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Measuring true temperature in lead-free assembly

Measuring true temperature in lead-free assembly Lead-free legislation is now law in Europe and China, forcing most manufacturers to move to lead-free manufacturing to comply with the legislation, or simply to avoid the rising cost of traditional SnPb components. The majority of printed circuit assemblies use SAC alloys that require reflow temperatures around 240°C and peak temperatures of 260°C. These higher reflow temperatures have created a narrower process window and an increased need for more accurate temperature measurement data. This article analyzes different thermocouple products and their suitability for modern lead-free manufacturing.

by Yoshinobu Anbe, Phil Harrison Keywords: Fine thermocouples, Sheet thermocouples, Conventional thermocouples, Process window, temperature measurement data

Measuring true temperatures The higher reflow temperatures necessitated

by SAC alloys narrow the process window, making accurate temperature measurement data more critical than ever. When we researched the response and accuracy of thermocouples, however, it became clear that the conventional (ball tip type) thermocouple cannot measure the correct temperature. This is largely due to the relatively large mass of the ball shape tip and the minimum contact area with the substrate being measured.

Three variables affect a thermocouple’s response time: • The contact area (or heat conductive area) between the tip and the objective material. It is important that the contact area between the thermocouple tip and the substrate is as large as possible, for maximum heat conductivity.

• The heat capacity of the tip. For the maximum response speed to the true temperature, the tip of the thermocouple must be as small as possible. The time taken to heat the ball shape of the conventional thermocouple will create a time lag between the true temperature and the recorded temperature.

• The thickness of the wire from the tip. The thermocouple tip wires will have the same damaging effect to the data being recorded as the size of the tip will, due to the coefficient of heat conductivity. The wires connecting the tip to the main thermocouple cable will draw heat away from the point of temperature measurement.

Equation 1 illustrates the interaction of these variables.

In the case of the conventional ball tip shape, (typically 0.5~1.0 mm diameter) thermocouples, we find a small contact area compared with the large mass of the ball shape, and a large crosssectional area of the tip wires typically 200 µm

diameter2 resulting in a slow response time and misleading data.

Research and development In order to overcome the problems associated with conventional thermocouples we have developed a range of thermocouples to deliver fast accurate data.

Fine thermocouples (Figures 1 and 2) are used to measure small components, commonly used tip sizes are 50µm and 25µm diameter; however, 13µm thermocouples are available for special applications. To assist in attaching to component leads, the tip can be pre wetted with solder and component test applications where a voltage is present the thermocouple wires, including or excluding the tip can be insulated.

Sheet thermocouples (Figure 3) are purpose built to measure the surface temperature of a substrate. We have developed a patented, onesided black body that will absorb the infrared energy at the same time that the conducted temperature of the surface being measured. This type of thermocouple would be best suited to applications where the temperature of a large bulky component is to be measured, such as a transformer or a heat sink etc.

The fine/sheet thermocouple, commonly used to measure the temperature of a silicon wafer, combines the best characteristics of both: the extremely fast and accurate response of the fine thermocouple’s small tip mass and small wire diameter, and the large contact area of the sheet thermocouple.

Performance testing To test the performance of the various thermocouple designs, four thermocouples were affixed to the side of a stainless steel pot (Figure 4). The thermocouples tested included a 40 µm diameter sheet thermocouple, a 25 µm diameter fine thermocouple, a 50 µm diameter thermocouple, and a conventional, ball-type thermocouple. The thermocouples were all

Equation 1 Speed/accuracy = contact area/tip heat capacity x wire diameter2

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Measuring true temperature in lead-free assembly

Figure 1. Fine thermocouple.

Figure 2. Fine thermocouple, close-up.

Figure 3. Sheet thermocouple.

Figure 4. Data were gathered by affixing various thermocouples to the side of an SUS pot.

attached at the same height using Kapton tape. Hot water was added to the pot at a steady rate. Sampling time was 100 ms.

