Method for manufacturing multilayer flexible printed circuit board

The present inventions relates to a method for manufacturing a multilayer FPCB having different number of layers in different areas. The method includes the steps of: providing a binder layer; removing a portion of the binder layer thereby defining an opening in the binder layer; forming a multilayer FPCB which having a first copper clad laminate structure and a second copper clad laminate structure disposed on two opposite sides of the binder layer respectively using the binder layer; cutting the first copper clad laminate structure; cutting the multilayer FPCB in manner that a portion of first copper clad laminate structure that is exposed to the opening is separated from the first copper clad structure thereby obtain a multilayer FPCB having different number of layers in different areas.

BACKGROUND

1. Technical Field

The present invention relates to a method for manufacturing a flexible printed circuit board, and especially to a method for manufacturing a multilayer flexible printed circuit board having different numbers of layers in different areas.

2. Discussion of Related Art

Flexible printed circuit boards (FPCB) have been widely used in electronic products such as mobile phones, printing heads and hard disks. In these electronic products, some movable parts are advantageously connected to a main body via a flexible printed circuit board. FPCB can assure power supply and signal transmission in such environment due to their excellent flexibility.

FIG. 5Fshows a multilayer FPCB structure, which has different number of layers in different areas; in other words, there are a thick area with a number of layers and a thin area with less layers in a same FPCB. The thick area can have a higher circuit density whilst the thin area exhibits higher flexibility.

FIGS. 5A to 5Fshow a conventional process for manufacturing such type of FPCB. As is shown inFIGS. 5A and 5B, a first copper clad laminate (CCL)41, a binder layer45, a second CCL42are laminated sequentially. As is shownFIG. 5C, dry films412,422are applied on the first CCL41and the second CCL42respectively, and then the dry films412,422are exposed and developed. Because there is a cliff-like thickness difference between the first CCL41and the second CCL42, a gap46is formed at the “cliff”.

As is shown inFIG. 5D, the first CCL41and the second CCL42are etched using an etchant and the dry films412,422are removed, the etching step, the etchant can seep into the gap46and react with dielectric layers in the first CCL41and the second CCL42. As a result the dielectric layers may peel off from the CCL.

Referring toFIG. 5E, a third CCL43and a fourth CCL44are laminated with the first CCL41and the second CCL42respectively. Referring toFIG. 5F, a through hole47is formed. The through hole47can be made by drilling or by laser ablation. After the through hole47is formed, a conductive layer is formed on a surface of the through hole47by electroless plating or electroplating. In the plating process the dielectric layer of the second CCL42is exposed to a plating solution thereby forming a number of copper lumps thereon. These copper lumps can pierce dry film that applied on the second CCL42in the next pattern-forming process, and etchant used for developing the dry film can react with dielectric layer or copper layer of second CCL42and cause poor quality product to be formed.

In the aforementioned process for manufacturing multilayer FPCB that has different number of layers in different areas, a fall structure between different CCLs can causes a series of quality problems, therefore there is a desire to develop a process that can solve aforementioned quality problems.

SUMMARY OF THEN INVENTION

This and other features and advantages of the present invention as well as the preferred embodiments thereof and a method for manufacturing a multilayer flexible printed circuit board having different number of layers in different areas in accordance with the invention will become apparent from the following detailed description and the descriptions of the drawings.

In one embodiment, a method for manufacturing a multilayer FPCB includes the following steps. Firstly, a binder layer is provided and a portion of the binder layer is removed thereby defining an opening in the binder layer. Secondly, a first copper clad laminate structure and a second copper clad laminate structure are attached on opposite sides of the binder layer, thus a multilayer FPCB is formed. Each of the first copper clad laminate structure and the second copper clad laminate structure includes at least one dielectric layer and at least one conductive layer formed thereon. The first copper clad laminate structure should have an excess portion, the excess portion being located above the opening of the binder layer. Thirdly, a first slit is made in the first copper clad laminate structure along a boundary of the excess portion. The first slit is in communication with the opening of the binder layer. Finally, the excess portion of the first copper clad laminate is removed along the boundary of the excess portion of the first copper clad laminate, thus a portion of the second copper clad laminate is exposed to an exterior through the opening of the binder layer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring toFIG. 1, a method for manufacturing a multilayer FPCB haying a different number of layers in different areas includes the steps in no particular order of providing a binder layer; removing a portion of the binder layer thereby defining an opening in the binder layer; attaching a first copper clad laminate structure and a second copper clad laminate structure on opposite sides of the binder layer, the first copper clad laminate structure having an excess portion; defining a first slit in the first copper clad laminate structure; and removing the excess portion of the first copper clad laminate structure, thus obtaining a multilayer FPCB having a different number of layers in different areas.

