Archive for category Vapor Phase Soldering
Thermocouple Attachment in Vapor Phase Soldering
Posted by Paul Austen in Epoxy, Kapton Tape, Thermocouple Attachment, Vapor Phase Soldering on February 8th, 2011
Attaching thermocouples in Vapor Phase Soldering (VPS) presents a potential problem that’s often overlooked. To illustrate this potential problem, let’s look at how VPS works. All the work needed to heat the components and solder joints to the melting temperature of the solder take place in the very thin “condensate” layer of liquefied vapor phase fluid. This 0.2 mm layer is very much like the layer of water that forms on a cold drink on a humid summer day. (See figure 1.) The vaporized fluid (cloud) is at or very near the boiling temperature of the fluid, which is typically 200ºC to 240ºC, depending on the fluid you choose to use.
Figure 1 – Vapor Phase Soldering and the condensation layer
When your room-temperature assemblies are immersed in the vapor cloud, the vapor quickly condenses onto the cold surfaces of all the parts, releasing the heat of vaporization, or the heat energy that was put into the liquid to evaporate it into a vapor. This energy release begins heating your assembly rapidly toward the 200+ºC vapor temperature.
To measure the thermal profile of the process to prove you are staying within the components’ thermal limits, (always a prudent step) you must attach a thermocouple to the component or solder joint you wish to profile. Here is where you must take care. (See figure 2)
Figure 2 – Typical thermocouple attachment to component lead
The thermocouple may be attached with solder or epoxy. However the thermocouple wires extend away from the point of measure and may be exposed to the vapor. The vapor will condense onto the thermocouple wires in the same way it condenses onto all the parts. Exposed bare wires will heat quickly because they are very small. These heated wires will conduct that heat toward the thermocouple bead, where it is sensing the temperature, and artificially heat the sensing point. This will actually show as a “false” temperature rise in the early part of the ramp up in temperature because the heat is conducted in to the part by the thermocouple wires. The insulated part of the wire will not heat as fast because the insulation slows the heat flow of the condensing vapor. .
So when connecting thermocouples to components to measure the thermal profile, it is very important to insulate any exposed thermocouple wire beyond the thermocouple sensing bead. (See figure 3)
Figure 3 – Insulated thermocouple wire
This is best done with a thin layer of epoxy covering the exposed bare wires of the thermocouple, up to and a bit past the insulation on the thermocouple wire, being careful not to cover the sensing bead of the thermocouple. The epoxy does not need to be very thick, just a thin covering. Using too much would have the “chilling” effect of preventing the heat from getting to the component or solder joint, giving an artificially cooler profile result.
You may also try Kapton® tapes or high-temp heat shrink around the end of the thermocouple. These may shrink back or come loose, however; and if you plan to profile more than once, these methods may not hold well over time.
References:
SMT Magazine, “Vapor Phase Soldering: The Comeback Kid” by Ray Prasad
Thanks to Kevin Syverson, Silicon Forest Electronics, Inc., for the use of their Vapor Phase soldering system.
Thermal Profiling and Vapor Phase Soldering
Posted by Paul Austen in Vapor Phase Soldering on September 9th, 2010
There has been some new talk by some of the best quality conscious electronic manufactures about the many benefits of an older soldering process: vapor phase soldering. Vapor phase soldering has a lot of good things to offer, now that we have gotten past the stigma of the old CFC fluids and moved on to newer chemistries. The maximum temperature that the assembly can be subjected to is dictated by the boiling point of the fluid being vaporized. Because the boiling point of the fluid is a physical constant, you might think, “Why bother running a thermal profile on the assembly being soldered.”
This idea should be considered carefully, and here are some reasons why thermal profiling in vapor phase soldering is still a very good idea:
1. Although the boiling point of the vapor phase fluids is a physical attribute that limits the maximum temperature, the condensation of the fluid onto the components can impart a lot of heat, real fast. This can subject components to the old thermal shock problem, and unless this heating rate is carefully controlled by the vapor phase machine, you may well be shocking the components. Thermal profiling is the only way to show this is under control.
2. The maximum temperature is a function of the fluid type, and one needs to be sure the correct fluid is being use. There is a fluid whose boiling point is hot enough for lead free soldering, and not too hot for leaded soldering, about 230ºC. This “happy medium” is a good compromise, so one does not have to own two different vapor phase machines, or change fluids from one process to the other, but it is another reason why thermal profiling is a good idea: to prove that the process is meeting the need of the solder paste and the limits of the components.
3. A process undocumented is a process out of control. Unless you have some evidence that the thermal profile is meeting the requirement of the solder paste and the limits of the components, you cannot prove the process is in control statistically. You can’t make process control charts if you don’t measure the process. This is at the heart of a good Thermal Quality Management (ThQM) program.
4. Your customer still wants to know what the thermal profile looks like. No matter how you solder your customer’s boards, they still want to know what they were subjected to, thermally. This is your assurance to them that you have treated their product properly.
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