1.1 PRE-HEATING -- THE PRE-REQUISITE 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.
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 "popcorning" has recently entered our vocabulary. Popcorning 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 all fairness, there exists one very real limiting factor which prevents rework temperatures ever achieving the same low temperatures as are possible with initial production reflow. Although one can approach the same low temperatures, one can never get down to the exact temperature level. This is because of the simple fact that all rework requires the localized application of reflow temperature to a targeted component and initial production reflow requires a generalized application of reflow temperature to the entire printed circuit board assembly whether it be 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.
And so, because of the fact that the rework process which is localized in nature prohibits the entire PCB from being heated up to reflow temperatures as with initial production reflow processes, an elevated initial temperature setting must be made which will be compensatory to the thermal load or sinking from the PCB assembly. In other words, the very nature of rework which is localized necessitates and therein, dictates a higher initial thermal range than those seen in production process in order to offset the load of the entire board assembly which can only be elevated to 338°F (170°C) max.
That said, there is still no reason that rework temperatures cannot be pushed towards the lower and safer thermal range typical in initial production reflow processes and thereby approach the recommended targeted temperatures which have been given by semiconductor manufacturers for decades.
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 should be reduced. The introduction of a brief pre-heating stage into the rework process makes this possible.
1.4 FOUR METHODS OF PRE-HEATING PCBA's AT THE BENCHTOP BEFORE OR DURING REFLOW:Today, pre-heating PCB assemblies appears to have fallen into four categories: use of ovens, hot plates, IR platess and the use of forced convection, the warm AirBath™.
The Oven: The use of an oven to pre-heat the substrate before reworking and initiating solder reflow for either removal and/or replacement of components can yield the most uniform temperature profile as it warms both the top and bottom of the PCB assembly as ovens also do in high volume production equipment such as conveyor ovens and wave solder machines. It's just a little problematic to crawl inside an oven with your PCB to preheat while your perform selective soldering or desoldering tasks on one side of the board. Of course, one can preheat in an oven and then race with it in your high-gloved hands back to the bench, but it's hardly a solution either.
The Oven: The use of an oven to pre-heat the substrate before reworking and initiating solder reflow for either removal and/or replacement of components can yield the most uniform temperature profile as it warms both the top and bottom of the PCB assembly as ovens also do in high volume production equipment such as conveyor ovens and wave solder machines. It's just a little problematic to crawl inside an oven with your PCB to preheat while your perform selective soldering or desoldering tasks on one side of the board. Of course, one can preheat in an oven and then race with it in your high-gloved hands back to the bench, but it's hardly a solution either.
Still, a pre-heating oven can also be a useful tool for the secondary purpose of baking out internal moisture within some delicate integrated circuits and preventing the "popcorning" concerns mentioned above. Such a bake-out of the PCB within a pre-heating oven typically can be as long as 8 hours.
However, as stated, the main drawback with a pre-heating oven is that simultaneous rework by a technician during the pre-heat application is not feasible. Also, quickly cooling for solder joint strength is nearly impossible with ovens.
The Hot Plate: The obvious limitation to the hot plate is that not all PCB assemblies are single sided. In fact, in today's world of hybrid and mixed technologies, the PCB that is entirely flat or plane on one side is an exotic and rare creature indeed. PCB's typically can carry heat sinks, connectors, jumpers and transformers on both sides of the substrate. These uneven surfaces on the board present an indirect path of heat conduction from the hot plate to the board assembly.
Another disadvantage of the hot plate is in that once the solder reflow has been achieved, a hot plate can still continue an undesirable heat application to the PCB and its components--- even if it is then turned off . This is because of the fact that a hot plate, even after being un-powered, can still have its residual stored heat within the plate which continues to conduct to the PCB. This continued application of heat to the PCB impedes the cooling down of the newly formed solder joint. Such hindering of the cool down of the joint can induce an unwanted lead pool formation resulting in a weaker and inferior solder joint.
IR Preheaters: There are many drawbacks to IR which is why it really never completely caught on. That is not to say that, as with hot plates, there are some applications that work with IR pre-heaters. 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 (this is a very common complaint). Still another great disadvantage to IR preheaters is that they can not ever truly be "temperature controlled" without the technician having to always pre-assemble an external thermocouple into 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.
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. And much like the newer, popular forced convection commercial ovens, the circulating warmed air is far more effective than static warm air.
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 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 for the PCB. A fanning of ambient air across the PCB as the board exits a reflow zone is standard practice. Post-cooling™ -- a key component in production -- has a place at the bench, too.