Data were logged using a KEYENCE GM3000. Chart 1 compares the response and accuracy of the thermocouple types.

At one second after pouring hot water, the temperature of fine and sheet thermocouples were all more than 30°C higher than the temperature of the conventional thermocouple. More than 20 seconds later, the conventional thermocouple caught up to the other three.

These results indicate the superiority of the new types of thermocouple over the conventional ball-type.

We repeated the experiment several times to ensure that the water was added at a uniform level; each time we recorded the same results Next, three sets of conventional thermocouples and three sets of fine thermocouples were soldered to PCB lands. A minimum high-melt solder was used, and surprising data was obtained.

The temperature of the conventional thermocouples showed a standard reflow profile, but the temperature of the fine thermocouples showed a steep rising in the preheat and reflow zones.

Each stage of the reflow process can be clearly seen (Chart 2) with the fine thermocouples, as the PCB passes through the reflow oven. By adjusting the process speed and the zone temperatures the errors due to popcorning were minimized.

With Pb-free profiles, a higher reflow temperature, like 240°C, is widely used.

Overheating could cause faults from steam eruption resulting from moisture in the PCB or the SMT package. This could lead to cracking (delamination, often

Figure 5. The sheet and fine thermocouples display higher accuracy and a quicker response than the conventional thermocouple.

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Figure 6. Three sets each of fine and conventional thermocouples were soldered to PCB lands.

referred to as popcorning).[1] These internal cracks (one kind of capillary) absorb moisture from the atmosphere and cause CAF (conductive anodic filament) type migration in the field.

Another very important consideration when reflowing electronic components is the thermal tolerance of each component.

By identifying the component with the lowest tolerance of thermal distress, we can easily attach a thermocouple to the component. For example, by using a high melting point solder, the thermocouple can be attached to a 0.3 mm pitch QFP lead, 1005 ceramic chip or flip chip underfill etc. Alternatively by using our Ag paste or Kapton (polyimide) tape, the thermocouple can be attached to the component body or the circuit board itself.

Whilst concentrating on the component with the lowest tolerance, we should also address the components with the higher mass and those nearest to the centre of the board, to ensure that they are being heated sufficiently to produce a fully reflowed solder joint. This is a balancing act, and like any task it can be achieved provided we have the tools and equipment to do the job. For further information regarding the fixing of thermocouples please visit http://www.anbetherm.com/tcuse.htm Conclusion The very low heat tip capacity of fine thermocouples provides the fastest response, and therefore a more accurate thermal profile. Sheet thermocouples, which offset their 200 μm metal wire diameter with a large contact area, also provide rapid temperature response. Both types, along with the combined fine & sheet thermocouple type, are more suited to the narrow process windows required in Pb-free environments than the slower-response, conventional ball-type thermocouple.

When selecting a thermocouple, it is true to say that fast response and very high accuracy are two of the most important www.globalsmt.net

Measuring true temperature in lead-free assembly

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Figure 3. Thermocouple measurements of a reflow process, fine thermocouples versus conventional.

characteristics. However, speed and accuracy will change for each thermocouple in different applications. It is therefore just as important to choose the correct thermocouple for the task at hand.

Although we manufacture a standard range of thermocouples, we are always working with our customers to produce thermocouples for special applications.

In the unlikely event that you cannot find what your looking for at http://www.anbetherm.com, we will be happy to discuss your application and how we can adapt to your requirements.

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References [1] ‘Printed Circuit Board Reliability: Needed PCB Design Changes for Lead-Free Soldering’ by Werner Engelmaier, Global SMT & Packaging Vol.5 No.8, Sept. 2005.

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Yoshinobu Anbe has a MS from Nagoya Institute of Technology. Yoshinobu has over 30 years of experience in reliability electronics to include processing and assembly.

Phil Harrison has worked in the electronics industry since leaving school in 1977.

Phil worked in a production environment before progressing into sales of SMT placement, reflow and wave soldering equipment.

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