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PROCESSING RECOMMENDATIONS

OHMEGAPLY® RESISTOR-CONDUCTOR MATERIAL

1. Prepare panel according to instructions of photoresist supplier.
   
   
2. Apply photoresist according to photoresist manufacturer's instructions. Any photoresist capable of the necessary resolution may be used.
   
   
3. Use exposure conditions specified by photoresist manufacturer to print the composite pattern artwork (First Print).
   
   
4. Develop per photoresist manufacturer's instructions.
   
   
5. Check developed images for proper size, freedom from pinholes and delineation of edges.
   
   
6. Etch copper using any conventional copper etchant capable of uniform etching with minimal undercut. When the copper is completely removed, the grey resistive layer will be visible. Do not attempt to etch the excess Ohmega resistive layer away with the copper etchant (First Etch).
   
   
7. Etch excess Ohmega resistive material away with a solution containing 250 grams per liter of copper sulfate pentahydrate and 2 milliliters of concentrated sulfuric acid per liter maintained at a temperature of 90+ degrees centigrade. (Second Etch) This should take from three to ten minutes. Certain stainless steel tanks, heaters, or board racks may interfere with the etching of the excess Ohmega because of their galvanic potential on the copper/Ohmega junction. Therefore, polypropylene or CPVC tanks, insulated heaters, and racks are recommended. The photoresist is left on the layer during this step to minimize oxidation or unexpected etching of the copper traces. It is also recommended that this step be done soon after the first etch (step F), while the layer is still wet, in order to prevent any air oxidation/passivation of the exposed resistive material which may increase the required dwell time for etching away the excess grey resistive material.
   
   Another method used to remove the unwanted resistive material is to mechanically scrub the board using a conveyorized brush roller machine. Make sure the resistive material in the fine spaces between the copper lines is removed and the fine copper traces are not damaged by the scrubbing. This step can be done as the board/copper cleaning or roughening step prior to the oxide step or prior to the lamination of photoresist for the printing and developing of the second print (conductor protect) image. Light scrubbing of the layer prior to the copper sulfate etchant will also reduce the etch time required to remove the excess resistive material.
   
   As the bath ages, the etch time increases for type "A" chemically resistive OhmegaPly® (e.g. 100, 250, 1000 ohm per square). If the etch time becomes excessive or if there is any residual resistive material left on the layer that will not etch off, the copper sulfate bath should be changed.
   
   The life of the bath depends on the usage, drag-in, drag-out and any containment build-up. The bath is inexpensive to change and is amenable to waste treatment removal.
   
   
8. Strip the resist in the stripper recommended by the photoresist manufacturer. Do not overstrip. Overstripping can undermine the dielectric supporting the edge of the resistors and thereby affect the resistor value.
   
   
9. Clean and dry panels after stripping operation.
   
   
10. If the OhmegaPly® core is an inner layer (buried resistor application) and the copper used is single treat, then an oxide treatment may be required to prepare the copper surface for bonding in the multilayer process. The oxide treatment can be done at this step of the process or can be done after the final etch and strip, just before multilayer pressing. The advantage of oxide treating at this step of the process is the avoidance of exposure of the resistors to the pre-treatment cleaners and the oxidizing bath itself. These baths may effect the resistive material resulting in an increase in the ohmic value of the resistors. If oxide treatment is done at the end of the process, then any added increase in resistor value should be characterized and compensated for in the original artwork.
    
    The disadvantage of oxide treating after step I) is the necessity of next applying another layer of photoresist over the core for final etching and stripping. The adherence of the photoresist to the oxide may require changes in developing or stripping.
    
    
11. Apply photoresist according to photoresist manufacturer's recommendations.
    
    
12. Print conductor protection pattern. (Second Print)
    
    
13. Develop per photoresist manufacturer's instructions.
    
    
14. Inspect the layer for freedom from pinholes, correct size and delineation of edges.
    
    
15. If the layer was oxided in step J, dip it in Sulfuric Acid or HCL solution (10-20% by weight) to remove the oxide from the copper over the defined resistor area.
    
