A photosensitive polyimide additive process for ultra-thin rigid-flex boards

Abstract

A photosensitive polyimide layer addition process for an ultra-thin rigid-flexible board, comprising the following steps: selecting a rigid metal plate as a substrate, and making openings in the rigid plate area; laminating a first insulating photosensitive PI layer on both sides of the metal plate; exposing, developing, and curing the first insulating photosensitive PI layer; making through holes, hole filling, and additional circuit plating; laminating a second insulating photosensitive PI layer on the back of the circuit board after plating; exposing, developing, and curing the second insulating photosensitive PI layer laminated in step 6); performing hole filling and additional circuit plating; etching to remove the metal plate in the flexible board connecting band area; and silk-screening to obtain an ultra-thin rigid-flexible board. The process of the present invention is simple and easy to process. The prepared rigid-flexible board has the characteristics of thinner thickness, smaller size, less deformation, and high wiring integration. At the same time, the production efficiency is high and the cost is low.

Description

Technical Field

[0001] The invention relates to the technical field of circuit board production, and in particular to a photosensitive PI addition process for an ultra-thin rigid-flexible board. Background Art

[0002] With the continuous advancement of technology, the functional requirements of electronic products are getting higher and higher, and the appearance is also very focused on being short, small, light and thin. For this reason, more and more multi-layer integrated functional circuit boards are being adopted, especially the rigid-flex board has developed rapidly in recent years. In the preparation process of the rigid-flex board, there are still many problems. Not only is the production difficult, the process is complicated, and the cost is high, but there are also defects such as poor bonding, deformation and delamination. There has never been a major breakthrough in the product qualification rate and performance.

[0003] After investigation, the existing Chinese patent number CN201520517802.X, "A Soft-Rigid Combination Board", includes: a flexible circuit board, the surface of which is provided with an insulating soft board covering film, and a conductive window is provided on the soft board covering film; a sintered coating is provided on the copper surface of the conductive window; a rigid circuit board, on which is provided a metal sintered layer corresponding to the conductive window, which is mutually melted with the sintered coating after heating and pressing or ultrasonic heating and welding; a bonding adhesive layer is provided on the flexible circuit board covering film. During preparation, a conductive window is opened on the flexible circuit board covering film, and a sintered coating is made on the copper surface of the conductive window; a bonding adhesive layer is attached: a metal sintered layer is made on the rigid circuit board; the metal sintered layer is connected to the sintered coating of the conductive window, and the two are mutually melted by heating and pressing to form a conductive layer, so as to achieve line conduction between the flexible circuit board and the rigid circuit board. This type of soft-rigid combined circuit board has improvements in thickness and wiring, but for chips, the thickness is not thin enough, the size is not small enough, and the wiring integration is not high enough, so it is necessary to design a preparation process for ultra-thin boards. Summary of the invention

[0004] The technical problem to be solved by the present invention is to provide a photosensitive polyimide addition process for an ultra-thin rigid-flex board which is simple and reasonable and easy to process. The prepared rigid-flex board has the characteristics of thin thickness, not easy to deform and high wiring integration.

[0005] The technical solution adopted by the present invention to solve the above technical problems is: a photosensitive polyimide addition process for ultra-thin rigid-flexible boards, characterized by comprising the following steps:

[0006] 1) Select a hard metal plate as the substrate, and use etching, mechanical drilling or laser drilling to open holes in the hard plate area;

[0007] 2) Plug the holes with resin and grind the holes in the hard board area;

[0008] 3) Laminating a first insulating photosensitive PI layer on both sides of the metal plate;

[0009] 4) exposing, developing and curing the first insulating photosensitive PI layer;

[0010] 5) Then do through-hole, hole filling and additional circuit plating;

[0011] 6) Laminating the second insulating photosensitive PI layer on the back of the circuit board after electroplating;

[0012] 7) exposing, developing and curing the second insulating photosensitive PI layer laminated in step 6);

[0013] 8) Then fill the holes and add circuit plating;

[0014] 9) Etching and removing the metal plate in the flexible board connection zone;

[0015] 10) Finally, solder resist screen printing is performed to obtain a solder resist screen printing layer, that is, an ultra-thin rigid-flexible board is obtained.

