The advantages and disadvantages of vapour phase soldering

17 November 2014

In the beginning of SMT, vapour phase soldering was the preferred reflow soldering technology because of its excellent heat transfer capabilities.

The process is widely used for assemblies with a large variable of thermal mass components on the PCB. It has the advantage that it can heat large masses evenly and quickly whilst the peak temperature is controlled. It emits thermal energy by phase change of the heat transfer medium from gas-like to liquid state. This phase change, known as condensation, on the surface of the solder piece lasts until the assembly has reached the temperature of the vapour. Another advantage includes no overheating of assemblies, so vapour phase soldering is ideal for lead free solder processes. It offers continuous heating-up of the complete assembly, an oxygen free soldering process without the use of any protective gas and finally the efficient use of energy because of the high heat transfer, therefore reducing operating costs. However there were some disadvantages, fast temperature rise and little influence on the temperature profiles. Due to this, vapour phase soldering was reduced to special applications with high mass or complex boards in low numbers. 

To combat these process challenges, SolderStar developed a new profiling system designed specifically for vapour phase machines. The latest evolution of these machines offer user controlled soldering, with freely adjustable temperature gradients and profiles. 

The original ‘dream’ sold by the manufacturers was that profiling was not needed; the reality is ‘you can't manage what you don't measure’. ‘Tomb-stoning’ is an evident problem; given the even heating of the vapour phase process this is unexpected. Yet PCB designers do not always adhere to machine manufactures recommendations so they then need to be set-up by an engineer and monitored if long term quality is to be maintained.

SolderStar developed a solution that gave users the ability to understand and control the production process. A two-piece, low mass heat-shield gives protection whilst minimising process impact, allowing operation across pre-heat, vapour and vacuum stages. Once retrieved from the process, the hot shield can be opened allowing data download and rapid air cooling, minimising the potential for instrument damage. Alternatively, captured data can be transferred by wireless telemetry to any selected PC station, allowing real-time profile viewing and analysis by the user.

A high temperature must be reached so that soldering can be accomplished even on sockets or large BGAs, but at the same time needs to be checked for overheating or delamination. With vapour phase soldering, even FR4boards, double sided densely packed, can be soldered reliably and with no danger of overheating.

The vapour phase heat transfer offers an excellent tool for easy and reflow soldering, independent whether lead free or leaded solders used.

Machine types and profiling methods

There are several machines that benefit from SolderStar thermal profiling, one being medium size production machines. This style of machine is harder to profile as some machines have internal thermocouple ports internally wired to the outside world. An external profiler can be used to capture the profile, although this is more difficult and not ideal for daily production testing. This machine also comes with a vacuum stage, if this is employed the soldering stage uses a sealed chamber and it is not possible to get wires inside, a true pass through profiler is needed, this is where SolderStar’s system comes into its own. 

Another machine process is inline, these are also difficult to profile with long wires as they snag and the results are not always accurate. Again, if vacuum is employed you cannot use long wires.

The SolderStar PRO - VP system allows users of batch and in-line vapour phase soldering systems to fully profile their products in the same way as a conventional reflow machine. The system includes the SolderStar PRO data-logger combined with a sealed light-weight heat shield capable of operating in the pre-heat, vapour reflow and vacuum stages of the machine. Data can be passed to a nearby PC via wireless telemetry allowing the profile to be viewed in real-time. Once data capture is complete the heat shield can be opened and the datalogger removed, allowing rapid cool down cycles and minimising the risk of overheating the profiling unit.

SolderStar’s product has an external data logger with long wires, making them a perfect choice for manufactures with different soldering processes as only one datalogger is necessary. There are also added ‘down the wire’ logging capabilities via a USB connection, with live data which can be viewed by the engineer as the PCB passes through the machine. The pass through profiling unit is SolderStar’s main unit and is the only way to profile machines with vacuum stages.

High thermal transfer

The use of galden vapour in the machines means the thermal transfer is high compared to a conventional reflow oven, and subsequently the thermal protection of the datalogger is paramount to survive overheating.  The vapour must not be allowed inside the profiling unit as it will ultimately damage the electronics and insulation used. As vacuums are employed the problems increase, the system must be able to withstand any pressure difference seen in the process, and a real-time wireless telemetry link is required to relay data back to PC from inside the machine and vacuum chamber.

There are two approaches to this problem, the one box approach - a datalogger electronic PCB inside a sealed box and the two box approach with a datalogger that can be separated from the protective shield when it exits the machine.

