Page 413 - Mechatronics for Safety, Security and Dependability in a New Era

Ch80-I044963.fm Page 397 Tuesday, August 1, 2006 4:54 PM
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plastic after a few usage times the joints will get broken. Therefore, spiral connections were tested
instead of knotting. However, the problem still partly remained and we concluded that the edges of
heat-shrinkable plastic could be too hard for fibres, which gradually abraded off and disconnected the
sensor from the system. We tested also small PWBs as combiners along the sensor connection bus.
Yarns were soldered directly to these PWBs. In this case the same problem existed and the yarn got
broken near the PWB's edge. The reason could be too hard PWB's edges or the interface between
solder and yarns.
We have also sewn metal clad aramid fibre yarns by hand to cloth to form electrodes (Rantanen at al
2001). These kinds of electrodes were used to replace commercial electrodes in the measurement of
skin resistance in the sensor shirt. The main function of this measurement was to indicate the sweating
level of the user. The simple measurement was not very demanding for the electrodes and they were
found to be sensitive enough for our purposes. However, it would be beneficial to be able to sew the
yarn by machine to ensure larger electrical conductivity and to ease and accelerate the implementation
of electrodes. In the bioimpedance measurement suit we have also tested fabric electrodes (Vuorela et
al. 2003). Suit's purpose is to measure the impedance of the user and based on that the suit can
indicate the total body water level. In this application our goal was to replace commercial silver
chloride electrodes with fibre electrodes, since commercial electrodes were found unsuitable for
moving people. At this time we have utilised polyamide based silver plated material (Finnesd data
sheet). New electrodes were found to be better than commercial ones however, long-term durability,
e.g. coping with continuous washings and abrasion has to be found out.

TEST METHODS AND RESULTS

Due to problems that have been occurring in ECF's connections we decided to find out whether we
could improve the reliability of these connections by proper connection mechanisms and materials. We
have studied the use of two kinds of silver clad aramid fibre yarns. The first yam is thinner and more
twisted than the other one. For a reference material we chose another easily available material made of
stainless steel, which is also very strength (Bekaert data sheet). Unfortunately, it has poor conductivity
and its joining to PWBs is difficult, because its melting point is much higher than in joining materials.
For metal clad aramid fibre yarn connections we used SnPb (tin-lead) solder and for stainless steel
yarn connections we used Sn (tin) solder. Lead will be forbidden in consumer electronics and
therefore, we also made joints by using electrically conductive adhesives. At first we used Loctite's
conductive adhesive (Loctite data sheet) and later we used also Electron Microscopy Sciences' (EMS)
adhesive (EMS data sheet). Both adhesives were isotropic and silver filled and they had to be cured by
heat. Their conductivity properties are almost similar, but thermal properties of EMS's adhesive are
better.

For test methods we chose accelerated environment reliability tests and a tensile strength test.
Accelerated environmental tests produce failures in joints by the same damage mechanisms than in
real use but in noticeably shorter time. Therefore, these tests can be utilised to estimate long-term
behavior of the electronics products. The most probable environment hazards causing damages are
temperature and humidity (Yoshinori & Yasuko 1996) and therefore, we decided to include these
parameters to our tests. We performed tests in specific test chambers in our institute and during the
tests temperature and humidity inside the chambers varied.

Ten pieces of one-type connections have been tested to ensure the reliability of the results. We have
tested SnPb and Sn solders and two kinds of isotropic adhesives as joining materials, surface mount
and leading through techniques, and also rubber sealant on some joints to prevent abrasion of the yarn
against hard solder connection. Each channel of the climate chamber measures continually the voltage