[0016] Preferably, the hard metal plate in step 1) is a steel plate, a copper plate or a nickel plate.

[0017] Furthermore, the opening in step 1) is obtained by etching, mechanical drilling or laser drilling.

[0018] Furthermore, the hard plate area in step 2) refers to the head and tail, and this product is explained using the head opening as an example; grinding is to grind the protruding part of the resin at the opening until the resin height is flush with the hard metal plate.

[0019] As an improvement, the exposure, development and curing in step 4) refers to removing the first insulating photosensitive PI layer on the front side of the soft board connecting band area and the tail hard board area of ​​the ultra-thin soft-hard combination board to obtain the exposure grounding point, i.e., the blind hole, and using exposure and development instead of laser drilling to improve efficiency and reduce cost.

[0020] Preferably, the steel plate, copper plate or nickel plate has an optimal thickness in the range of 0.1-0.3 mm.

[0021] Preferably, the first insulating photosensitive PI layer has an optimal thickness in the range of 10-40 um.

[0022] Preferably, the second insulating photosensitive PI layer has an optimal thickness in the range of 10-40 um.

[0023] Furthermore, the step 5) is followed by through-hole, hole-filling and additive circuit plating. The through-hole is to machine an opening with a diameter smaller than that of the metal plate at the center of the head resin plug hole, thereby forming a new conductive hole; the hole-filling is to fill the blind hole position at the exposure and development opening on the first insulating photosensitive PI layer with copper; the additive method is to make the first circuit plating layer on the first insulating photosensitive PI layer, and the new conductive hole is filled with a layer of copper material on the hole wall by electroplating, which is connected to the first circuit plating layer; the blind hole is filled with copper material by electroplating, which is connected to the first circuit plating layer.

[0024] Finally, the step 8) is followed by hole filling and additive circuit plating. The hole filling is to fill the blind hole positions at the exposure and development openings on the second insulating photosensitive PI layer with copper; the additive method is to make a second circuit plating layer on the second insulating photosensitive PI layer, and the blind holes are filled with copper material by electroplating to connect with the second circuit plating layer.

[0025] The product described in this article is a three-layer plate stack. If multiple layers are to be prepared, the above steps 6), 7), and 8) can be repeated to obtain the product.

[0026] Compared with the prior art, the advantages of the present invention are: using steel plate, copper plate or nickel plate as the substrate can not only increase the support force during chip mounting, but also reduce the deformation of the chip; the insulating material uses photosensitive PI, which is easy to process, has a single material, and has a higher CTE matching degree; using photosensitive PI material, based on additive circuit technology, makes the wiring integration higher; selecting photosensitive PI to use exposure and development technology instead of traditional laser drilling, which is efficient and low cost, can avoid the poor residual glue at the bottom of the conventional laser drilling hole, and has better reliability. In addition, the conventional steel sheet is bonded and connected to the circuit board through conductive glue, and the conductive glue has the disadvantages of moisture absorption, oxidation of conductive particles, and thermal expansion after high temperature, resulting in large grounding resistance, insufficient bonding strength, and even the risk of explosion; using an embedded steel sheet structure, the circuit and the steel sheet are directly connected through the blind hole filling formed after the photosensitive PI development, with good conductivity and good bonding force, reducing the risk of explosion; the process of the present invention is simple and easy to process, and the prepared soft-hard combination board has the characteristics of thinner thickness, less deformation, and high wiring integration, while the production efficiency is high and the cost is low. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 This is a flow chart of the photosensitive PI layer addition process provided in an embodiment of the present invention, wherein a is steel plate input and hole opening, b is photosensitive PI layer lamination, c is photosensitive PI layer exposure, development, and curing, d is through-hole making, hole filling, and circuit electroplating, e is photosensitive PI layer lamination, exposure and development, f is hole filling and circuit electroplating, and g is steel plate etching and silk-screen printing. DETAILED DESCRIPTION

[0028] The present invention is further described in detail below with reference to the accompanying drawings.

