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Tech-Etch Inc.

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Flexible Circuit Design and Manufacturing

Minimums and Tolerances


Positional or pitch tolerance is heavily dependant upon flex material properties and circuit construction. Designs with ground planes and stabilizing Cu layers can result in lead pitch tolerance of +/- .0015" per inch (4 sigma process).

Description
Standard
Advanced
Via size
.015"
.002"
Minimum Lead Width
.004"-.008"
.0015"
Minimum Lead Spacing
.004"-.008"
.002"
Cu-Cu layer Registration(2 layers)
.002"
.0005"
Cu Layer Count
1-2
5-6
Pitch Tolerance (windowed leads)
  • Coverlayed
+/- .002"/in.
+/- .0005"/in.
  • Non-Coverlayed
+/- .0015"/in.
+/- .0005"/in.
Die-cut Coverlay Opening to Copper
+/-.015"
+/-.005"
Etched Coverlay Opening to Copper
+/-.010"
+/- .005"
Laser Coverlay Opening to Copper
+/-.002"
+/-.001"
PTH Capture Pad Size (buried)
Via Dia +.020"
Via Dia +.004"
Edge Seal
.020"
.005"
Outline Excise
  • Edge to Edge
+/-.010"
+/-.002"
  • Feature to Edge
+/-.015"
+/-.003"


Construction

Adhesiveless - Adhesiveless materials fall into 2 general categories cast dielectric on copper and sputtered copper on dielectric film. These materials have gained popularity for several reasons. The absence of an adhesive layer results in a thinner, more flexible construction and in some cases the ability to manufacture flex using different techniques. Adhesiveless materials generally have superior electrical properties but that advantage is negated if an adhesive based coverlayer is applied.

A. Cast Dielectric The manufacture of cast dielectric materials starts with .5, 1 or 2 oz. (rolled annealed or electro-deposited) copper and a thin layer of polyimide is cast onto the surface. It is available with copper on one or both sides of .001 or .002" polyimide. Fabricating - Printed Circuit Boards
B. Sputtered Cu Sputtered materials start with conventional .001 or .002" polyimide film. Angstrom thick copper is sputtered over a Ni-chrome or monel barrier layer. Copper is then plated to a thickness of 200 microinches. Sputtered adhesiveless material has a unique advantage in that it can be used in the manufacture of semi-additive circuits. Fabricating - Printed Circuit Boards

Adhesive Based - Adhesive based materials fall into two main categories, acrylic and epoxy.

A. Cover Layers Polyimide with acrylic or epoxy adhesive are the most common choices for coverlayer material. Polyimide thickness ranges from .0005" to .005" are available with .001 or .002" thickness as typical. Adhesive thickness is a function of circuit material thickness and is usually left to the discretion of the manufacturer. Photoimagable coverlayers (PIC) can be used in applications requiring high density access to circuit layer.
Fabricating - Printed Circuit Boards
B. Stiffeners Areas of a flex circuit can be stiffened for component attach, strain relief or mounting surfaces. Polyimide, Cirlex or FR-4 can be used to produce a range of flexibility or stiffness. Standard flex circuit adhesives or pressure sensitive adhesives can be used to provide varying performance alternatives. Fabricating - Printed Circuit Boards
C. Alternate Materials
Exotic and thick metal Tech-Etchs photo-etching experience gives us the ability to manufacture flex using a variety of conductor materials, including copper alloys, nickel, kovar, steel, and resistance alloys. Consult with engineering if you have specific requirements.
Fabricating - Printed Circuit Boards

Circuit Layer Generation

Circuit layer conductors can be created using an additive or subtractive manufacturing process. The choice between additive or subtractive process is generally left to the discretion of the manufacturer.

A. Additive The creation of additive circuitry starts with very thin (5 micron or thinner) copper layers on one or both sides of polyimide film. Resist is first coated on the surface of the copper, and then it is imaged and developed. What remains are channels in the resist in which copper will be plated. This plated copper becomes the traces, pads and other conductive regions of the circuit. Next the resist is removed and the background 5 micron copper is etched away to isolate the newly created copper features. There are more processing steps in creating additive circuitry than subtractive circuitry, making it more costly. However, additive processing provides resolution of .003" wide (or smaller) leads and a rectangular lead cross section.

B. Subtractive Subtractive circuitry starts with copper bonded to one or both sides of polyimide film. The copper thickness is the desired final thickness. Resist is applied to the copper surface and then the resist is exposed and developed. What remains are channels or openings in the resist, through which the copper can be attacked using an etchant. After etching, the resist is removed and the finished circuitry remains.

Drilling

Flexible circuits may be pre-drilled to create plated through hole interconnections or simple mounting holes. Holes smaller than .008" will be laser drilled.

Outlining

Individual parts can be de-panelized using several different methods.

Steel rule dies are the most common and economical means of de-panelizing flex. High quality plywood is laser cut in the pattern to be outlined. Steel blades and spring-loaded datum pins are inserted into the laser cut path. The flex panel is registered to the blades using the datum pins and a flat platen is brought into contact with the blades, cutting the outline. Advantages of steel rule dies include low initial cost, fast delivery and the ability to excise many parts in one strike.

Class A hardtools are used for tight tolerance requirements. They typically consist of a punch and die mounted in a precision die set. Dies can be built to cut single or multiple parts. Advantages of hardtools include tight tolerances, process consistency and long life.

Polyimide etch Polyimide (and adhesive) etch processes are commonly used to create access to conductors and create part outlines. Similar to metal etching, resist is imaged and exposed to create an etching mask and the polyimide is chemically attacked. When defining the outline, small tie tabs are left behind to suspend the part in the panel. These tie-tabs are usually cut by hand during final inspection of the product. Advantages of using polyimide etch to outline parts include cost efficient sheet processing, quick lead-time and intricate outlining at no additional cost.

CO2 or UV-YAG lasers can be used to outline flex circuits. Advantages include minimal tooling, short lead-time and intricate outlining.

Finish Min Thickness (in.) Max Thickness (in.) Tolerance
Hard Gold .000010 .000200 Min.
Soft Gold .000010 .000200 Min.
Tin .000050 .000200 +/- 25%
Tin-lead .000050 .000500 +/- 25%
Cu-Tin - - -
Nickel .000050 .000200 +/- 40%
Hot Air Level N/A N/A N/A


Flexible Circuit Glossary

Annular Ring - The ring of exposed solder or copper around a through hole.

Buried Via - A plated through hole buried within internal layers of a circuit. There is no direct access to the via.

Blind Via - A plated interconnection from one layer to an adjacent layer through a fixed depth LASER drilled opening.

Cantilevered leads - Unsupported conductors extending from an edge of a flex circuit.

Coverlayer - Insulating layer usually bonded with adhesive.

LCP - Liquid Crystalline Polymer, a relatively new dielectric substrate used in the manufacture of flex circuits.

PTH - Plated through hole. Used as a means of creating an electrical connection from one circuit layer to another.

SMT - Surface Mount Technology

Steel Rule Die - A tool used to cut flex circuits (and other materials) from a panel.

Stiffener - A rigid or semi-rigid material that is bonded to a flex to facilitate component attachment. Typically made of polyimide or epoxy glass.

Windowed leads - Conductors that are unsupported by insulation. Typically running across a window, the pitch can be quite tight allowing high density mass termination.

ZIF (Zero Insertion Force) termination - A style of termination that allows a flex circuit tail or tab to be inserted into a circuit board mounted connector. After insertion a mechanical actuator locks the flex in place.

This technical information has been contributed by
Tech-Etch Inc.

Click here to find suppliers

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