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Materials
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A typical flexible circuit consists of a layer of copper sandwiched between layers of polyimide or photoimageable coverlay.
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COPPER
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Copper is the most common metal found in flexible circuits and it comes in two forms: rolled annealled or electrodeposited.
Rolled annealled is the more common product used in North America and is the material of choice in dynamic applications. If a circuit is to be used in a dynamic setting the grain direction of the finished circuit must be specified. Electrodeposited copper is less expensive than rolled annealled. Copper thicknesses are always specified in weight per square foot. For example one square foot of copper which is .0014 or 35 microns thick weighs one ounce. The table below shows common copper thicknesses. Coppers between 7 and 14 ounces are available. Once a flexible circuit becomes thicker than 14 ounces a customer should begin to talk to a buss bar manufacturer.
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COPPER THICKNESS
(1000's of an Inch)
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COPPER THICKNESS
(Microns)
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WEIGHT
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.00035
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9
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¼ oz.
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.0007
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18
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½ oz
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.0014
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35
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1 oz.
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.0028
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70
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2 oz.
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.0042
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105
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3 oz.
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.0056
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140
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4 oz.
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.0070
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175
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5 oz.
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.0098
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245
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7 oz.
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.0196
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490
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14 oz.
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Other metals can also be used to manufacture flexible circuits including Beryllium Copper and Nickel alloys.
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POLYIMIDE
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There are 14 different types of covercoat materials listed in IPC-231/C but polyimide dominates the market. There are various polyimide
manufacturers in North America, DuPont's Kapton is the most common (A registered trademark of DuPont). In order for a covercoat to be used it must be coated with an adhesive. IPC-232/C lists some 21
types of glues or adhesives but the most commonly used are acrylics, epoxies, and phenolic-butyral.
Each has its own strengths and weaknesses. Acrylics are used for most military applications and flex to install. Phenolic-butyrals dominate the dynamic flex applications and epoxies are generally used in reel-to-reel applications. PFC Flexible Circuits Limited used primarily Kapton/Acrylic systems from DuPont which are known as the Pyralux group of products.
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CLADS
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Clads consist of copper bonded to Kapton with or without an adhesive. Typical constructions include:
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TYPE
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SINGLE SIDED WITH ADHESIVE
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SINGLE SIDED CAST ON ADHESIVELESS
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DOUBLE SIDED WITH ADHESIVE
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DOUBLE SIDED ADHESIVELESS
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ORDER
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COPPER
ADHESIVE
KAPTON
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COPPER
KAPTON
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COPPER
ADHESIVE
KAPTON
ADHESIVE
COPPER
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COPPER
KAPTON
COPPER
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NOTE
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MOST COST EFFECTIVE
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PRODUCT OFFERINGS ARE STILL LIMITED
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MOST COST EFFECTIVE - TRY NOT TO USE IN RIGID-FLEX
CONSTRUCTIONS
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MANY TYPES: CHROME SEED, NICKEL SEED, CAST, AP
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PHOTOIMAGEABLE COVERLAYS
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Photoimageable coverlays are basically flexible solder masks. They come in two forms.
DuPont offers a dry film soldermask which has excellent properties, similar to Kapton, and can be used in many applications but is not applicable for dynamic builds. Coates, Rogers and Peters as well as many other companies offer screen floodable soldermasks or spray on types. The advantages of photoimageable soldermasks include: 1) lower cost than a Kapton covercoat and, 2) the ability to image surface mount openings or fine features on the solder side of the circuit unlike Kapton whose opening shapes are limited by drilling parameters or the use of expensive die systems.
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© 2001 PFC All Rights Reserved Tel:+1-416-750-8433 Email:pfc@baxter.net
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