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[TABLE OF CONTENTS]
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IV) PCB RESISTOR LAYOUT AND ARTWORK
A) ORIENTATION OF RESISTORS
Sheet resistance varies slightly with different orientation of the resistor element. The resistance tolerance can be improved by orienting all the resistor element axes in the same direction or 45 degree from laminate edges.
B) SELECT A FIXED WIDTH AND VARY LENGTHS
The resistance tolerance depends on the accuracy and repeatability of the etching process which defines the lengths and the widths of resistor elements. It can be improved by choosing a fixed width or minimum numbers of different widths and adjusting the length accordingly to obtain the required ohmic values.
C) THERMAL MECHANICAL ISOLATION
Fig. 11
The primary purpose of isolation is to reduce (thermal stress) the flow of heat from a plated through via or surface mount pad during drilling, soldering process, reflow, hot air leveling, etc.
Secondarily, it also acts as chemical and mechanical isolation which avoid the possibility of chemical contamination attacking resistor element directly such as flux contaminants and mechanical stress created at the junction between copper and resistor element directly e.g. drilling process respectively.
It is recommended to have a minimum copper connection of 10mils length from the edge of the plated through hole to where the resistor element begins.
D) ARTWORK LAYOUT
OHMEGA-PLY® processing consists of two prints and three etches. The two prints are as follows:
- 1st print--COMPOSITE image of conductors and resistors
- 2nd print--RESISTOR DEFINE image of resistor elements,which is commonly used for voltage or ground plane with most of the copper preserved
or
CONDUCTOR PROTECT image of conductor,commonly used for signal plane
Fig. 12
COMPOSITE (negative film) RESISTOR DEFINE
COMPOSITE (negative film) CONDUCTOR PROTECT
The three etches are as follows:
- 1st etch--COPPER removal
- 2nd etch--OHMEGA removal
- 3rd etch--COPPER removal to form resistor element
E) DESIGN ELEMENT TERMINATION AREA AND OVERLAP AREA
Fig. 13
1) ELEMENT TERMINATION AREA--design element length longer (5-10 mils) so that any misregistration or misalignment of artwork can be compensated. In figure above, two different resistor designs in which one is with a termination area and other one is without termination area. The one with a termination area still preserves the element length and shape even though there is misregistration.
Fig. 14
2) OVERLAP AREA--design the overlap area (5 to 10 mils beyond resistor's width) so that any misregistration of artwork in image printing process can be compensated as is shown in fig above.
-Design overlap into artwork of the resistor pattern to avoid misalignment
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