Thermal Imaging Certifies Reflow Soldering Process

To meet this new requirement, a joint venture consortium has successfully devised a thermal imaging technique that controls and monitors the temperature within reflow ovens. The consortium includes reflow oven manufacturer Quad Europe (High Wycombe, UK) and Loughborough University (Loughborough, UK). The new technique, called Adaptive Intelligent Reflow (AIR), integrates a high-performance infrared line scanner with high-speed data acquisition.

In a typical reflow soldering oven, a production engineer performs an initial thermal set-up of the oven’s various temperature controllers. The engineer accomplishes the set-up by attaching four thermocouples to a test PCB, and logging temperatures as the board passes through the oven. This set-up enables the oven manufacturer to optimise the temperature profile in the oven zones that make up a complete oven (see Figure 1).

Figure 1. The temperature of the soldering paste on a PCB follows a typical profile as it passes the four zones of a reflow soldering oven.

When a PCB manufacturer later uses the oven in production, the temperature profile can vary for several reasons. Firstly, different PCB layouts have different thermal capacities, which, in turn, apply a different load to the oven. Secondly, the frequency with which you pass PCBs along the oven also varies thermal loading. Thirdly, oven operators can arbitrarily adjust conveyer speed or temperature control. Also, ageing variations in sensors or controllers may vary temperature. Currently, you have no guarantees that even a complete batch of PCBs (let alone any one PCB) has been heated correctly. Equally, you have no way to be sure that the reflow oven has not subjected key components to excessive temperatures.

Thermal Image Controls Oven

At the heart of the AIR system is a FLIR International infrared line scanner that provides non-contact, high-resolution thermal images of PCBs passing through an oven (see Figure 2). The scanner produces an image from 2500 samples/scan at a 20 Hz scanning speed. (see, “IR Scanner Basics” explains scanner operation more fully.)

Figure 2. You can use white crosses to select points of interest on the thermal image of a PCB as it passes through reflow soldering. Temperature graphs with upper and lower limits show trends over several boards at selected points.

The AIR system employs a dedicated PC, which uses a 250k measurements/s data acquisition board to collect data from the scanner. During the whole process, the line scanner continuously scans the moving conveyor belt. A photocell inside the oven instructs the PC when to begin and end the acquisition of an image. This mode of data acquisition forms thermal images of the PCBs which are passing along the conveyer.

As part of the initial set-up for each PCB layout that you manufacture, the system provides a thermal image at peak reflow temperature for a reference board. You can then identify up to ten points on the reference image for the system to correct for emissivity as it monitors every subsequent board that you process. You can also set the upper and lower process limits and alarms.

In operation, the system monitors your selected points for deviations from the programmed limits on every PCB it processes. The system automatically adjusts the oven’s reflow zone temperatures when the average of your ten points exceeds the set limits. If the recorded temperature of any selected point deviates from the set limits, the system provides a warning display. A similar warning indicates if the system has reduced the temperature so low that it impairs the soldering process.

Under normal conditions, the system compares the average temperature of the selected points with the average temperature of the five previous PCBs in the oven. The system then compares the resultant average with measurements on the reference PCB. If the result is less than 38C, then the system adjusts the reflow zone temperature. Averaging in this way minimises the possibility that a single PCB could initiate a change in oven temperature, which may occur, for example, if there is a missing component.

The system displays and stores data from each PCB that you process and passes it to statistical process control software. The software displays a trend analysis based on the reference image made of the test board at the commencement of the production run. The system also stores thermal images of the last twenty PCBs which have undergone the process. An optional bar code reader is available to record bar codes from the PCBs. The system saves the bar code data, along with the time, date, and temperature information to an archive file for the complete traceability of boards processed in that oven.

Dave Sibley is product manager for reflow and AIR systems with Quad Europe in High Wycombe, UK. Steve Green is business development manager for thermal imaging with FLIR Systems International, West Malling, UK. You can contact the authors at dave.sibley@quad-europe.co.uk   and steve.green@flir.uk.com.

For more information: Quad Europe, High Wycombe, UK. +44-1494-464088, www.quad-sys.com;  FLIR Systems, West Malling, UK. +44-1732-220011, www.flir.com.

A single IR scanner can have a 908 scanning sector, 400 mm object distance, and 75 to 600°C temperature range.

IR scanners use electro-opto-mechanical infrared sensing heads to convert thermal energy emitted from objects into an output video signal. Scanners can sense temperatures over the range 08C to 16008C. Scanners use mercury cadmium telluride (MCT) and indium gallium arsenide (InGaAs) detectors with scanning rates from 8 to 40 Hz. The scanning sector is 908 with a focusing range 400 mm to infinity
(see Figure, right).