Method for manufacturing printed circuit board with cavity

A method for manufacturing printed circuit board includes steps below. An inner substrate including a first electrically conductive layer is provided. A first electrically conductive pattern is formed in the first electrically conductive layer. The first electrically conductive pattern includes an exposed region and an attaching region. A protective layer is formed on the entire exposed region. A first adhesive layer and a second electrically conductive layer are laminated on a surface of the first electrically conductive pattern in the attaching region and a surface of the protective layer. A slit along a boundary of the exposed region passing through the second electrically conductive pattern and the first adhesive layer is defined. The second electrically conductive layer corresponding to the exposed region, the first adhesive layer corresponding to the exposed region and the protective layer is removed.

BACKGROUND

1. Technical Field

The present disclosure generally relates to printed circuit boards (PCBs), and particularly, relates to a method for manufacturing a printed circuit board with a cavity.

2. Description of Related Art

To accommodate development of miniaturized electronic products with multiple functions, multilayer printed circuit boards are widely used due to their special characteristics such as micromation, lightness, and high-density interconnectiblity.

Many electronic elements need to be packaged on the PCB, in electronic communication with the electrically conductive traces of the PCB. In order to reduce packaging height, a cavity for receiving the electronic elements is formed when manufacturing the PCB. Some of the electrically conductive traces in an inner layer in the cavity are exposed. A method for manufacturing the PCB with cavity generally includes a step of laminating an adhesive sheet and an outer electrically conductive layer on an inner layer substrate having electrically conductive traces. The adhesive sheet is generally pre-preg. The pre-preg includes B-stage resin. When the pre-preg is heated and pressed, the resin in the pre-preg can flow and have an adhesive ability, thus the inner layer substrate and the outer electrically conductive layer are adhered by the adhesive sheet. When the adhesive sheet is hot pressed, the overflow of resin in the adhesive sheet is hard to control. The overflow of resin on the electrically conductive traces is hard to remove, such that the PCB with a cavity has to be discarded.

What is needed, therefore, is a method for manufacturing a printed circuit board with cavity to overcome the above-described problems.

DETAILED DESCRIPTION

Embodiments are now described in detail below and with reference to the drawings.

Referring toFIGS. 1-9, a method for manufacturing a PCB provided in an illustrative embodiment includes the following steps.

Referring toFIG. 1, in the illustrative embodiment, the inner substrate110is a double sided copper clad laminate. The inner substrate110includes a first electrically conductive layer111, a second electrically conductive layer112, and an insulating layer113sandwiched between the first electrically conductive layer111and the second electrically conductive layer112.

Step2, referring toFIGS. 2 and 3, a first electrically conductive pattern1111is formed by removing part of the first electrically conductive layer111, and a second electrically conductive pattern1121is formed by removing part of the second electrically conductive layer112.

The first electrically conductive pattern1111and the second electrically conductive pattern1121may be formed by using a photolithography process and an etching process. The first electrically conductive pattern1111includes a number of electrically conductive traces1112and at least one pad1113.

The first electrically conductive pattern1111includes an exposed region1114and an attaching region1115surrounding the exposed region1114. The exposed region1114is used as a bottom portion of a cavity (i.e. a cavity102) formed in the following steps. The attaching region1115of the first electrically conductive pattern1111will be laminated on an outer layer. Portions of the electrically conductive traces1112and the pad103are arranged in the exposed region1114of the first electrically conductive pattern1111.

Step3, referring toFIG. 4, a solder mask layer114is formed on a surface of the first electrically conductive traces1112and a surface of the insulating layer113in the exposed region1114. At least one opening1131is defined in the solder mask layer114. The at least one pad1113is exposed to the opening1131. The solder mask layer114may be formed by printing and baking liquid solder resist on the first electrically conductive pattern1111and the insulating layer113. The solder mask layer114is configured for covering and protecting the first electrically conductive traces1112in the exposed region1114.

It can be understood, when there are no first electrically conductive traces1112arranged in the exposed region1114, the step of forming the solder mask layer114may be omitted.

Step4, referring toFIG. 5, a protective layer115is formed over the entire exposed region1114.

The protective layer115is adhered to the surface of the solder mask layer114and the surface of pad1113for protecting the pad1113from being corroded by a reagent in a hereafter process. The protective layer115is a removable layer. The protective layer115may be made of peelable thermally curable printing ink or a photo resist ink. The protective layer115should have enough mechanical strength, heat resistance, and be acid and alkali resistant. A softening point of the protective layer115is higher than 200. The protective layer115can bear a pressure force larger than 25 kg/cm2.