The method will be described in detail with the following preferred embodiments:

FIGS. 2A to 2Lshow a process for manufacturing a FPCB that has different numbers of layers in different areas in accordance with a first preferred embodiment.

Referring toFIG. 2A, a binder layer15is provided and a portion of the binder layer15is removed thereby forming an opening152in the binder layer15. The opening152can be formed by cutting, stamping, laser ablation or etching. The binder layer15has a side-wall154exposed in the opening152. In this preferred embodiment, the opening152has a rectangular shape. Alternatively, the opening152can be other shapes, for example, trapezium, triangle etc.

Referring toFIG. 2B, a first copper clad laminate11and a second copper clad laminate12are provided. The first copper clad laminate11includes a dielectric layer111and a conductive layer112formed on the dielectric layer111. The second copper clad laminate12includes a dielectric layer121and two conductive layers122and124formed on two opposite sides of the dielectric layer121respectively. The dielectric layers111,121and the binder layer15can be made of one material selected from the group consisting of polyimide, polytetrafluroethylene, polythiamine, polycarbonate, polycarbonate ester, polyester, and copolymer of imide, ethylene and dimethyl terephthalate. The conductive layers112,122,124can be made of a conductive material, such as copper, silver or aluminum.

Referring toFIG. 2C, the first copper clad laminate11, the binder layer15and the second copper clad laminate12are laminated. In detail, the binder layer15is laminated so as to be sandwiched between the dielectric layer111of the first copper clad laminate11and the conductive layer124of the second copper clad laminate12. A portion of the conductive layer124is exposed in the opening152. The conductive layer112of the first copper clad laminate11and the conductive layer122of the second copper clad laminate12are exposed to the exterior. Advantageously, a laminating machine is used in this step.

Referring toFIG. 2D, conductive patterns are formed in the first copper clad laminate11and the second copper clad laminate. That is, the conductive layer112of the first copper clad laminate11and the conductive layer122of the second copper clad laminate12both have conductive patterns formed therein. In this step, a sub-slit114is formed at a predetermined position in the conductive layer112of the first copper clad laminate11. Specifically, the sub-slit114is corresponding to and aligned with the side-wall154of the opening152.

Referring toFIG. 2E, a third copper clad laminate13and a fourth copper clad laminate14are laminated on the first copper clad laminate11and the second copper clad laminate12, respectively. The third copper clad laminate13includes a dielectric, layer131and a conductive layer132formed on the dielectric layer131. The fourth copper clad laminate14has similar structure to the third copper clad laminate13. The fourth copper clad laminate14includes a dielectric layer141and a conductive layer142formed on the dielectric layer141. The third copper clad laminate13and the first copper clad laminate11are bound via a binder layer16. In detail, the binder layer16is sandwiched between the dielectric layer131of the third copper clad laminate13and the conductive layer112of the first copper clad laminate11. The second copper clad laminate12and the fourth copper clad laminate14are bound via a binder layer17. In particular, the binder layer17is sandwiched between the dielectric layer141of the fourth copper clad laminate14and the conductive layer122of the second copper clad laminate12. That is, the conductive layer132of the third copper clad laminate13and the conductive layer142of the third copper clad laminate14are exposed to the exterior.

Referring toFIG. 2F, a through hole18penetrating through the first, second, third and fourth copper clad laminates11,12,13and14is formed and a conductive layer182is coated on an inner side-wall of the through hole18for providing conduction between the conducive layers112,122,124,132, and142in the first, second, third and fourth copper clad laminates11,12,13and14.

Referring toFIG. 2G, conductive patterns are formed in the conductive layer132of the third copper clad laminate13and in the conductive layer142of the fourth copper clad laminate14. In this step, a sub-slit134is formed at a predetermined position in the conductive layer132of the third copper clad laminate13. Specifically, the sub-slit134is corresponding to and aligned with the side-wall154of the opening152. In other words, the sub-slits114,134are aligned with each other. After this step, a four layer flexible printed circuit board substrate19is formed.

The first copper clad laminate11, the binder layer16, and the third copper clad laminate13constitute a first copper clad laminate structure110. The second copper clad laminate12, the binder layer17, and the fourth copper clad laminate14constitute a second copper clad laminate structure120. The conductive layers132,142are at the outmost side of the FPCB substrate19. The first copper clad laminate structure110includes a first end1101and a second end1102. An interface1103between the first end1101and the second end1102is corresponding to and aligned with the side-wall154. The sub-slits114,134are all formed close to the interface1103. Specifically, a portion of the interface1103is exposed in the sub-slits114,134.

