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Vapor Phase Making Strong Comeback
VAC645-A provides vacuum and vapor phase reflow.
By Reyna Mora, IBL Technologies, LLC, El Paso, TX
Vapor phase, also known as condensation soldering achieves a very gentle, reliable, and repeatable process. Vapor phase reflow was used back in the 80s — during the early days of surface mount technology — due to its excellent heat transfer capability. Heat transfer in vapor phase is independent of the size or shape of the mass. The entire PCB or component is heated evenly and equally.
Over the years new technology and new vapor phase machines have been developed and perfected. One major advantage of this technology is that there is no overheating of components because the maximum soldering temperature is never higher than the boiling point of the vapor. The vapor creates an inert atmosphere area without the use of nitrogen, and can process leaded and lead-free with fast changeover time. In addition, the latest technology in vapor phase provides all the benefits of the Soft Vapor Phase and vacuum vapor phase to reduce voids and obtain low delta T's.
What has changed in the last 30 years? Vapor phase reflow started back when surface mount technology was in its infancy and slowly disappeared due to various issues such as uncontrolled thermal capacity, extreme ramp rates that resulted in failure modes, the machines were all batch-mode, and finally the chemistry was hazardous to both environment and operator. Today, although vapor phase is gaining huge popularity due to its advantages over convection and wave soldering, some people remember only the negatives from that era.
That is why we ask this question, what has changed? Principally, the chemistry is no longer solvent based, it is stable and environmentally friendly. The designs of the machine are much more flexible; today we have in-line systems with relatively high throughput; cost of operation is much less than most people expect — very competitive when compared with a convection oven because of no need for nitrogen and low electricity use.
Finally, it is a better process overall, providing a much better heat transfer coefficient, and all production runs automatically in an oxygen-free (inert) area. The operator can manage peak temperatures very precisely and get almost any desired profile. Because of all of these changes and advantages, there is less potential for PCB damage and less thermal stress to components.
The Vapor Phase Process
In today's manufacturing environment, the best soldering method to achieve a gentle and optimum quality and heating is vapor phase. The demands of lead-free soldering's higher soldering temperatures, are coupled with ever-smaller components, and far more complex PCBs. In addition, boards can contain very different masses of components. The result is that many companies are facing soldering problems, especially with lead-free processes.
A chemically inert fluid called galden is used for the heat transfer. Galden LS/HS is a line of fluorinated fluids specifically designed for vapor phase soldering. Some of the features of galden include: wide choice of grades with different boiling points, narrow molecular weight distribution, low heat vaporization, excellent thermal and chemical stability, no explosion hazards. This fluid is non-corrosive, electrically non-conductive and does not deteriorate over time. The physical and chemical properties provide excellent conditions for a reliable and repeatable heat transfer to all types of materials.
The boiling point depends on the application; most galden liquids have boiling points between 200 up to 235°C. However, it is possible to obtain fluids with a higher boiling point. Obviously, the fluid that has a boiling point of 235°C means that this is the maximum temperature in the vapor phase oven. For this reason, overheating is physically impossible. The bottom chamber is filled with the liquid; on average you 20kg is needed to fill a medium-size machine. The liquid is then heated up to its boiling point and the vapor rises over the liquid. The part to be soldered is then lowered to the vapor blanket and it is evenly heated up. There is no shadowing effect, with IBL's Soft Vapor Temperature Control, it is possible to run equal profiles, and the size of the component does not matter.
Optimal Heat Transfer
With the introduction of the patented soft vapor phase (SVP) technology, the process of controlling the heat transfer is much more efficient and precise. This is achieved by heat level adjustment. The way it is controlled is by the height level of the PCB in the vapor blanket, and the more power the heaters have, more vapor is produced. The SVP allows adjustment of temperature profiles to nearly any required temperature gradient.
Model SLC509i is an example of an in-line vapor phase machine.
The assembly is lowered into the vapor blanket, where the operator can program into the machine the profile he wants to run. By setting this profile, the system provides the same profile every time, it is reliable, and completely repeatable, providing perfect solder joints every time, and no cold solder joints.
