Welcome to Zephyrtronics! The World's First Low Temp Approach to the Electronic Benchtop.

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In hindsight, it is really quite remarkable that it took so long to happen! And to the shame of the larger, established rework equipment manufacturers, it is even more remarkable that this small, upstart entrepreneurial company in Southern California would be rewriting all the rules for electronic rework. Rework tools should duplicate the thermal conditions found in the reflow oven. The PCBA should be ramped up at 2-4°C per second and preheated to a temperature of 300°F for a period of up to 1 minute; just like the reflow oven. By inserting a preheating step in the rework cycle, the benefits of preheating can be realized. "Preheating the assembly activates the flux, removes extraneous volatiles from the flux, brings the metals to be soldered up to solder wetting temperatures, and elevates the temperature of the assembly in order to prevent thermal shock during exposure to the molten solder." Accordingly, this cleansing from the activation of the flux just prior to reflow will enhance the wetting process.

1.4 THE IMPORTANCE OF PRE-HEATING IN ALL REWORK:

"Preheating of the board at a lower temperature will prevent thermal shock, eliminating the fracturing which may otherwise result from the higher heat required to remove the target component.” Different components on the board expand at different rates when high heat is applied. The resulting thermal discrepancies within the assembly create thermo-mechanical stresses which can and do cause embrittling, fracturing and cracking of those materials of lower thermal expansion rates such as chip capacitors and resistors. For more on this subject, please refer to the more detailed technical paper “Two Critical Benchtop Processes: Preheating and Post-Cooling.”

Recently, the introduction of the portable, convective bottom-side pre-heater, now known generically as an "air bath" has made the theory of pre-heating during rework a reality. Where hot plates had failed, the air bath quickly succeeded. There was no longer any excusing the lack of pre-heating a printed circuit board assembly prior to rework. With the air bath, the pre-heating revolution had begun and the paradigm was shifting, but there was still yet another positive contribution to made to the rework and repair regimen for today's PCBA's: the use of low temperature solder alloy to remove the SMD.

1.5 THE USE OF LOW TEMPERATURE SOLDER ALLOYS FOR REMOVING SMD’S:

Removing electronic components through the introduction of a targeted application of low temperature melting solder alloys had first been heralded and successfully proven practical as early as in the 1970's. Joseph Funari and William C. Ward are the most notable early proponents of the concept. Essentially, they proved that the removal of any component on the assembly can be accomplished using metallurgy rather than high heat as in the past. Metallurgy is the science of mixing different metals together. A metallurgical phenomenon can be used to remove any SMD from any assembly.

In an IBM publication, Ward described the concept of "low temperature soldered component removal" and later for a national trade journal wrote that low temperature alloy "can dissolve conventional solders and metals by the same metal solvent action that is effective in securing attachment in conventional soldering operations. Common soldering hierarchy involves the use of a solder alloy melting at about 183°C, that at a suitable elevated temperature (typically 260°C), secures attachment by dissolving small amounts of metals to be joined at temperatures well below their melt points. Copper that melts at 1083°C and nickel that melts at 1455°C are probably the most notable examples of the materials commonly joined by the 260°C solder melt-solvent phenomenon."

Simply put, a single, targeted SMD could be reflowed and removed at temperatures below the melting point of the any of the other SMD's on the very same board at temperatures less than 150ºC. And as SMT popularity was increasing in the United States, Russian engineers in 1986 had begun to use "a mixture of tin, lead, and bismuth as an alloy" for selected component removal from printed circuit boards and perform quality rework at the same time.

Finally, at the 1996 Surface Mount International Exposition in Silicon Valley, the synergistic combination of convective bottom pre-heating (air bath) coupled together with a targeted application of low temperature melting solder alloy was first demonstrated to the world resulting in an instant recognition by industry leaders of the relevance to the discipline of rework and repair. Indeed, the patent pending AirBath product and concept from Zephyrtronics was chosen as "The Best New Product" of the year winning the prestigious Vision Award for its innovative contribution to Surface Mount Technology.

It has now been proven that by preheating a localized area of the assembly to be reworked, to a temperature well below the reflow temperature of eutectic solder, but above the melt temperature of a special low temperature alloy (typically lead, tin, bismuth and indium), the eutectic solder adhering the targeted SMD to the assembly can be co-metalized permitting quick and selective SMD removal at previously unheard of temperatures. (See Figure 4)

The introduction of low temperature melting solder alloys to an existing PCBA is no cause for concern. In fact, after component removal is achieved, the pads are easily cleaned and polished with flux and swab removing all previous solder from the pad (both eutectic and the lower temp also). Pads are then re-tinned exclusively with eutectic solder and the board is as good as new.

