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Page 69 The Impact of Thermocouple Attachment Methods... Continued from previous page


were included in the PCB to allow for routing and wire stress relief. Thermo couples were attached to points on the PCB using each of the attachment methods listed above. A new PCB was used for every meas-


using high-temperature solder, returned an average tempera- ture of 257.8°C with a standard deviation of 0.5°C. Cp and Cpk values of 1.0


were reported with upper and lower control limits of three stan- dard deviations (+/-1.50°C). Based


dimensions, the high-temperature solder method offered the most consistent and accurate results


is the attachment method. Notice the reduction in meas- ured temperature with Kapton


Figure 3: attachment method processing order.


compared to the initial capability study.


All other attachment types


returned lower values. The fol- lowing tables illustrate and quantify the deviations. Figure 4 shows that all con-


tape, aluminum tape, and SMD adhesive. This indicates that the attachment method may inter- fere with the thermocouple/sol- der pad interface. It is likely that the mass of the tape/adhesive is the cause. As discussed above,


Figure 2: control thermocouple capability study chart.


urement and a control thermo- couple, employed to provide a baseline, was attached to the same 1.0 mm pad on every PCB using high-temperature solder (Figure 1). A reflow soldering oven was


set up to produce a typical lead- free solder profile. No changes to the machine parameters were made during the experiments, and a capability study using 30 sample boards was conducted to ensure the settings were stable and repeatable. The peak tem- perature of the control pad, with the thermocouple tip attached


on these results, the test proce- dure was deemed valid (Figure 2). All attachment types, except


for the UV adhesive, offered good mechanical strength with no detached thermocouples recorded. The UV adhesive suffered four detachments throughout the 11 cycles, making it difficult to collect any meaningful data for the com- parison tests. Therefore, this eval- uation suggests that UV adhesive may not be a viable attachment method. Analysis of the peak temper-


ature measurements revealed that in all cases and on all pad


Figure 4: peak temperature comparison by type, 1 mm pads.


trol thermocouple measurements fall within the required limits. This is also true of all readings where high-temperature solder


the UV adhesive results are inconclusive, as two thermocou- ples became detached. The measurements taken on


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5.0 mm solder pads exhibited a similar trend as the 1.0 mm pads (Figure 5). This is true on all pad sizes. The non-high-temperature solder methods produced lower results, confirming that the attachment medium is interfer- ing and affecting the tempera- ture readings of the pads. Note here that the high-temperature solder result also shows a decrease in temperature caused by the increased mass of the measured pad. Figure 6 quantifies the aver-


age deviation experienced for each attachment method. (Again, the UV adhesive results should be dis- regarded as the readings are influenced by the detachments experienced during the trial.) These temperature readings


suggest that high-temperature solder provides strong mechani- cal and thermal bonds between the thermocouple and the tested area, and it returns the most accurate results. All other meth- ods delivered temperatures well outside the required specifica- tion for accuracy and this should be considered when choosing an Continued on next page


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