TECHNIQUES FOR AVOIDING LIFTED SOLDER TABS
Indeed, of the steps necessary for a successful strain gage installation, soldering usually requires the most practice to become proficiently skilled. Of all the faults in gage soldering, lifted or dislodged solder tabs are perhaps the most common. This unwelcomed event is always frustrating because, when it occurs, the affected gage must be removed and replaced.
Why does this occur, and how can it be prevented?
Here are the most common causes of lifted soldering tabs, and the remedies for preventing them from occurring in your future installations.
Excessive soldering Heat. Most Micro-Measurements strain gages are limited to use at temperatures of +500°F (+260°C) or lower. Yet the temperature of the soldering iron tip typically must be in the range of +600F (315°C) to +700F (370°C) for the lowest temperature solder, M-LINE 361 and 361A lead-tin eutectic solders, which melt at +361F ( +183°C). Tip temperatures must be even higher for other solders. We typically recommend the soldering pencil tip temperature be the melt point of the solder plus 150°F so for example when using the 361A, the tip temperature (at least the initial temperature) should be around 511°F. The temperature may have to be higher depending upon the iron, substrate material, etc.
That means the dwell time of soldering iron tip on solder tab must be limited to 2-3 seconds. (1 second is often too short for adequate heat transfer.)
The following should help you to accomplish the soldering operation in this period of time:
Use the correct soldering pencil tip. Use of conical tips should be avoided except when soldering to the very smallest gages. Chisel tips are preferred since the heat the solder tab uniformly and quickly. Conical tips apply heat at a small contact area and by the time sufficient heat has transferred to the gage tab, the contact point has been overheated which can cause it to blister and lift. Make sure the cladding on the tip is intact. A corroded tip will not properly transfer heat to the solder tab.
· Pre-tin gage tabs and terminals before attempting to attach leadwires. Make certain the gage tabs are not coated with tape mastic or bonding adhesive. If necessary, clean the tabs with a soft “pink” pencil eraser. ( Be certain to avoid the coarse ink erasers which will damage the tabs).
· Properly tin the soldering iron tip. A pool of molten solder on the tip is essential for rapid heat transfer. Keep the tip tinned at all times to protect the thin layer of cladding.
· Ensure that adequate amounts of solder flux are present. Wetting of the solder tab requires flux. Use either solder with flux core ( M-LINE 361, 361A, and 570) or apply external flux (M-LINE AR or SS). (The SS flux is necessary only on the tabs of D- and K- alloy gages without copper dots or pads.)
· Wire Size - Wires that are too large place excessive stress on the solder tab/gage backing interface.
· When soldering to EA-, N2A-, EP-, ED-, or EK- Series strain gages with 0.125 –in ( 3-mm) or larger gage lengths, use 30- to 36-AWG single-conductor copper wire between that solder tab and the terminal strip. For shorter gage length ( and smaller tab sizes) use 34- to 36-AWG wire.
· When soldering the leadwires directly to CEA- Series gages, or those with Option W terminals with 0.12-in (3-mm) or larger gage lengths, use 26- to 30-gauge standard wire. For shorter gage length ( and smaller tab/terminal sizes) use 30-gauge standard wire.
· Strain Relief. Failures can occur at the solder tab from excessive movement of the wires after attachment. Securing the leadwire cable to test specimen and the incorporation of strain-relief loop in the leadwires between that gage tabs and point of attachment will help ensure that the movement of the wires at the gage tabs or terminals is minimized.
And, of course, use of C2A, C4A and other strain gages with Option P ( preattached leadwires) can eliminate the need for soldering to the gage or terminal altogether in many applications.
With the proper strain gage accessories and installation techniques, lifted soldering tabs can be virtually eliminated. Should you have any questions concerning your specific applications, our Applications Engineering Department is always available by phone or e-mail to help ensure your success in stress/strain measurements.