Reducing Soldering Defects With Thermal Imaging

We’re getting bad solder joints on our high-density circuit boards. Can thermal imaging be used to locate these bad connections? … We’re using hot-bar soldering for some fine-pitch devices and we’re not getting good solder connections on all the leads. Can thermal imaging help us find the bad solder joints? … We build hybrids and need to be able to determine the quality of the bond between the semiconductor die and the substrate. Can thermal imaging help?

These are the questions we hear as PCB designs become more densely populated. Bad solder joints are occurring more frequently on these congested designs and thermal problems are developing in hybrids when voids form in the eutectic bond between the die and substrate.

Common to each of these scenarios is heat and temperature. Heat is an integral part of the manufacturing process and improper setup of process equipment can lead to manufacturing defects.

In reflow soldering, temperature variations across the board may result in faulty solder connections. With hot-bar soldering, nonuniform heating of the bar may lead to uneven temperatures and poor solder connections.

In hybrid manufacturing, bad process control may result in improper reflow of the solder which bonds the die to the substrate, allowing voids to form. These voids, in turn, may lead to higher thermal resistance between die and substrate and higher junction temperatures. These higher junction temperatures can adversely affect performance and reliability.

In the first two scenarios, finding the bad connections isn’t the real solution–understanding their root causes and taking corrective action is. Finding solder voids in hybrid assemblies is tougher and requires tight process control.

In each of these scenarios, improper application of heat can lead to subtle but substantive problems, and traditional troubleshooting tools may not find the problems. Thermal imaging is a tool which can help solve these problems.

What are Thermal Imagers?

Thermal imagers are instruments that create pictures or maps of heat rather than light. An imager scans an object, measures its radiated infrared (IR) energy and converts the data to thermal maps.

There are many systems on the market, and the target application(s) determines which system is best. Some systems are really viewers which create pictures based on temperature, and are frequently used in maintenance applications or for providing quick checks on system conditions.

Measurement systems, on the other hand, have radiometric capability as well as the capability to present information in image form. These systems provide detailed thermal images which include temperature data at each pixel of the image.

Often, cursors can be positioned to a point within the image and then the underlying temperature is displayed on screen in alphanumeric form. Images may be digitized, stored to disk, compared, manipulated, processed and printed. Standard image formats, such as the tagged image file format, permit files to work with a wide array of software programs.

How Does IR Technology Work?

IR imaging technology relies on the fact that any object whose temperature is above 0(degree)K radiates IR energy. The amount of radiated energy is a function of the object’s temperature and its relative efficiency of thermal radiation, known as emissivity.

The amount of radiated thermal power is determined by this equation:

W = E O T 4 in watts/cm2

where W = Spectral radiant exitance (radiation)

E = Emissivity

O = Stephan Boltzmann Constant

T = Temperature

This energy can be measured with a calibrated instrument which converts the energy to a temperature which can be displayed.

Many scanning techniques and IR detectors are used today. Typical thermal detectors operate in the 2- to 5-micron and 8- to 13-micron wavebands. There are applications where each range offers advantages; however, either range will work well for applications calling for measuring temperatures above 0(degree)C, those typically encountered in electronic engineering and manufacturing environments.

Temperature resolution, the capability to measure small temperature differences, can be as fine as 0.1(degree)C. Spatial resolution, the capability to measure temperatures on small areas, can be as fine as 15 microns.

Applications extend from microelectronic thermal analysis to mapping earth resources from space. Scanning rates can vary from hundreds of scans per second to tens of seconds per scan. System costs can range from approximately $10,000 to more than $100,000, depending on features.

In thermal testing, particularly reliability testing, temperatures change relatively slowly, so scan speed needn’t be a major concern and lower-cost systems can be used.

Thermal Imaging and Profiling Reflow Ovens

Locating faulty solder connections on circuit boards is difficult, as the resistance of the bad connections can be low. This low resistance means that I2R losses are small, and the heat generated when the current flows is hard to detect. Thermal imagers have difficulty measuring these very small temperatures.