[0029] like Figure 1 As shown, a photosensitive polyimide addition process of an ultra-thin rigid-flexible board is shown. The structure of the ultra-thin rigid-flexible board of this embodiment is divided into three parts, namely the head rigid board part A, the flexible board connection zone B and the tail rigid board zone C. The thickness requirement of the head rigid board part A of the conventional board is 0.3-0.4mm, the thickness requirement of the tail rigid board zone C is 0.3-0.45mm, and the thickness requirement of the flexible board connection zone B is 0.08-0.12mm; the thickness requirement of the head rigid board part A of this ultra-thin rigid-flexible board is 0.2-0.3mm, the thickness requirement of the tail rigid board zone C is 0.15-0.25mm, and the thickness requirement of the flexible board connection zone B is 0.04-0.08mm. The specific preparation process includes the following steps:

[0030] 1) A hard metal plate is selected as the substrate. Usually, in step 1), the hard metal plate is a steel plate, a copper plate or a nickel plate, and the optimal thickness is in the range of 0.1-0.3 mm. Etching, mechanical drilling or laser drilling is used to obtain an opening 1.1 in the hard plate area. The metal plate of this embodiment is preferably a steel plate 1;

[0031] 2) Fill the opening 1.1 in the hard board area with resin and grind it; this product takes the opening in the head as an example, fill the opening 1.1 with resin, which is usually a plugging epoxy resin material, and then grind the protruding part of the resin at the opening until the height of the resin is flush with the hard metal plate;

[0032] 3) Laminating the first insulating photosensitive PI layer 2 on both sides of the steel plate; the first insulating photosensitive PI layer 2 is made of polyimide material, and the optimal thickness is usually in the range of 10-40um;

[0033] 4) exposing, developing, and curing the first insulating photosensitive PI layer 2; removing the first insulating photosensitive PI layer 2 on the front side of the soft board connection zone area B and the tail hard board area C of the ultra-thin soft-hard combination board, and obtaining the exposed grounding point, i.e., the blind hole 2.1, using the exposure and development method instead of laser drilling to improve efficiency and reduce cost;

[0034] 5) Then make through holes, fill holes and add circuit plating. The through hole is to process a hole with a diameter smaller than the metal plate opening 1.1 in the center of the head resin plug hole, so as to form a new conductive hole 1.2; the filling hole is to fill the blind hole 2.1 position at the exposure and development opening on the first insulating photosensitive PI layer 2 with copper. The additive method is to make the first circuit electroplating layer 3 on the first insulating photosensitive PI layer 2. The new conductive hole 1.2 is filled with a layer of copper material on the hole wall by electroplating, and is connected with the first circuit electroplating layer 3; the blind hole 2.1 is filled with copper material by electroplating, and is connected with the first circuit electroplating layer 3;

[0035] 6) The second insulating photosensitive PI layer 4 is pressed on the back of the circuit board after electroplating; the second insulating photosensitive PI layer 4 is usually made of polyimide material, and the optimal thickness is usually in the range of 10-40um. During the pressing process, part of the photosensitive PI material fills the lower part of the through hole 1.2 and the copper foil gap of the first circuit electroplating layer 3, so that the first insulating photosensitive PI layer 2 and the second insulating photosensitive PI layer 4 under the steel plate 1 are pressed together;

[0036] 7) exposing, developing and curing the second insulating photosensitive PI layer 4 laminated in step 6); obtaining an exposed grounding point, i.e., a blind hole 3.1, in the second insulating photosensitive PI layer 4;

[0037] 8) Then, hole filling and additive circuit plating layer 5 are performed; hole filling is to fill copper at the blind hole 3.1 position at the exposure and development opening on the second insulating photosensitive PI layer 4, and the additive method is to make the second circuit plating layer 5 on the second insulating photosensitive PI layer 4, and the blind hole 3.1 is filled with copper material by electroplating to communicate with the second circuit plating layer 5;

[0038] 9) Etching and removing the metal plate in the flexible board connection zone B;

[0039] 10) Finally, solder resist screen printing is performed to obtain the solder resist screen printing layer 6, and after surface treatment, testing, appearance, etc., the product ultra-thin rigid-flex board is obtained.

