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38 The PCB Magazine • June 2014 BASIC PRINCIPLES oF PoLyMER THICK FILM FLExIBLE CIRCUITS continues In general, resistors can be printed to within ±20% of desired value without trimming, however if tighter control of the tolerance of resistor value is required, laser or mechanical trimming of the resistor can be employed. " " liohms/square/25 µm of ink thickness, how- ever, patent literature reports values of 0.100 ohm/square/25 µm for copper particles plated with silver. While conductive ink prices may run high owing to the price of silver, the overall processing cost should reduce the over- all cost of production. That aside, care should be used when at- tempting carry higher currents with metal filled polymers. Hot spots within the conductor/ polymer matrix can cause rapid degradation of the conductor and possible failure. Also in general, the operating temper- ature of the circuit should not exceed 90°C and the base ma- terial choice may suppress that limit even further depending on what material is used. Screen printed resistors are familiar and fundamental de- vices commonly found incor- porated into many PTF circuit designs. When planning to use them in a design, the resistors should be kept to a minimum of one or two bulk resistive values to facilitate processing. In general, resistors can be printed to within ±20% of desired value without trimming, however if tighter control of the tolerance of resistor value is required, laser or mechanical trimming of the resistor can be employed. It is also possible for the producer to mix resistive and conductive inks to tune the resistor value if desired, but this takes time and care to assure consistency. When considering terminations, the design rules for circuit pads or lands for PTF circuits are similar to those used for rigid printed wir- ing boards, however, the termination features should be discussed with the manufacturer. Also while polymer thick film inks are not directly solderable, conductive adhesives can be suc- cessfully used to surface mount components. In such cases the component land design for surface mounting is basically the same as it is for traditional PCBs. Device attachment must be accurate, however, because the components will not center themselves in the same way that soldered components do when the solder melts during reflow. As is the case with other more standard type flexible circuits, PTF flex circuits such as those used for membrane switches often have need for edge contacts which allow them to be plugged into connectors or mated to another surface. The finish of choice for PTF is commonly carbon. The thin graphite coating is both low friction and sufficiently con- ductive to carry power and signals at the higher voltages that are typically used where PTF technology can serve and it works well with the ZIF-type connectors that are common- ly used with other types of flexible circuits. Summary Polymer thick film tech- nology is a very useful solu- tion for the manufacture of flexible circuit designs which can take advantage of its ben- efits while tolerating its more limited abilities. It facilitates the production of highly cost-effective as- semblies using simple processing methods and low cost materials. While PTF may not be suit- able for every application, it should not be over- looked or automatically ruled out without hav- ing given it at least a moment of consideration whenever cost consideration is a high-level concern. PCB joseph (joe) Fjelstad is the founder and president of Ver- dant electronics and a four-de- cade veteran of the electronics industry, as well as an interna- tional authority and innovator in the field of electronic inter- connection and packaging technologies. Fjel- stad has more than 250 u.s. and international patents issued or pending and is the author of Flexible circuit technology.