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Flex & Rigid-Flex Circuits

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Flex Circuit Layer Generation, Drilling & Plating

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.

Additive Process
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. Tech-Etch can produce fine line circuits with trace and space patterns as small as .001" in production volumes. Actual size of part shown in photo at right is .5" wide.

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

fine holes
Flex and Rigid-Flex circuits may be drilled to create plated through hole interconnections or simple mounting holes. Holes smaller than .008" in a flex circuit substrate will be laser drilled. Tech-Etch has the ability to drill vias as small as .001" diameter using laser drill technology.

For Rigid areas, Tech-Etch employs the use of a state of the art vision drill system which supports registration to within .001”, hole diameters down to .004” (substrate dependent), and blind via drill capability using surface sensing technology.



Plating

Plating
With over 30,000 square feet of our facility dedicated to finishing, Tech-Etch can provide both conventional and exotic surface finishes to flex circuits. Exposed pads will typically need a finish for corrosion resistance or compatibility with the assembly process or application. Electroplating is the most common finishing process. However, it can only be accomplished when all the conductors are connected to a common external bus during fabrication. Tech-Etch offers in-house electroplating of Tin, Tin Lead, Copper, Nickel, Soft Gold, and Hard Gold.

For designs where the traces cannot be bussed into the panel, electroless finishing options such as hot air solder leveling, immersion gold over electroless nickel (ENIG), and immersion tin are available as alternatives. Tech-Etch flexcircuits can be made RoHS compliant by selecting the proper finish. Our product engineers can help ensure the finishselected supports the intended application.
Selective Plating


Selective Plating
By combining our in-house electroplating and precision imaging capabilities, Tech-Etch can provide circuits with selective finish plating as pictured here. This process allows for different finishes plated on separate areas of the part. This is particularly useful when multiple assembly processes will be used (e.g. solder assembly and wire bond).

Plating Tolerances

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%
Nickel .000050 .000200 +/- 40%
Hot Air Level N/A N/A N/A
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