The present invention relates to a method of forming a multi-layer printed circuit board (PCB) and the product thereof, and more particularly to a multi-layer PCB that has inner circuit layers formed with a resin build-up process and outer circuit layers formed with a lamination process to eliminate problems caused by forming the multi-layer PCB with only one single type of manufacturing process and therefore has effectively upgraded quality and reduced manufacturing cost.
To meet the requirements of various kinds of existing electronic apparatus to be thin, light, small, and compact, all electronic components and the printed circuit board (PCB) in the electronic apparatus for the electronic components to mount thereon are correspondingly reduced in size and weight. In other words, it is an increasingly urgent requirement for the PCB to have highly densely distributed circuits thereon. Currently, there are two ways for increasing the circuit density on the PCB. The first way is to increase the density of circuits on each circuit layer of the PCB, and the second way is to form a multi-layer PCB by stacking additional circuit layers on one or more cores of the PCB. The second way of forming a multi-layer PCB may be further divided into two types, namely, a lamination process, in which copper clad and dielectric material are laminated, and a resin build-up process, in which no copper clad but only bare dielectric material is coated or laminated. More particularly, the lamination process includes the steps of forming circuit layers on one or more cores; applying a laminating dielectric, such as a film formed of epoxy resin and fiberglass, between two inner cores or at an inner side of each outer layers of copper clad to form a dielectric layer; and heating and laminating multiple layers of cores to form a multi-layer PCB. And, the resin build-up process includes the steps of forming circuit layers on one or more cores; applying a dielectric, such as resin, on the cores to form the dielectric layers through liquid epoxy coating or dry film type epoxy laminating; forming another circuit layer on each dielectric layer; and repeating the steps of forming the dielectric layer and the circuit layer so that the dielectric layer and the circuit layer are alternately stacked to form a multi-layer PCB.
Technically speaking, the above-mentioned two processes for forming the multi-layer PCB have their respective advantages. For example, the resin build-up process using the resin material as the dielectric layers is more useful in the refinement of the circuit layers. On the other hand, the lamination process using the reinforced-fiber-contained dielectric, such as film formed of epoxy resin and fiberglass, as the laminating dielectric may advantageously improve the peel strength, thermal stress reliability, and size stability of the produced multi-layer PCB.
Moreover, in the circuit design for multi-layer PCB, it is necessary to establish interconnection between two adjacent circuit layers at some specific contacts thereof to enable electrical conduction. Currently, there are many different ways for establishing the above-mentioned interconnection and electrical conduction between two circuit layers on the multi-layer PCB. For example, laser or mechanical drilling may be employed in the multi-layer PCB forming process to form conductive holes at interconnected areas, and then, one of many known ways may be employed to form an electrically conductive plating layer on each hole. Alternatively, in the lamination process, solid copper column plating may be implemented at an area on one circuit layer to directly connect with a contact on another circuit layer.
A multi-layer PCB having good quality must have predetermined thermal resistance, copper peel strength, and stiffness. Since outer circuit layers on the multi-layer PCB are generally applied with a layer of solder mask, on which electronic components are mounted by way of insertion or surface mounting technique, they must have relatively enhanced copper peel strength to avoid undesired peeling of the electronic components and circuits off the PCB. When the conventional resin build-up process is employed to form the outer circuit layers, the latter shall have lower thermal resistance, stiffness, and, particularly, copper peel strength as compared with that formed with the lamination process. Moreover, the application of solder mask and the mounting of electronic components on the outermost circuit layers (or the first layers) results in limited space and area for the circuits. Under this condition, a part of the connecting circuits must be moved to inner circuit layers, that is, the second or even the third inner circuit layers. This necessitates the refinement of circuits on the second and the third inner circuit layers.
When the conventional lamination process is employed to form the inner circuit layers, it is necessary to form an electrically conductive plating layer on all conductive holes formed on the previously laminated copper clad, resulting in an increased thickness of the finished circuit layers to cause difficulties in the refinement of circuits on the second and/or the third inner circuit layers. On the other hand, since it is not necessary to refine circuits on other layers at inner sides of the third layers or the inner circuit layers directly formed on the cores, these inner circuit layers are usually formed with the lamination process without adversely affecting the structural quality of the multi-layer PCB.
Currently, most multi-layer printed circuit boards are manufactured with only one type of process to form multiple layers. For instance, in a VIL process developed by JVC of Japan, the way of liquid epoxy coating is employed to form the inner and the outer circuit layers of the multi-layer PCB. That is, both the outer and the second and third inner circuit layers are formed with the resin build-up process. After all the layers of the multi-layer PCB are formed, the outmost layers are applied with the solder mask. While the outer circuit layers formed with the VIL process through liquid epoxy coating, drying, and copper plating do satisfy the requirement of circuit refinement, they have a reduced reliability as compared with the outer circuit layers formed with the lamination process. In other words, in the multi-layer PCB produced with the VIL process, since both the inner and the outer circuit layers are formed through liquid epoxy coating, the completed PCB has apparently lower peel strength and thermal strength reliability as compared with that formed with the lamination process. The multi-layer PCB made with the VIL process also has reduced stiffness to result in user's doubt and accordingly lowered acceptance.