The advantage of the one box approach is its smaller size, yet as it is perfectly sealed to withstand the vacuum, it needs to be strong and hence thermal mass/weight is high. When it exits the machine it becomes hot, if simply left it will continue to heat and damage the electronics. Force cooling is needed to restore its temperature, such as a water bath.
The advantage of the two box approach is it has better thermal protection and has rapid cool down. It uses SolderStar’s generic datalogger which can be used for multiple processes. Only one calibrated unit is needed each year, rather than multiple units. However, it is physically larger. Most engineers are concerned that the measurement instrument involved in the process will take energy away from the vapour cloud and affect the measurement, therefore not a true reflection of the PCB profile. The larger it is, the more concern there is. SolderStar did not find the one box approach to be the favoured process.

SolderStar’s approach to the problem of ‘physically larger’ was to look laterally at the problem. The actual problem is the thermal mass of the object. They produced a ‘thin’ wall protective shield which has a large surface area, but exceptionally low thermal mass. It is teflon coated to resist the galden build up, and has proven to have virtually no impact on the process. The lightweight skin of the shield heats rapidly, and once up to temperature the condensing on the shield is minimal. The carefully ribbed and strengthened design includes a pressure release system to ease opening after the vacuum stage.

Internal testing

SolderStar undertook internal testing of the system on an Asscon Quicky 450. The test was done using the smallest scale machine, as an ‘impact’ would more likely be seen. It allowed a profile to be captured with long wires, so no instrument in the process chamber, and then with the instrument inside.

The purpose of the test was to further validate the thermal performance of the new VP heat shield, perform a ‘leak test’ evaluation of the new ‘O’ ring seal and monitor any effect of the thermal shield on the process.

Conclusions were reached that thermal performance is adequate for most vapour phase processes, with the original spec of 250C for nine minutes likely to be achieved. The new ‘O’ ring provides sufficient seal and both the red and green traces almost exactly overlay in the ramp up and peak stages, the green trace has a longer peak as it was left in the process longer. The ramp up slope and time are the same with the system inside and outside the barrier.  

The vapour energy available within a machine like the Quicky 450 is lower than what the profiling system is programmed for. The light weight shield is heated rapidly and energy then thermal absorption minimised, at no point did the vapour ‘collapse’ and the process be impacted.

The SolderStar system has been proven not just through in-house testing, but also through use by manufacturers. Rehm Thermal Systems and IBL Technologies, both leaders in vapour phase technology, recommend it, with IBL now offering the system with their machines.

Real-time telemetry
Real-time telemetry reliably functions within the vacuum stages of the process. For this type of process, engineers like to see the profile in real-time. This is hard in any industrial machine as the RF energy is greatly attenuated by the metalwork. With a vacuum stage it is even more difficult, as it is essentially a sealed metal box, the system relies on wiring and other components in the chamber to allow any RF energy to be in/out.

SolderStar’s approach is innovative and has proved an exceptional solution. The system is designed using ‘mesh networking’ technology which provides a ‘self-healing’, very reliable, network/data link. The approach allows routers/repeaters to be used as necessary to relay the signal in machines where other systems would simply not work. Normally no repeater is necessary if the PC receiver is within 10m of the machine. Additionally, as no signal is 100 per cent guaranteed they employ an intelligent ‘two way‘ protocol, if a ‘dead spot’ is encountered for a few seconds in the machine, the datalogger buffers then offloads once the link is re-established, continuing in real-time.

Vapour phase soldering has become competitive for high volume productions. The vapour, like all gases, is equally filling out a given sphere. The equal temperature distribution over the boards is automatically provided by this physical effect, while in a convection oven the cross profile can vary in temperature. The physical limitation of the maximum temperature in a vapour phase system does not require further controlling mechanisms to avoid overheating. This quality feature is providing the highest benefits in long-term reliability of the soldered electronic boards. 

The excess heat needed to ensure a perfect solder, vapour phase soldering is only 5 to 10°C over the melting point of the solder paste. Other reflow methods require 30 to 35°C excess heat for the same task due to its lower heat transfer rate. 

The vapour phase process provides an oxygen free atmosphere at no extra cost leading to the best possible wetting. The energy consumption (typically 5-6kW for a large inline system) is much lower than convection as the energy remains in the hot liquid. Optimised insulation reduces the introduction of heat into the surrounding and thus saves air conditioning. Vapour phase ovens are smaller than convection ovens, thus they save space in production. Vapour phase is a perfect choice and together with SolderStar profiling systems and its dedicated software, it allows vapour phase machines to be defined, enabling the process to be controlled to the same extent as a conventional reflow oven.

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