Mixed Technology — lead process with lead-free components — is getting a bigger concern every day. With the patented soft vapor temperature control, IBL vapor phase machines can provide impressive solutions. The maximum temperature is close to the melting point of the leaded chemistry due to a very low Delta T.
The Syncro Mode offers a wider range in the setting up of soldering processes. With a sensor control, each process cycle is continuously monitored and matched against a reference cycle. The resulting exact process repeatability provides, in particular for complex PCB boards, a continuous and precise soldering process.
Reducing Solder Joint Voids
Due to the air and flux in the solder paste, reflow solder joints and solder areas may contain voids. With the combination of the soft vapor phase soldering process and a vacuum process, the reliability of the finished product increases. The new vacuum vapor phase soldering machines are based on the company's premium series. The vacuum vapor phase soldering system operates in a completely inert atmosphere through the entire reflow and vacuum process. Not only does the vacuum vapor phase machine reduce voids, the premium machines also have the capability of reducing voids by using a profile with a thermal soaking zone.
With the growing demand in electronics to have void-free solder joints, the patented technology vacuum vapor phase machine has been a reliable and proven method of reaching such goals. The technology to have the vacuum chamber work inside the vapor helps to ensure a defensive atmosphere during the process. This machine does not require additional heat up of the assembly because there is no temperature drop during the different steps of the process. These new machines — VAC 645 and VAC 665 — can be operated as single or series production in-line systems to fit any production line. The vacuum series can also be utilized without vacuum if desired, simply by changing the program.
The idea that vapor phase is much more costly than a conventional convection reflow due to the use of galden is very far from the truth. In reality, the cost savings could be much higher with a vapor phase machine. First of all, a vapor phase machine does not require any nitrogen. The power consumption of a vapor phase machine is much lower than with a convection oven. IBL's premium machines use an average of 10kVA versus a much higher number for any other type of reflow oven. A vapor phase machine utilizes 98 percent of the heat to heat the assembly wherein other reflow processes waste a great deal of energy by heating up the factory, adding to the energy cost plus increasing the load on the factory's air conditioning.
Recommended maintenance for an IBL machine should be performed once-a-year for one shift operation, twice a year if there are two shifts, three times a year for three shifts. Cost of this preventive maintenance is very low.
While the galden liquid utilized in vapor phase is very costly, all of the machines have a very good vapor recovery system, which leads in very low losses of galden. However, at the end if all the cost savings from electricity and nitrogen from a vapor phase machine are added, owning a vapor phase machine is still more cost-effective.
On a reliability test performed by a customer, a reflow analysis was performed on Package on Package (PoP) assemblies using vapor phase technology. The objective of this test was to verify that the solder joint formation conformed to IPC standards. The solder joint being tested was between PoP BGAs and the PoP assembly and board pads. The maximum temperature was controlled by the boiling point of the liquid. The results demonstrated that the solder joints that were analyzed met all the requirements of IPC.
Voids were also measured by an x-ray inspection.The voiding area was less than 25 percent. This reflow operation was performed using a regular IBL machine. The company also provides the Premium Vacuum machine which can guarantee minimal voiding, as described earlier. In a comparison performed by one of IBL's customers between vapor phase and convection reflow system showed that solder joint quality was high and free of voids. Intermetallic thickness and older joint strength was verified through pull and shear tests, demonstrated excellent joint strength. Vapor phase solder joints appeared more shiny and produced excellent solder wetting. Small delta T was achieved between high thermal mass components and small components. The PCB tested was able to withstand more than 8 cycles thorough the vapor phase without any evidence of delamination..
There are many different machine types and sizes that offer different solutions — from a small laboratory system perfect for small production, prototypes, and rework; mid-size premium systems; and inline machines for high volume production. With the vacuum machine we have also a great solution for void reduction.
Contact: IBL Technologies, LLC, 1086 Whirlaway Dr., El Paso, TX 79936 Web:
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