The Importance of Preheating and Post-Cooling™

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TWO CRITICAL BENCHTOP PROCESSES

Copyright © 1996, 2001, 2007, 2008, 2009, 2010, 2011, 2012 by David Jacks

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.

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.

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.

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.

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.

The ZT-2-MIL AirPencil Shown With A Zephyrtronics AirBath™ Bottom-Side Preheater Producing the Optimum Solder Reflow Profile.

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.

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.

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.

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.

 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.

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.

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.

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.

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.

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.