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Attaching Thermocouples for Solder Reflow Board Profiling Using High Temperature Solder

Application Note #00001 Rev: 98-05

Overview
Requirements
The Attachment Process
Troubleshooting
LIST OF FIGURES
- Figure 1 : Standard KIC TC's
- Figure 2 : TC harness
- Figure 3 : Stress relief attachment

Overview

This Applications Note covers the attachment of thermocouple (TC) wires to a PCB for part profiling using high temperature solder.

We recommend 10-88-2 solder for TC attachment because it does not start melting until 268°C, which is above the temperatures the board will see during the reflow process. 10-88-2 solder also creates a solid thermal connection between the TC bead and the temperature measurement point.

Requirements

To complete the operations described in this paper you need to know how to use a soldering iron and solder wick. Required tools and materials are: 10-88-2 high temperature solder, a soldering iron rated above 300°C, a solder wick, and the appropriate thermocouples (TC's). KIC sells thermocouples specifically designed for solder reflow and wave solder applications. These TCs are 30 AWG, have a thick coating of FPA Teflon insulation (260°C), and are colored to reduce the chance of confusing TC locations on the board with the TC number (see fig. 1). Common, single color, 36 AWG, lower temperature insulation thermocouples can be used, but will often provide sporadic results. If you are attaching 6 thermocouples or more, you may want to use a "TC Harness" with 9 or 12 thermocouples in one connector (see fig. 2). The TC Harness eliminates the possibility of plugging thermocouples into the wrong numbered connector.

Figure 1: Standard KIC Thermocouples

Figure 2: TC harness, available in 9
and 12-channels. TC wires can be made
from 1' to 9' (.3-3 meters) long.

The Attachment Process

We recommend using a fully populated board for thermal profiling whenever possible. This will provide a profile that will be very close to actual production conditions and allow you to develop more efficient production recipes.

TC Locations: TC location is usually determined by identifying critical or sensitive components and attaching the TC’s to the appropriate pads. Additionally, it is best to place the TC’s so that you receive temperature readings from the hottest and coldest points on the board. The measurements received from these three to twelve TC’s (depending on the size of the board) will allow you to determine whether the thermal profile is consistent throughout the product, and also to measure the heating and cooling profiles of heat sensitive components. In placing your TC’s, it is important to keep in mind that areas that are densely populated or have larger components will take longer to heat up and also will hold heat longer. It is also important to place TC’s at the edges of the board and in any areas with small or non-existent components that will heat up faster than the rest of the board.

Surface Preparation: Thoroughly clean your selected TC locations, taking care to remove any residual low temperature solder and other contaminants that might prevent complete high temperature solder wetting.

Use alcohol or a suitable solvent to scrub the attachment surface. Then place the solder wick on the surface and press the soldering iron into the wick, heating both the wick and the surface to be cleaned. Use as much wick as necessary to remove all free flowing solder droplets. When solder has been completely removed, re-scrub the attachment surface to remove any remaining contaminants.

Attachment: Do not attempt to attach the TC by melting solder into the spot and then shoving in the bead. Place the TC bead on the attachment surface and heat both evenly, then touch the solder to the heated TC bead and let the heat from the TC bead melt the solder. This method gives you superior wetting and a stronger solder attachment to the pad or lead.

Your temperature reading will come from the first point of contact between the two wires leading from the TC. To insure accurate readings, it is critical to carefully separate the two wires all the way up to the TC bead after soldering.

High temperature solder is an efficient heat conductor, so if a tiny bit gets between the TC bead and the lead or pad, you will still get an accurate reading. However, too much solder at the measurement point will increase the heat capacity of the TC and cause your peak temperature measurement to read low.

To keep from accidentally breaking the TC attachments or stressing the solder joint when moving the profiler and the attached board, wrap your TC wires through a hole in the back of the board so that the TC attachment point is never stressed. If you are using colored thermocouples purchased from KIC, you will find that you can hang the entire board from two or three of these wires without damaging the attachment or the wire. (see fig. 3)

Figure 3: TC wires threaded through holes for stress relief

Troubleshooting

PROBLEM 1:

In profiling the test board, one or more of the TC’s experiences a step change in temperature and then consistently reads differently from the other TC’s (generally higher on the heating side of the profile and lower on the cooling side.)

PROBABLE CAUSE: The TC attachment to the board has failed and the TC is floating in air. This can happen if the attachment area was not thoroughly cleaned and/or not all low temperature solder was removed. Contaminants interfere with the solder wetting and residual low temperature solder reduces the melting temperature of high temperature solder. Possible other causes are improper application of the solder to the TC attachment point or the pad separating from the board.
SOLUTION: Since the test board is still in the oven, there is nothing you can do to remedy the situation until the run is complete. All data before the step change is valid for analysis purposes, and all data after that point is not. For 100% accuracy, you must reattach the TC and then repeat the profile.

PROBLEM 2:

During a profile run you get an erratic or "jumpy" profile from one or more of the TC’s.

PROBABLE CAUSE: The two wires leading to the TC bead are touching at some point before the bead.
SOLUTION: Examine all TC’s and separate any wires that are touching before re-profiling. Most common thermocouples are not designed to be used in solder reflow applications, the wire is too small (36 gauge) and breaks easily, and the insulation is very thin or rated to too low a temperature. If your TCs are suspect, we suggest you contact KIC Thermal Profiling to receive quality TCs that have been designed for the reflow process.

PROBLEM 3:

You are following all the instructions but it is difficult to establish solder connections and your TC’s will not stay connected to the board.

PROBABLE CAUSE: If you are using fresh, good quality solder, then the problem is probably a soldering iron that is not rated for high temperature solder. Typical rework soldering irons do not get hot enough to melt 10-88-2 solder.
SOLUTION: Use a soldering iron that is rated for 10-88-2 solder. It must heat to a minimum of 286°C, and 300°C is preferable.

If you follow the steps given in this paper and use a KIC profiler, you will get the most accurate profile possible of your production boards.

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