Referring toFIG. 2H, a laser beam130is applied to the first copper clad laminate structure110and aligned with the sub-slits114,134to cut the dielectric layer131of the third copper clad laminate13, the binder layer16, and the dielectric layer111of the first copper clad laminate11. The laser beam130can be produced by a carbon dioxide laser. When the laser beam130reaches the conductive layer124, the conductive layer124prevents the laser beam130from cutting deeper. In other words, the conductive layer124acts as a protective metallic block, which prevents the laser beam130from cutting any deeper. Referring toFIGS. 2I and 2J, a slit191penetrating through the first copper clad laminate structure110is formed after the laser beam-cutting step. An area192defines a shape of a FPCB. In the illustrated embodiment, the area192is rectangular, and has a first side193and a second side194opposite to the first side193. The slit191extends from the first side193to the second side194. The interface1103is exposed to the slit191.

Referring toFIG. 2K, the flexible printed circuit board substrate19is cut along the boundary of the area192into a shape. Stamping is a preferred process for this step. A portion or the boundary of the area192coincides with that of the opening152. The flexible printed circuit board substrate19is cut so that a portion198of the first copper clad laminate structure110, which is exposed to the opening152and the slit191, is separated from the first copper clad laminate structure110. That is, the portion198of the first copper clad laminate structure110is defined by the opening152, the slit191and a portion of the cutting route of this step. In other words, the slit191is defined along a portion of the boundary of the portion198, and the portion of the cutting route of this step coincides with the other portion of the boundary of the portion198.

Referring toFIG. 2L, the portion198is removed and a multilayer flexible printed circuit hoard100having different number of layers in different areas is obtained. A portion of the second copper dad laminate structure120is exposed to an exterior through the opening152of the binder layer15.

In the preferred embodiment, no stepped structure between the first copper clad laminate structure110and the second copper clad laminate structure120of the multilayer flexible printed circuit board100is formed, therefore the aforementioned problems in conventional methods can be avoided.

FIGS. 3A to 3Brelate to a second preferred embodiment of a method for manufacturing a multilayer flexible printed circuit board having different number of layers in different areas. Referring toFIG. 3A, similar to the first embodiment, a first copper clad laminate21, a binder layer25, and a second copper clad laminate22are provided. The first copper clad laminate21has a dielectric layer211and a conductive layer212formed on the dielectric layer211. The binder layer25has an opening252and a side-wall254exposed in the opening252. The second copper clad laminate22includes a dielectric layer221and a conductive layer222formed on the dielectric layer221. However, unlike the first embodiment, a protective coating224is formed on the dielectric layer221of the second copper clad laminate22. Specifically, the dielectric layer221is sandwiched between the conductive layer222and the protective coating224. The protective coating224can be a film of metal, such as copper, aluminum etc. The protective coating224can be made by sputtering, plating or laminating preformed protective coating224on the dielectric layer221. The protective coating224corresponds to the side-wall254of the opening252of the binder layer25.

Referring toFIG. 3B, when a multilayer FPCB substrate29is formed as steps similar to that described in the first embodiment, a first copper clad laminate structure210is cut by a laser beam230, and then a slit291penetrating through the first copper clad laminate structure210is formed therein. The slit291is defined along the side-wall254, and the protective coating224is below the slit291. Portion of the protective coating224is received in the opening252, and the other portion of the protective coating224is in contact with the binder layer25. That is when the first copper clad laminate structure210is cut, the laser beam230corresponds to the protective coating224. When the laser beam230reaches the protective coating224, the protective coating224prevents the laser beam230from cutting further deeper, so in other words, the protective coating224acts as a protective metallic block.

A method for manufacturing a multilayer flexible printed circuit board in accordance with a third embodiment is similar to that of the second embodiment, except that there are no sub-slit formed in the conductive layers of the first copper clad laminate structure.

Referring toFIGS. 4A to 4D, after forming a multilayer FPCB substrate39, a first laser beam381and a second laser beam382are alternatively used for cutting the conductive layers and the dielectric layers in the first copper clad laminate structure310. The first laser beam381can be an Nd:YAG laser and is used to cut the conductive layers of the first copper clad laminate structure310. The second laser beam382can be a carbon dioxide laser and is used to cut the dielectric layers and a binder layer of the first copper clad laminate structure310.

In this preferred embodiment, no sub-slit needs to be formed on each conductive layer of the first copper clad laminate structure310, so therefore the process is more simple.