Resistor trimming

Resistor trimming with laser cutting (continuously adjusting resistor values). 

Resistor Trimming Comparison ofThick Film and Thin Film Technologies The Hybrid Assembly Process... 

Resistors buried in LTCC are out of direct access. In general, the following three strategies of embedded resistor trimming are available ([39] Figure 9.37) ... 

Energy of high- Buried resistor trimming High impact on layout. 

Tobita, T, Takasago, H., and Kariya, K., Investigation of conduction mechanism in thick film resistors trimmed by pulse voltage method, IEEE Transactions on CHMT, Vol. 15, No. 4, August 1992. 

The issue of output aeeuraey from unit to unit usually requires that the referenee used be trimmed to two pereent variation or less, and the resistors within the resistor voltage divider be one pereent. The aeeuraey of the outputs then are these toleranees added together plus any inaeeuraeies within the transformer turns. 

It would be helpful to add an ignition switch to the coil - avoids case heating from ON all the time since no series resistor is used, also allows you to handle the card to trim the pot without getting zapped, BTDT. 

A final alternative for metal patterning is the use of lasers to selectively and accurately etch small metal areas. Lasers are routinely used to trim thin-film resistors (126) they may also be used to repair defects or etch conductor lines by direct ablation in an inert atmosphere (127,129) or by laser-initiated etching in a reactive gas or liquid (128). 

Figure 5.2.1 shows a photograph of an integrated surface-micromachined accelerometer, and Figure 5.2.2 is a close-up of the surface-micromachined polysilicon MEMS structure near the center of Figure 5.2.1. The structural material is 3 pm thick polysilicon and the IC process is bipolar and CMOS (BiCMOS) with thin film resistors. This structure thus combines bipolar transistors, CMOS, precision laser-trimmed resistors, and mechanical polysilicon [4],...

Analog trimming concepts, in which the offset is measured online while resistors are adjusted by laser trimming, are widely used. This type of trimming has to be done before deposition of the passivation layer, to ensure that the laser cut will be covered by the passivation layer. The laser has to be chosen and adjusted carefully to avoid damage to the insulation layer, which could reduce its long-term stability. 

The gain of the non-inverting final amplifier (Fig. 6.2.9) is adjusted with an trim-mable resistor network in the feedback loop. To detect short circuits between the output and ground or supply voltage two active output limiters are included in the final stage. 

A trimmable resistor network in the feedback loop of the operational amplifier OP2 is aligned during the calibration process. The total resistance of the resistor network is increased by opening fusible links (Fig. 6.2.11). A test calibration is not possible. At the end of the hole trimming a special link is fused to deactivate the complete control logic for safety reasons. Consequently, accidental trimming is prevented. 

Calibration of heater temperature by laser trimming of resistor Rv... 

The calibration of the characteristic curve is done by a current divider with a trimming resistor in the sensor connector, for example. This sensor type has been in mass production since the mid 1990s, with production volumes of millions. 

The thick-film design consists of four layers, to be separately screen printed and fired on a 1 in square alumina substrate (figure 14.9). Commercial formulations were used for electrodes, bridge trimming resistors, and passivation layers. The first attempted sensor layer was a commercial silver/palladium paste modified by the addition of palladium powder. Based on the performance of the first thick-film sensors, DuPont Electronics (Research Triangle Park, NC) specifically formulated a palladium-based thick-film paste for this application. 

Resistor patterns are designed so that the average value of the fired resistor is lower than the required value. The final resistor is then trimmed using a laser, or an abrasive jet, to remove material and increase the resistance to within 1% of the desired value. 

The distribution of the conductive particles and the contact between them also determines the resistance of the deposited film. Final resistance values are obtained either by sand blasting, where the thickness of the film is reduced, or by laser trimming, to increase the effective length of the resistor. These procedures would be performed after the resistor has been fired, but before the application of the protective glaze coating. 

Other laser-materials interactions applied in the electronics industry involve the unique ability of laser light to be focused to very small areas and to heat only the areas irradiated. This allows lasers to serve a wide range of resistor and capacitor trimming applications. Also, when properly focused, lasers are able to remove unwanted bridges in microcircuits. If the atmosphere above the substrate is selected to form desired deposits when the substrate or the gas is heated, then lasers can be used to personalize microcircuit masks and to repair damaged circuits. Work with ultraviolet lasers may make it possible... 

For most SMD resistors the substrate is a rectangular piece of ceramic, usually alumina. At the upper side conductors are deposited along two edges. The resistive film is deposited between and partly over these electrodes. The resistance value is adjusted by laser trimming. A protective coating is appHed over the resistive film. Contacts are made over the edges these usually extend over the sides of the ceramic and even some distance over the lower side to improve the soldering quality. 

Variable film resistors are made with carbon composition films, usually on a paper laminate substrate, or with metal glaze films on a ceramic substrate. It is complicated to trim these resistive elements. Therefore, the tolerance on the resistance of film variable resistors is usually high, in the order of 20%. For most applications, however, this is no problem. 

Some variable resistors are only used for trimming circuits during manufacture or repair. They maybe replaced by fixed resistors that are trimmed by the customer. Although the components are cheaper, the expensive trimming equipment makes this only attractive in special cases. Other devices are used many times during the operation of the equipment. For the latter types, minimum wear of the resistive element and the wiper is important. 

For thin-film resistors the heat diffusion into the substrate is dominant for aU but extremely short pulses. The effective heat capacity is proportional to the surface area thus, large resistors here ako are at an advantage. Breakdown usually occurs at spots where the current density is high, for instance at the end of trimming groove. Irregularities in the film may compHcate matters even more. Nevertheless, it k possible to make mathematic models of the pulse behavior of such resistors that accurately predict the maximum allowed pulse power under specified conditions. An example of such a calculated curve is given in Fig. 2.23. 

D/A converter ICs often include special features that enable them to be interfaced easily to microprocessors or other systems. Microprocessor control inputs, input latches, buffers, input registers, and compatibility to standard logic families are features that are readily available in D/A ICs. In addition, the ICs usually have laser-trimmed precision resistors to eliminate the need for user trimming to achieve... 

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