In one embodiment, the protective layer115made of peelable thermally curable printing ink can be formed by a method described as follows. Liquid peelable thermally curable printing ink is applied onto the entire surface of the pad113and the entire surface of the solder mask layer114. Then the liquid thermally curable printing ink is cured by using a baking process, thereby obtaining the protective layer115. A model of the liquid thermally curable printing ink may be LM-600 PSMS. In an alternative embodiment, the protective layer115made of photo resist ink can be formed by a method described as follows. Photo resist ink is applied onto the entire surface of the pad113and the entire surface of the solder mask layer114. Then portions of the photo resist ink is cured by using an exposing process, thereby obtaining the protective layer115. A model of the photo resist ink may be PR 2000SA.

Step5, referring toFIGS. 6 and 7, a first adhesive layer120is formed on the of the attaching region1115of first electrically conductive traces1112and the protective layer115, and a third electrically conductive layer130is formed on the first adhesive layer120. A second adhesive layer140is formed on the second electrically conductive pattern1121, and a fourth electrically conductive layer150is formed on the second adhesive layer140. The third electrically conductive layer130includes a first region131and a second region132. An arrangement of the first region131and the second region132is similar to that of the attaching region1115and the exposed region1114. The first region131surrounds the second region132. The first region131is superposed on the attaching region1115of the first electrically conductive pattern1111. The second region132is superposed on the exposed region1114of the first electrically conductive pattern1111. A third electrically conductive pattern1301is formed by removing part of the third electrically conductive layer130in the first region131, and a fourth electrically conductive pattern1501is formed by removing part of the fourth electrically conductive layer150.

Prior to forming the first adhesive layer120and the second adhesive layer140, the surface of the first electrically conductive traces1112and the surface of the second electrically conductive pattern1121can be roughened by brown oxide treatment or black oxide treatment.

In one embodiment, referring toFIGS. 7 and 8, prior to forming the third electrically conductive pattern1301and the fourth electrically pattern1501, a number of first plated vias1201are defined in the first adhesive layer120and the third electrically conductive layer130in the first region131and a number of second plated vias1401are defined in the second adhesive layer140and the fourth electrically conductive layer150. A first plating metal layer1202in each of the first plated vias1201are electrically connected with the first electrically conductive pattern1111and the third electrically conductive layer130. A second plating metal layer1402in each of the second plated vias1401is electrically connected with the second electrically conductive pattern1121and the fourth electrically conductive layer150. After the third electrically conductive pattern1301and the fourth electrically pattern1501are formed, the first plating metal layer1202is in electric communication with the first electrically conductive pattern1111and the third electrically conductive pattern1131. The second plating metal layer1402is in electric communication with the second electrically conductive pattern1121and the fourth electrically conductive pattern1501.

The first adhesive layer120and the second adhesive layer140may be pre-preg. The third electrically conductive layer130and the fourth electrically conductive layer150may be copper foils. The third electrically conductive pattern1301and the fourth electrically conductive pattern1501are formed using a photolithography process and an etching process. The third electrically conductive layer130of the second region132is remaining to form an alignment mask1302. Part of the third electrically conductive layer130between the third electrically conductive pattern1301and the alignment mask1302is removed.

Step6, referring toFIGS. 8 and 9, a slit101is defined in the first adhesive layer120along the boundary between the exposed region1114and the attaching region1115. The third electrically conductive layer130, the first adhesive layer120and the protective layer115in the exposed region1114surrounded by the slit101is removed along the slit101, thereby forming a cavity102and exposing the pad1113to the cavity102.

The slit101is formed by using a laser cutting process or a punching process. The slit101is defined along a boundary of the exposed region1115. In the illuminated embodiment, the slit101is formed along the boundary of the alignment mask1302and only passes through the first adhesive layer120. When part of the third electrically conductive pattern1301along the boundary of the exposed region1115remains, the slit101needs to pass through the third electrically conductive layer130and the first adhesive layer120. One surface of the first adhesive layer120adheres to the protective layer115, an opposite surface of the adhesive layer120adheres to the alignment mask1302. A material of the protective layer115is different from that of the alignment mask1302. An adhesive force between the first adhesive layer120and the protective layer115is smaller than that between the first adhesive layer120and the alignment mask1302, such that the alignment mask1302and the first adhesive layer120can be peeled from the surface of protective layer115together. The first adhesive layer120is isolated from the first electrically conductive pattern111in the exposed region1114by the protective layer115when removing the adhesive layer120, thus the first electrically conductive pattern111in the exposed region1114will not be affected when forming the adhesive layer120.

The protective layer115is removed after the adhesive layer120is removed, thus the cavity102for receiving electrical elements is formed. According to different materials of the protective layer115, methods for removing the protective layer115can also be different. When the protective layer115is made of peelable thermally curable printing ink, the protective layer115can be removed by peeling. When the protective layer115is made of photo resist ink, the protective layer115can be removed by striping in a corresponding reagent. In the striping process, a chemical reaction will occur between the reagent and the protective layer115, such that the protective layer115is dissolved and is separated from the surface of the first electrically conductive pattern111and the solder mask layer114.