The real key to solving solder-joint problems is to prevent them from occurring in the first place. This means the soldering process must work properly and the machine operator must have fully characterized the boards being soldered. The oven profile must be set as dictated by characteristics of the boards, and this is where thermal imaging can make a significant contribution.

Oven profiling typically involves sending boards with thermocouples attached, through the reflow machine. The temperatures indicated by the thermocouples are used to set each stage of the reflow soldering cycle.

This approach assumes the circuit boards and components reach uniform temperatures at each stage of the process. They don’t. This means the placement of the thermocouples is very important. If they are placed at the wrong locations, the profile may be incorrect for other areas of the board.

Thermal imaging can be used to monitor the heating of boards as they come directly from the oven. These images can be used to determine if the thermocouples are correctly placed, and the extent of temperature uniformity across the boards. Large temperature variations can lead to solder defects caused by improper flux activation or solder reflow.

Figure 1 shows a thermal image of circuit boards exiting a reflow solder machine. The exit fans had been disabled and the scan was made shortly after the boards left the oven.

Note the variability of temperature across the surface of the boards. We have found instances where temperatures on a board can vary by as much as 40 to 50(degree)C. This much variation could compromise the quality of solder connections on the board.

What causes these variations? They are influenced by a number of factors, most related to board design. Component mix and layout, thermal mass, thermal resistance, and emissivity of materials all play a role. These factors can affect the way heat is absorbed and conducted by the boards and components as they pass through the oven.

If certain areas take on too much heat and have higher-than-desired temperatures, the solder may be too liquid or components may float. If areas absorb heat more slowly, temperatures may not reach the point for flux activation or proper solder reflow. This can lead to improper reflow and poor solder connections.

For maximum effectiveness, a direct line-of-sight for the camera must be maintained. Installing a viewing port with an IR transparent window above the exit from the heating chamber is one way to accomplish this.

Hot-Bar Profiling

Hot-bar soldering entails heating a ceramic blade to a temperature hot enough to activate flux and reflow solder. The edge of the blade is then brought down onto the leads of the device which is positioned on the lead pads. The hot blade solders the leads simultaneously and uniformly. If the temperature along the blade’s edge isn’t uniform, uneven reflow occurs. This results in bad solder connections between the leads and the circuit board pads.

Looking for Voids in All the Wrong Places

The solder used to attach bare die to the substrate in multichip modules (MCMs) and hybrids provides a path for heat flow. Voids in the solder, caused by process problems, increase the thermal resistance. Higher thermal resistance leads to a higher operating temperature for a device, which, in turn, leads to reduced reliability and lower performance. The capability of thermal imaging to map temperatures across the device can be useful in identifying situations where these voids may exist.

Figure 2 is the image of a small hybrid assembly. In this case, the die attach was good and no thermal problems existed. The image shows the degree of detail available on the flow of heat and overall thermal management of the assembly. If a bad bond between any die and the substrate had existed, the image might have shown hot spots on the problem die. By comparing the image of a “golden” device against an unknown device, thermal differences would have been accentuated.

Conclusion

Although different, these applications share one thing in common: They use heat as an element of the process. Heat can be part of the problem or it can be part of the process.

Thermal imaging provides a comprehensive map of the thermal factors affecting the device or process being monitored. If you were to try to gather similar levels of information using thermocouples or temperature probes, it would be a major undertaking requiring many hours of technical labor.

With its speed and comprehensiveness of measurement, thermal imaging can be a cost-effective tool for troubleshooting thermal problems and for monitoring processes which are based on heat. The payback from using thermal imaging will come in the form of reduced scrap and more reliable products.

About the Author

Jim Walcutt is Vice President of Sales and Marketing at Compix. Before joining the company in 1989, he worked for Tektronix and Wavetek. Mr. Walcutt received a B.S.E.E. degree from Fairleigh Dickinson University and an M.B.A. from the University of Portland. Compix, Inc., P.O. Box 885, Tualatin, OR 97062-0885, (503) 639-8496.

Copyright 1995 Nelson Publishing Inc.

March 1995

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