(For a helpful reference to the above described process, please see our full-color, pictorial step-by-step SMD removal process combining preheating and low temperature solder, LowMelt® Desolder).

As lead content becomes more and more an environmental issue, the use of alternative solder alloys has become a hot issue. Typically, it is bismuth and indium that are the likely candidates for lead replacement. In fact, Bismuth is now a common component in solder alloys currently used in PCB production. One need only look at the popular and highly effective production process of "Step-Soldering" to see a clear example of this.

As PCB assemblies become smaller and the components on them more powerful, the need to be able to produce double sided PCBA's using automated equipment has become acute. The problem arises when you manufacture the boards with the same composition of eutectic solder on both sides. The same thermal profiles must be used in the reflow ovens. Using the same thermal profiles leads to shifting components on the assembly causing more rework and lower quality. To avoid this problem assemblers ran the process of "step-soldering". Its premise was to process one side of an assembly with eutectic Sn63/Pb37 solder (melting point 183°C), flip the board, re-profile the oven and solder the reverse side with Bi14 low temperature solder (Bi14/Sn43/Pb43 melting point 165°C). The result is less thermal shock to parts and lower operational costs with adequate solder-joint strength."

In the above example, we find assemblers are depositing 14% Bismuth permanently into the assembly during the production process that result in "adequate solder-joint strength". In the case of the low temperature solder alloy being used for component removal during rework, there is should be no bismuth left on the pad at all after cleanup and, of course,  the new solder joint is made exclusively (100%) with eutectic solder.

A final note in the paradigm shift in the rework regimen of SMT concerns replacing a new component onto the site where the old one was removed. Again, this is accomplished using the same processes as when the board was originally manufactured.

The assembly should begin at room temperature. An application of eutectic solder paste to the lead-land interface is made. Next, the new component is aligned and oriented properly. (It is recommended to use a vacuum-handling tool during placement).

After alignment is made, begin to preheat using an airbath and ramp up the temperature at 2-4°C. per second and let the assembly "soak" for a few minutes. After soaking, use a hot air pencil (not a hot air jet) with pin-pointed heated air to the individual lead/land interface to heat and reflow the solder paste (480°F).

The reflowed solder paste will wick to the individual lead-land interface because that is the only place where the solder will adhere. Afterwards, simply clean up what little residual flux there was in the paste and inspect the solder joints for full reflow and/or bridges. The assembly is now ready to be tested and put back into service. (See Figure 5)

1.6 SUMMARY:
The convective bottom preheating of PCBA's coupled with the use of low temperature melting solder is the "paradigm shift" for SMD removal and replacement. Production quality temperature profiles are easily achieved. By duplicating the production heat profiles when the PCBA's were originally produced, SMD rework can be accomplished safely and effectively. Most impressing is how little time the entire process takes from removal to replacement and clean-up.

However, time can no longer be the only consideration in calculating effective and quality rework. The integrity of the rework, the assurance of low temperature profiles must also be considered along with costs. In calculating costs, it is important to not only factor time, but also material costs, the cost of damaged substrates and components (scrap), and the reliability and longevity of the reworked board once it is put back in service. In the past, rework has been, at most, an afterthought; a necessary evil as it were. However, in the future, as PCBA's become smaller, denser and more expensive, rework will become an integral part of every production process. As assemblies become more expensive, a safer and more reliable method must be used to insure that expensive components and PC boards can be safely recycled.

At the 1997 NEPCON West Exposition, David Jacks challenged the industry at large in his technical presentation there as part of the official proceedings: "The time has long since passed for toleration of the double standard where PCBA production requirements are, on one hand, exacting while, on the other hand, those related to rework are lax. The time proven thermal profiles that became the industry standard for production within the electronics industry for the greater part of the last half of the 20th Century must finally be brought to bear in the realm of rework and repair. The ending of the "Great Double Standard" requires only one thing: that manufacturing and quality engineers around the globe insist that the high temperatures long associated with rework become a relic of the past."

After a decade of irrelevant SMD contact "tools" that relied on hand pressure and technique or a thermally overloading and non-discriminating application of reflow temperature air gusts to the PCB, a new paradigm has emerged that is sensible, practical, easy to understand and implement, and best of all brings all of the positive elements of high volume PCBA production into rework and repair. The lowest temperature rework solution turned out to be the easiest and most cost effective of them all. The paradigm has shifted.