    
16. Etch away the copper over the resistors using a conveyorized alkaline ammoniacal etching system (Third Etch). This critical step requires tight process controls on the alkaline etching system. The alkaline etchants will begin to slowly remove the Ohmega resistive material (nickel alloy) as soon as the overlying copper is etched away. The offset in resistance value will be determined by the length of the time of exposure to the alkaline etchant. The magnitude of the resistance change can be compensated for by including this offset with the etch factor and changing the photomasks according. Depending on the alkaline etchant used, the following ranges of operating conditions have been used successfully in etching the resistor layer:
    
    Etchant temperature: 120 - 130 F (48-54 C) Etchant pH 7.6 - 8.4 Specific Gravity (Copper Loading) 1.17-1.19 Etch lower (bottom) spray pressure 18 - 22 psi (124 - 151 kPa)
    
    Regardless of the type of alkaline etchant used, the etcher should have a pH controller to maintain the pH within a + 0.1 during the resistor etching. This can be done by installing an automatic ammonia feed pump/pH controller.
    
    Observe the following practices:
    
    
    1. Minimize exposure of the OhmegaPly® resistors to the etchant solution by making sure the end point has been optimized. This means the etcher speed should be adjusted so that final removal of the copper off the resistors occurs just before the circuit board exits the etch chamber.
       
       
    2. If the panel being processed has the OhmegaPly® on one side only, then the resistor side of the laminate should be "face down" as it goes through the conveyorized spray etcher. This will avoid pooling of the etchant on the OhmegaPly® resistors.
       
       
    3. Do not touch or abrade the exposed OhmegaPly® resistors.
       
       
    4. Rinse the panel thoroughly and strip the photoresist off the panel as soon as possible after alkaline etch to prevent the attack of any entrapped etchant on the resistors.
       
       
    5. Some board shops have found that reducing the spray pressure of the etchant (without significantly decreasing the etch rate) often gives them less resistance value variation.
       
       
    6. Before running a large batch of panels, one panel should be run first and its resistors measured to make sure the etching conditions are optimal.
       
       
17. Inspect etched resistors for cleanliness and etched dimension.
    
    
18. Strip photoresist according to photoresist and stripper manufacturers' recommendations. Some resist strippers may attack the resistors; especially those containing ethylene glycol butyl ether. Check the effect of the stripping solution on resistor values by spot checking the values of several resistors before and after the stripping step. If the resistance values are higher (3-10%) after stripping, the exposed resistors are being attacked by the stripping solution. The attack of the stripping solution can be minimized by reducing the stripping temperature or concentration.
    
    
19. Measure resistor values according to test level requirements.
    
    
20. Clean panels using a method that has been determined not to change the resistor values.
    
    
21. Visually inspect resistor elements to insure that the panel does not contain flawed resistors or other layer defects.
    
    It is important to use ohms per square values, not just resistance readings, when measuring the resistor values: i.e., the actual resistors in question should be measured, the number of squares calculated, and the resistance reading divided by the number of squares to obtain the actual ohms per square. A significant increase in the material sheet resistivity (ohms per square reading) from the original nominal sheet resistivity value as supplied by Ohmega Technologies (i.e., 25 ohms per square, 100 ohms per square) may indicate chemical attack of the resistive material. A process characterization study should be implemented to determine which process or chemical bath caused the change in the resistive material value. Undercut at the first copper etch (Step F) can appreciably narrow the resistors while undercut during the second copper etch (Step P) can lengthen the resistors. Both changes in dimension can seriously affect the resistance value by increasing the number of squares of resistance resistive material making up the resistor element.
    
    
22. Buried Resistor Application: The layer is laminated using standard multilayer procedures, being careful to avoid abrasion of the resistive elements during the layup operation. Very high lamination temperatures (above 480F or 249C) and excessive shear pressure during multilayer bonding may cause a slight increase in the resistance values. The use of a "kiss" cycle, wherein the prepreg is heated to slightly above its Tg temperature at 0 - 50 psi, is recommended prior to the application of high lamination pressures. Use light glass cloth prepregs with high resin content (1080 or lighter is preferred) against the resistor layer.
    
    Surface Resistor Application: Protect the copper circuitry leaving the 5 to 10 mils of pad immediately adjacent to the resistor unprotected so that the coating material will cover the resistor entirely as well as part of the pad or trace. If the coating material is applied selectively without masking the circuit, then care must be taken to assure 5 to 10 mils overlap of coating onto the pad or trace edge. Use a solder mask or another conformal coating which by actual test has proven to be equivalent. Coat the resistors in such a way as to achieve a cured coating thickness of 0.5 to 1.5 mils.
    
    The information and data contained herein is based on sources we believe to be reliable, but no warranty thereof is given. Users are cautioned to confirm this data using their own tests. No statements contained herein should be considered as recommendations to use these materials in violation of any patents without a license from the owner of the patents indicate chemical attack of the resistive.