the bench.
Especially for semiconductors!
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, may be OK for more robust thru-hole chips, but never for SMD's
and BGA's. What's the solution? Introducing a brief pre-heating
stage into your benchtop soldering process.
1.4 FOUR METHODS OF PRE-HEATING PCBA's AT THE BENCHTOP BEFORE OR DURING REFLOW
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
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
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.
A NASA Survey by JPL spotlighted more IR
"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.
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.
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
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.
Why Hot Air Preheaters Are More Efficient Than Other Methods?
1.5 POST-COOLING™ the PCBA FOR ROBUST SOLDER JOINTS
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 these simple two processes
have traditionally been equally ignored within most lab
prototype soldering and rework
processes. However, the swift migration to SMT from
thru-hole technology -- along with the miniaturization of
delicate components -- makes both preheating and post-cooling
more necessary than ever before.
The Model: All high-volume
:PCB production solder reflow equipment -- such as conveyor ovens
and wave solder machines --
incorporate a final cooling stage after the reflow
stage. Fanning ambient air across a PCB as it
exits a reflow zone is not just a common practice, it is an
industry standard. Post-cooling™ is also essential at the bench,
There are two important points I want to make here.
Cooling Prevents Quality Problems: A slow cool down of a PCB assembly after solder reflow is
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. Slow cooling
lead-free solder alloysresult in solder joints filled with pits,
voids and stretch marks. This is easily prevented with accelerated cooling. Quick cooling yields tighter grain structure,
higher quality and robust solder joints.
Quick Cooing Prevents Solder Bridges & Moving Chips: A
quicker rate of solder solidification by rapid cooling reduces the time for accidental movement or vibrations to the
PCB after solder reflow, which cause a host of quality troubles
like solder bridging, especially with Ball
Grid Arrays (BGA's) or tomb-stoning chip capacitors and
resistors. This is why 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.
The AirBath's™ Mode for
Cools Down Your BGA's
Soldering in Seconds, Solidifying the Joints &
Allowing Lifting Nozzles
Sooner Without Accidental Bridging.
The benefits from a
proper preheating 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.
Effective Preheat is Essential for all
BGA rework and repair.
In fact, even
BGA and CSP re-balling
requires preheat. Bottom-side,
is your best solution. And preheating is even more
important with lead-free soldering or rework. 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.
Prevent Scrap Scrapped PCB's cost money and time.
Substrates with delamination, measling, bubbling, warping,
discoloration, scorching or with cracked ceramic chips,
degraded semi-conductors, bridged solder joints, weak solder
joints with pits and voids can set your company back all
because proper pre-heating and post-cooling were not
performed. These two processes are the two simplest and yet,
perhaps the most necessary benchtop processes of all.
-- David Jacks, Los Angeles, 1996.
POSTSCRIPT AND BIO
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
Micro BGA rework
SMT rework requires preheat.
And yes, if you really want quality solder joints,
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.
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®
He has designed
products, tools and appliances marketed by Sears®,
Black & Decker®,
Stanley Tools, Snap-On Tools®,
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.