Thermal Profiling in Reflow Soldering

by Rick Eissinger

When you can measure what you are speaking about, and express it in numbers, you know something about it: but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meager and unsatisfactory kind: it may be the beginning of knowledge, but you have scarcely, in your thoughts, advanced to the stage of science.

William Thomson, Lord Kelvin
Popular Lectures and Addresses (1891-1894)

Lord Kelvin would advocate thermal profiling, for only by profiling do we have any idea of what is actually occurring in a reflow soldering process. Profiling has two primary objectives; determining the correct process settings for a given assembly and verifying the process consistency for repeatable results.

There are a number of steps in the production of a surface mounted assembly and each of those steps has an acceptable "process window." With the increasing complexity of modern surface mounted assemblies, those process windows are narrowing and the margin of error at each stage is decreasing thereby increasing the necessity of process accuracy and stability at each step.

What are the ingredients in a Reflow Process?

• The Printed Circuit board - which can be made of a variety of materials, FR4 is common.
• Components - an application specific mix of active and passive parts
• Solder Paste - sometimes referred to as "a mysterious gray sludge." Solder paste is made up of small solder balls and flux. The metal portion may be around 90% of the paste weight and 50% of the volume. The flux is an inert solid at room temperatures and turns to liquid and becomes active at an elevated temperature.
• Heat Transfer Device - the oven, usually with controllable temperatures in multiple zones, adjustable conveyor speed and convective heat transfer.

The Mechanics of Profiling

Thermocouples
Type K thermocouples are most commonly used in the electronics industry.
The method of attaching the thermocouple to the assembly to be profiled can be specific to the assembly and situation as well as preference of the user.

Thermocouple Attachment
High Temperature Solder provides a very robust physical connection to the pcb. This method of thermocouple attachment is commonly used in an operation that can afford to sacrifice a dedicated reference board for profiling and process verification. Caution should be used to ensure the minimum amount of solder is applied to avoid affecting the profile.

Adhesives can be used to secure the thermocouple to the assembly. This usually results in a positive physical connection of the thermocouple junction to the assembly. Drawbacks are the possibility of the adhesive failing during the heating process, removal at the conclusion of the profile and residue left on the assembly. Again, caution should be taken to use the minimum amount of adhesive since adding thermal mass can affect the results of the profile.

Kapton Tape is easy to use, but probably provides the least secure method of attachment. Profiles done with tape used to attach the thermocouples will often show a rather jagged profile due to the thermocouple junction lifting from the contact surface during heating. Ease of use and lack of residual or destructive effects to the assembly make this is a popular method.

The Temprobe™ is a device manufactured by ECD. It clamps to the edge of a circuit board and by spring tension holds a thermocouple junction firmly in contact with he assembly being profiled. The Temprobe is quick and easy to use as well as non-destructive. Drawbacks are that the Temprobe is relatively expensive and small boards may require use of a board carrier.

Thermocouple Placement
Obviously the outside edges and corners of an assembly will heat up faster than the center and components of greater thermal mass will heat more slowly than components of lesser thermal mass. At the least therefore, placing a thermocouple toward an edge or corner of the assembly being profiled on a small component and another toward the center on a component of higher mass would be a good starting point. Most profilers have five or six channels and additional thermocouples can be placed on other parts of the board of interest or on components most subject to thermal shock or high temperature damage.

What to look for in your profile
Solder paste manufactures typically have profile recommendations specific to their paste formulations. These should be consulted to determine the best profile for your process.
The first step in the profile is a preheat period that brings the assembly from room temperature to a preheat temperature and evaporates the solvents from the solder paste.
The second step in the process is to bring the assembly up to a temperature at which the flux in the paste turns from a solid to a liquid and becomes active. In order for the flux to do it's job well, it must remain within a temperature window for a certain period of time. This is referred to as "soak time." During this time, board temperatures also have an opportunity to equalize. Caution must be taken not to extend the soak time too long or achieve too high of a temperature or the flux can be used up prematurely. The third step in the process is to melt the solder.
The most common solder alloy is Sn63Pb37 (63/37), also called "eutectic, which abruptly turns from a solid to a liquid at 183C. At the end of the soak time, the assembly must be brought above the melting point of the solder for a period, typically between 30 - 90 seconds. Again, your paste vendor may have recommendations as to a temperature window for the reflow phase.
Caution must be used not to exceed maximum temperatures and heating rates of any temperature sensitive components on the board. For example, a typical tantalum capacitor has a maximum temperature rating of 230C. Throughout the process, "slopes" (rate of temperature change) must be observed with an eye toward component specifications as well. A maximum slope in the range of 2-4C per second seems to be a common rule of thumb. The final step in the reflow process is cooling. An often neglected phase of the process, be sure to observe maximum slopes in the cooling period of the assembly.

When to profile?
The most obvious need for profiling is when presented with a new assembly. Oven settings must be determined to optimize the process for good results. As a diagnostic tool, a profiler is invaluable in helping determine the causes of poor yields and high rework. Profiling can uncover inappropriate oven settings or assure that the thermal profile is appropriate to the assembly .
Many facilities run a profile on a standard reference board every day or at the beginning of each shift to verify oven operation and avoid potential problems before they happen. These profiles can be stored in hard copy or electronic format and used as part of an ISO program or to perform SPC operations on machine performance over time.

Summary

• Thermal profiling is a key element in the assembly of pcb's both to determine process machine settings and to verify process consistency.
• Without measurable results you have no control of the reflow process.
• Consult your paste vendor and review component specifications to determine the best profile parameters for your process.