[0040] The product is a three-layer plate stacked structure. If multiple layers are required, the above steps 6), 7), and 8) can be repeated to obtain the product.

[0041] The ultra-thin rigid-flexible board prepared by the present invention has the following main characteristics:

[0042] 1. The product thickness in the Z direction is thinner and the size in the XY direction is smaller;

[0043] Second, the use of photosensitive PI materials and additive circuit technology make the wiring integration higher;

[0044] 3. The insulation material is pure photosensitive PI, which is not only easy to process, but also has a single material and a higher CTE matching degree;

[0045] 4. Embedding the steel plate directly under the top layer can increase the support force during chip mounting and reduce the deformation of the chip;

[0046] 5. Choose photosensitive PI and adopt exposure and development technology instead of traditional laser drilling method, which can avoid the bad adhesive residue at the bottom of conventional laser drilling holes, with high efficiency, low cost and better reliability.

Claims

1. A photosensitive polyimide addition process for ultra-thin rigid-flex boards, characterized in that The following steps are involved: 1) A hard metal plate is selected as a substrate, and a hole is formed in the hard plate area by etching, mechanical drilling or laser drilling; the hard metal plate in step 1) is a steel plate, a copper plate or a nickel plate; 2) Plug the holes with resin and grind the holes in the hard board area; 3) Laminating the first insulating photosensitive PI layer on both sides of the metal plate; 4) exposing, developing, and curing the first insulating photosensitive PI layer; the exposure, development, and curing in step 4) refers to removing the first insulating photosensitive PI layer on the front side of the soft board connection zone area and the tail hard board area of ​​the ultra-thin soft-hard combination board to obtain an exposure grounding point, i.e., a blind hole; 5) Then make through holes, fill holes and add circuit plating; the step 5) then make through holes, fill holes and add circuit plating, the through hole is to process an opening with a diameter smaller than the metal plate in the center of the resin plug hole, so as to form a new via hole; the filling hole is to fill the blind hole position at the exposure and development opening on the first insulating photosensitive PI layer with copper, and the additive method is to make the first circuit plating layer on the first insulating photosensitive PI layer; the new via hole is filled with a layer of copper material on the hole wall by electroplating, which is connected with the first circuit plating layer; the blind hole is filled with copper material by electroplating, which is connected with the first circuit plating layer; 6) Lamination of the second insulating photosensitive PI layer on the back of the circuit board after electroplating; 7) exposing, developing and curing the second insulating photosensitive PI layer laminated in step 6); 8) Then fill the holes and add circuit electroplating; 9) Etching to remove the metal plate in the flexible board connection area; 10) Finally, solder mask screen printing is performed to obtain the solder mask screen printing layer, that is, the product ultra-thin rigid-flex board is obtained.

2. The photosensitive polyimide addition process according to claim 1, characterized in that: The thickness of the steel plate, copper plate or nickel plate is in the range of 0.1-0.3 mm.

3. The photosensitive polyimide addition process according to claim 1, characterized in that: The thickness of the first insulating photosensitive PI layer is in the range of 10-40 μm.

4. The photosensitive polyimide addition process according to claim 1, characterized in that: The thickness of the second insulating photosensitive PI layer is in the range of 10-40 μm.

5. The photosensitive polyimide addition process according to claim 1, characterized in that: The step 8) then performs hole filling and additive circuit plating. The hole filling is to fill the blind hole position at the exposure and development opening on the second insulating photosensitive PI layer with copper; the additive method is to make a second circuit plating layer on the second insulating photosensitive PI layer, and the blind hole is filled with copper material by electroplating to connect with the second circuit plating layer.

6. The photosensitive polyimide addition process according to claim 1, characterized in that: The product is a three-layer plate stacked structure. If multiple layers are required, the product can be obtained by repeating the above steps 6), 7), and 8).

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