From the above analysis, it is found that any multi-layer PCB formed with only one of the two conventional processes could not satisfy the requirements for thermal resistance, copper peel strength, stiffness, and refinement of circuits on the second and third inner circuit layers at the same time.
It is therefore tried by the inventor to develop a method of forming a multi-layer PCB that would satisfy the requirements for thermal resistance, copper peel strength, stiffness, and refinement of circuits on the second and third inner circuit layers at the same time.
In one aspect of the present invention, there is provided a method of forming a multi-layer printed circuit board (PCB) that combines advantages obtainable from the resin build-up process and the lamination process.
In another aspect of the present invention, there is provided a multi-layer PCB having combined advantages obtainable from the resin build-up process and the lamination process.
According to the method of the present invention, different processes are employed for forming inner and outer circuit layers of a multi-layer PCB. More particularly, the second and third inner circuit layers of the multi-layer PCB are formed with the resin build-up process using liquid epoxy coating or dry film type epoxy laminating, and the outmost circuit layers of the multi-layer PCB are formed with the lamination process. Thus, circuits on the second and third inner circuit layers formed with the resin build-up process have upgraded refinement to satisfy the circuit design requirement of a multi-layer PCB, and the completed multi-layer PCB has improved overall thermal resistance, copper peel strength, structural stiffness, thermal stress reliability, and size stability.
The multi-layer PCB manufactured with the method of the present invention has quality and reliability superior to that of a multi-layer PCB having inner and outer circuit layers completely formed with only one type of process, and may be manufactured at reduced cost.
The structure and the technical means adopted by the present invention to achieve the above and other objects can be best understood by referring to the following detailed description of the preferred embodiments and the accompanying drawings, wherein
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The present invention is characterized in that the resin build-up process and the lamination process are employed to form the inner circuit layers and the outer circuit layers, respectively, of the 8-layer PCB 100. More specifically, the inner circuit layers, such as the second inner circuit layers 5a and 5b, which require refinement of circuits are formed by using a resin material, such as epoxy, as the dielectric to form the resin layers 4 with the resin build-up process through liquid epoxy coating or dry film type epoxy laminating, and then forming the inner circuit layers 5a and 5b. Thereafter, the dielectric layers 6 using the prepreg or the aramid fiber material as the dielectric and the outer circuit layers 7a, 7b are formed with the lamination process.
By using the resin build-up process and the lamination process to form different layers on the same one multi-layer PCB, the completed multi-layer PCB 100 is able to include advantages obtainable from the two processes. For example, the second inner circuit layers 5a and 5b of the multi-layer PCB 100 formed with the resin build-up process have upgraded circuit refinement to satisfy the circuit design requirement of the multi-layer PCB, and the outer circuit layers 7a and 7b of the multi-layer PCB 100 formed with the lamination process have improved thermal resistance, copper peel strength, stiffness, thermal stress reliability, and size stability. Therefore, the fully completed 8-layer PCB 100 has a quality reliability superior to that of a multi-layer PCB formed with only one of the conventional forming processes. Moreover, since at least some of the inner circuit layers, for example, the second inner circuit layers 5a and 5b, use resin layers 4 as the dielectric layers, and the cost for the resin material is much lower than the cost for the prepreg or the aramid fiber material used as the laminating dielectric layers 6, the multi-layer PCB made according to the method of the present invention has reduced material cost to lower the overall manufacturing cost thereof.
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The method for forming the 8-layer PCB 200 has characteristics generally the same as that of the 8-layer PCB 100. More specifically, the resin build-up process and the lamination process are employed to form the inner circuit layers and the outer circuit layers, respectively, of the 8-layer PCB 200. Wherein, the inner circuit layers, such as the second inner circuit layers 12a and 12b, which require refinement of circuits are formed by using a resin material, such as epoxy, as the dielectric to form the resin layers 15 on the cores 9 and 10 with the resin build-up process through liquid epoxy coating or dry film type epoxy laminating, and then forming the inner circuit layers 12a and 12b. Thereafter, the dielectric layers 16a using the prepreg or the aramid fiber material as the dielectric, and the outer circuit layers 11a, 11b are formed with the lamination process.
By using the resin build-up process and the lamination process to form different layers on the same one multi-layer PCB, the completed multi-layer PCB 200 is able to include advantages obtainable from the two processes. For example, the second inner circuit layers 12a and 12b of the multi-layer PCB 200 formed with the resin build-up process have upgraded circuit refinement to satisfy the circuit design requirement of the multi-layer PCB, and the outer circuit layers 11a and 11b of the multi-layer PCB 200 formed with the lamination process have improved thermal resistance, copper peel strength, stiffness, thermal stress reliability, and size stability. Therefore, just like the 8-layer PCB 100, the fully completed 8-layer PCB 200 has a quality reliability superior to that of a multi-layer PCB formed with only one of the two conventional forming processes, and have a reduced manufacturing cost.
The method of forming a multi-layer PCB and the produced multi-layer PCB according to the present invention have at least the following advantages:
Number | Date | Country | |
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Parent | 10623592 | Jul 2003 | US |
Child | 11270490 | Nov 2005 | US |