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 for the PCB. A fanning of ambient air across the PCB as the board exits a reflow zone is standard practice. Post-cooling™ -- a key component in production -- has a place at the bench, too.
A slower cool down rate of the PCBA after reflow can allow for an unwanted precipitation of lead-rich crystals from the liquid solder; and such a lead-rich pool formation at the metallurgical interface can result in a physically weaker solder joint. However, with the application of a moderately accelerated cooling rate, such Pb formation is hindered which yields a tighter grain structure and thus, a stronger, more robust solder joint.
Additionally, a quicker rate of solidification of the solder joint translates into minimizing the time for accidental movement or vibrations to the PCBA during reflow which can result in a host of quality troubles. The reduction of any potential misalignment of tiny SMD's and tomb-stoning situations is another benefit from post-cooling the PCBA for both initial production and rework.
Additionally, a quicker rate of solidification of the solder joint translates into minimizing the time for accidental movement or vibrations to the PCBA during reflow which can result in a host of quality troubles. The reduction of any potential misalignment of tiny SMD's and tomb-stoning situations is another benefit from post-cooling the PCBA for both initial production and rework.
1.6 SUMMARY:
The benefits from a proper pre-heating and a post-reflow cool down of the PCB assembly are many. The time involved to include these two simple procedures into a technician's rework routine is negligible. In fact, while the PCB is preheating, the technician can be busily 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 throughole soldering station or a soldering iron, reflowing tiny chips and SMD soldering with a non-contact, hot air pencil, desoldering through-hole connectors, desoldering with low melting solder that co-metalizes for SMD removal, or performing BGA rework and repair. In fact, even BGA and CSP reballing requires preheat. Bottom-side, effective pre-heat is your best solution. And preheating is simply imperative with all lead-free rework and/or soldering
The benefits from a proper pre-heating and a post-reflow cool down of the PCB assembly are many. The time involved to include these two simple procedures into a technician's rework routine is negligible. In fact, while the PCB is preheating, the technician can be busily 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 throughole soldering station or a soldering iron, reflowing tiny chips and SMD soldering with a non-contact, hot air pencil, desoldering through-hole connectors, desoldering with low melting solder that co-metalizes for SMD removal, or performing BGA rework and repair. In fact, even BGA and CSP reballing requires preheat. Bottom-side, effective pre-heat is your best solution. And preheating is simply imperative with all lead-free rework and/or soldering
Certainly, the assurance of not having to replace lifted traces or lands, or needing to troubleshoot the newly reworked board because it will not pass a circuit test also translates into a 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.
Accordingly, eliminating excessive scrap due to substrate delamination, measling or bubbling, warping, discoloration, and scorching means both time and money. Proper pre-heating and post-cooling down of the PCB assembly are the two simplest and yet, perhaps the most necessary benchtop processes of all.
Accordingly, eliminating excessive scrap due to substrate delamination, measling or bubbling, warping, discoloration, and scorching means both time and money. Proper pre-heating and post-cooling down of the PCB assembly are the two simplest and yet, perhaps the most necessary benchtop processes of all.
It is no wonder the term "popcorning" has recently entered our vocabulary. Popcorning 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. |
1.2 THE BENEFITS OF PRE-HEATING ARE MULTIPLE AND COMPOUNDING: First of all, preheating or "soaking" the assembly prior to initiating reflow helps to activate the flux and removes oxide and/or surface films from the metal surface to be soldered along with extraneous volatiles from the flux itself. Accordingly, this cleansing from the activation of the flux just prior to reflow will enhance the wetting process. Preheating also raises the entire assembly to a temperature slightly below that of the melting point of solder or the reflow point. Because this substantially reduces the delta in temperature between assembly temperature and the final reflow temperature application, the risk of thermal shock to the substrate and its components is greatly minimized. Thermal shock occurs when a rapid heating rate increases temperatures within the assembly at different rates. The resulting thermal discrepancies within the assembly create thermomechanical stresses which can and do cause embrittling, fracturing, and cracking to those materials of lower thermal expansion rates. 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. And a startling discovery for many engineers and technicians is when they learn that a preheated PCB will permit the use of solder paste and lead-free solder paste along with pin-pointed, precision hot air reflow by a hot air pencil. |
Because the two fundamental processes of pre-heating and post-cooling are often ignored by benchtop technicians, many problems arise. And worse, with rework tasks, costly PCB's which are sometimes considered "repaired" are, in fact, worse off after the rework than they were before it began. 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 address the discipline of rework and repair in this paper, please keep in mind that everything applies just the same with prototype and design tasks and to the low volume production runs of PCB assemblies.
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