Introduction

Manufacturing a multi-layer flex circuit board starts with a base material of copper clad flexible laminate. This is typically an unreinforced film coated with adhesive on both sides, with the outside surfaces covered with a thin sheet of copper. A lamination process bonds these elements together under heat and pressure.

The Materials

Such copper clad flexible laminates come in large sheets such as 24x36 inches—one of the standard sizes. The laminate material must be of the correct size and thickness, and be free from imperfections in the copper surface, such as pits and dents. After certification of the size and quality, a shearing machine cuts the full-size sheets down to more usable panel sizes, such as and typically 18 x 24 inches. To remove moisture and balance any internal stresses evenly, the panels are baked in ovens and then cooled before use.

The Process

1.    Using the computer-generated drilling information provided, the computerized drilling machines will drill holes in the panel as per specification. At the same time, the machines drill two or more ‘locating’ or ‘registration’ holes on the periphery of the panel to enable lining up the different films for further processing.

2.    The panel now undergoes a preparation process for the application of the conductive pattern. Usually, the chemical process starts with the panel being dipped in an acid bath, followed by the application of an anti-tarnish agent, ‘micro-etching’, or ‘chemical cleaning’. The next step involves creating the conductive pattern on the copper surfaces of the panel according to the circuit image.

3.    Following these steps, the image is transferred using a dry film. This is achieved through thedry film laminator using heated rollers to press a layer of photoresist onto each copper surface.Next, a sheet of film containing the negative image of the desired conductor pattern is placed on this layer of photoresist, and the sandwich is exposed to UV light inside an exposure chamber. UV light passing through the clear areas of the image sets up a chemical reaction in that area of the photoresist layer it touches. The unexposed areas remain relatively soft and a developing process in the followingstep removes this soft and unexposed resist, exposing the unwanted copper.

4.    Next follows an etching process, where a chemical solution removes the unwanted exposed copper from the surface of the panel. The required copper circuitry remains on the panel underneath the photoresist. Another chemical strips the hardened photoresist to expose the remaining copper.

5.    After a thorough inspection of the copper pattern, a very thin film of electroless copper is chemically deposited over both the surfaces of the panel, including inside the holes, also called vias. The metalized vias now connect one side of the flexible circuit to the other. Subsequent copper layers are added to the core over a layer of insulation called prepreg, and bonded to it using heat and pressure.

6.    Depth-controlled drilling then creates the blind and buried vias, followed by the same process of image transfer, etching, and electroless copper deposition on all the added layers.

7.    Finally, all the areas that will not be coated with solder are masked off with a coverlay. The finished flexible circuit board then undergoes an inspection process.

Conclusion

For more information on this particular process or for any advice on if your PCB specification should incorporate a flexible circuit element, please don’t hesitate to contact the team at PCB Global.  Please email you design file to sales@pcbglobal.comfor a rapid and competitive quote.