Internal conductor

Figure 12. Thick-film multilayer multichip micropackage for the Honeywell DPS 88 mainframe computer. Top, completed package populated with chips bottom, internal conductor layer with signal lines.

The contact problems are mitigated in the hybrid ion sensor by making the internal conductor shorter and shorter until it is more natural to talk about its thickness rather than its length. The material of this internal contact has not changed during this transition and neither has the electrochemistry at the interface. Thus, the only difference between the coated wire and the hybrid sensor is the length (or thickness) of the contact. We therefore skip it and go directly to the solid-state ISFET, in which the thickness of the internal contact is zero. In other words, the ion-selective membrane is placed directly at the input dielectric of the field-effect transistor (Fig. 6.20). 

If, for any reason, there was a breakdown of insulation in a part of an electric circuit or in any apparatus such as, say, a hand-held metal-cased electric drill, it is conceivable that current would flow external to the closed circuit of the conductor of the drill, if a path were available. For example, the metalwork of the drill may be in contact with a live internal conductor at the point of insulation breakdown (an example of indirect contact). Or, take the example of someone working at a switch or socket outlet from which the cover had been removed before the electricity supply had been isolated. In such cases the person concerned could touch live metal or a live terminal and, if the conditions were right, would thereby cause an electric current to flow through his body to earth (an example of direct contact). If the total resistance of the earth fault path were of a sufficiently low value, the current could kill or maim. 

Charts have been in existence for many years. They have been adequate for most applications, although they have their limitations. The external and internal conductor-sizing charts in IPC 2221, Generic Standard on Printed Board Design, are the standard for sizing traces and are included as Figs. 16.1 and 16.2, respectively. 

The internal conductor chart is conservative for traces with cross-sectional area up to 700 sq. mil. It becomes less conservative at high current levels or with larger cross-sectional areas. The internal conductor-sizing chart represents half the current from the external conductor chart. It does not represent internal conductor test data. The internal trace chart has significant margin and should be used for sizing both internal and external conductors when possible. When that is not possible, a more detailed approach can be taken.The first step is to use new charts that focus on the baseline set of charts. 

The internal conductor chart is best for sizing both internal and external traces that must operate in space or vacuum environments. 

One-oz. Internal Conductor Current Carrying Capacity in Air for Polyimide PCB Material... 

FIGURE 16.5 Baseline (Jouppi) 1 oz. internal conductor chart. [The baseline (Jouppi) charts represent the author s personal data collection.]... 

Several key points can be observed when looking at this chart and comparing against the IPC-2221 internal conductor-sizing chart. 

The predicted temperature rise of the same trace using the internal conductor-sizing chart in IPC-2221 is 266°C. 

The purpose of through-hole drilling printed circuit boards is twofold (1) to produce an opening through the board that will permit a subsequent process to form an electrical connection between top, bottom, and internal conductor pathways, and (2) to permit through-the-board component mounting with structural integrity and precision of location. 

Internal conductor attributes including over- and under-etch, conductor cracks and voids, and foil thickness... 

Etchback. Etchback is a process for the controlled, positive removal of non-metaUic materials from side walls of holes to a specified depth. It is used to remove resin smear and then to remove the reinforcement material laterally to expose additional internal condnctor snrfaces.The degree of etchback is critical to function. Too much etchback creates excessively rough hole side walls and causes weak FTH structures. Etchback requirements range between 0.005-0.08 mm. The degree of etchback is measured by evaluating vertical PTH cross sections of multilayer PCBs and is determined effective if at least one of the top or bottom surfaces of the internal conductor is attached to the plated copper on both visible sides of every conductor layer. (See Fig. 51.23 for an example of acceptable etchback for all classes.)... 

Internal Conductor Imperfections in the Microsection. In theory, circuits should be rectangular. Circuits are usually designed with this rectangular shape in mind. In practice, circuits are trapezoidal. Depending on how the circuit is formed, this shape could be close to... 

FIGURE 52.34 Crazing in base laminate. Crazing exceeds 50 percent of the physical space between internal conductors. (IPC)... 

Terminals and Internal Conductors. Indications of arcing damage and/or overheating, such as discoloration and melting of insulation, require the replacement of damaged parts. 

Internal interlayer delamination is where delamination occurs at the interface between the internal conductor and the ceramic, although the cracks do not reach to the outside of the substrate. The cause of the problem is an area of poor adherence between the green sheet and conductive paste in the laminated body. When there are many layers, or when the conductor on each layer is thick, there is a great difference in the thickness of the parts including the conductor and the parts with ceramic only, so that the laminated body is like a sandwich with a lot of filling (refer to Figure 7-17). Since this delamination occurs in order to release the stress within the laminated body, it takes a similar form to stepped interlayer delamination and circular delamination. 

Damage to the power cable or lead which exposes the colours of the internal conductors, which are brown, blue and green with a yellow stripe. 

Poor cable retention. The outer sheath of the power cable must be secured and enter the plug at one end and the equipment at the other. The coloured internal conductors must not be visible at either end. 

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