A multi-layer flexible circuit board preparation process avoiding pad PAD bumps

Abstract

The present invention discloses a process for preparing a multi-layer flexible circuit board for avoiding bumps of pads PAD. By changing the design of the process flow of the prior art, the blind hole design of the upper and lower outer layers of the multi-layer board is changed from the traditional outer layer laser to the sub-outer layer to the sub-outer layer laser processing and manufacturing. Therefore, after changing the product stacking structure and process, the SMT welding end will not be affected by the electroplated PAD left by the graphic electroplating process, and the quality of the product after SMT is well guaranteed, avoiding the occurrence of defects such as device tilting and cold soldering in SMT, and completely avoiding the quality risks of voids and cold soldering caused by solder paste printing, thereby greatly improving the reliability of product welding.

Description

Technical Field

[0001] The invention relates to the technical field of flexible circuit boards, and in particular to a preparation process for a multilayer flexible circuit board avoiding pad (PAD) convex points. Background Art

[0002] Due to the limitation of product application space, the functional requirements of flexible circuit boards are becoming more and more complex. When designing the circuit routing of the circuit board, it is necessary to use the HDI (High Density Interconnector) board with micro-hole connection design. Ordinary through holes can no longer design all the circuit routing layouts in the existing structure and space. Most of the industry uses the laser blind hole process design to make this type of FPC, and the products of this type of structure are used in the multi-layer FPC process. For the conduction between layers, graphic electroplating and whole board electroplating processes are generally used to complete the direct circuit connection and conduction of each layer. The above-mentioned existing technology has the following shortcomings:

[0003] 1. The existing traditional process mode, direct whole-board electroplating process will improve the problem of graphic PAD protrusion caused by the graphic electroplating process, but because the copper thickness of the circuit surface will increase after the whole-board electroplating, it will greatly increase the difficulty of etching precision circuit processing, resulting in the inability to process fine circuits or the product yield being seriously affected. At the same time, due to the difference in current size in the whole-board electroplating process, the thickness of the electroplating layer will be uneven, and for the circuit board with impedance control, its impedance value will be subject to abnormal fluctuations;

[0004] 2. The use of outer layer pattern electroplating process cannot guarantee the flatness and consistency of the outer layer pads, and the differences in the welding height of the device's welding pins may lead to risks such as device tilting and cold soldering;

[0005] 3. Traditional multilayer board (reference Figure 1 , taking three-layer FPC as an example) the process flow is to first use the double-layer substrate 10 of the double-layer board to make the L1 layer circuit 11 and the L2 layer circuit 12 on both sides respectively, then paste a layer of pure glue 13 on the L2 layer circuit 12, paste the L3 layer substrate after pre-pressing, and then laminate the entire Base, and then laser the blind hole from the L1 layer circuit 11 to the L2 layer circuit 12 and laser the blind hole from the L3 layer circuit 14 to the L2 layer circuit 12, perform the hole cleaning process on the blind holes on both sides, perform the electroplating process on the blind holes on both sides to obtain the upper blind hole copper plating 16 and the lower blind hole copper plating 17, and paste pure glue and coating 15 corresponding to the L3 layer circuit 14 and the lower blind hole copper plating 17, and then make the pad solder paste 18 corresponding to the upper blind hole copper plating 16..., the products produced by such a process flow will appear like Figure 1The blind hole depression at position A shown in the figure and the copper plating 16 of the upper blind hole protruding from the surface of the pad at position B shown in the figure are defective. Therefore, in the later SMT process, after printing the solder paste at position A, in addition to forming the soldering void phenomenon at position A, there is also the hidden danger of false soldering due to the raised pins of the soldered device caused by the protruding electroplating layer of the pad. Summary of the invention

[0006] In order to solve the above technical problems, the present invention provides a multi-layer flexible circuit board preparation process for avoiding pad PAD bumps, comprising the following steps:

[0007] S1. Provide a double-sided substrate, and make L1 layer circuits and L2 layer circuits on both sides of the double-sided substrate;

[0008] S2. Laser blind via from L2 layer circuit to L1 layer circuit;

[0009] S3. The upper blind via is formed by electroplating process to conduct and connect the L2 layer circuit and the L1 layer circuit;

[0010] S4. Apply a layer of pure glue on the copper-plated surface of the L2 layer circuit and the upper blind hole;

[0011] S5. After pre-pressing, apply L3 layer of substrate and laminate;

[0012] S6. Make L3 layer circuit on L3 layer substrate;

[0013] S7. Laser blind vias from L3 layer circuit to L2 layer circuit;

[0014] S8. The electroplating process forms a copper-plated blind hole in the lower layer to connect the L3 layer circuit and the L2 layer circuit;

[0015] S9. Repeat the above steps S4-S8 to form a multilayer structure;

[0016] S10. The copper-plated surface of the last layer of circuits and blind holes is coated with pure glue and film;

[0017] S11. Use the flat L1 layer circuit surface as the pad to print solder paste.

[0018] After the laser blind hole is formed in step S2, a plasma process is used to clean the inside of the hole, and then an electroplating process is performed in step S3.

[0019] After the laser blind hole is formed in step S7, a plasma process is used to clean the inside of the hole, and then an electroplating process is performed in step S8.

[0020] Through the above technical scheme, the present invention changes the design of the blind hole design of the upper and lower outer layers of the multilayer board from the traditional outer layer laser to the sub-outer layer to the sub-outer layer laser processing and manufacturing through the change design of the existing technical process flow. Therefore, after changing the product stacking structure and process, the SMT welding end will not be affected by the electroplating PAD (protrusion around the conductive blind hole) left by the graphic electroplating process, and the quality of the product after SMT is well guaranteed, avoiding the occurrence of device tilting, cold solder joints and other defects in SMT, and completely avoiding the quality risks of voids and cold solder joints caused by solder paste printing, thereby greatly improving the reliability of product welding. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art are briefly introduced below.

[0022] Figure 1 It is a schematic diagram of the structure of a multi-layer flexible circuit board in the prior art;

[0023] Figure 2 The present invention is a schematic diagram of a multi-layer flexible circuit board structure for avoiding pad PAD bumps disclosed in an embodiment of the present invention.

[0024] In the figure: 10. Double-sided substrate; 11. L1 layer circuit; 12. L2 layer circuit; 13. Pure glue; 14. L3 layer circuit; 15. Lamination; 16. Upper layer blind hole copper plating; 17. Lower layer blind hole copper plating; 18. Solder paste. DETAILED DESCRIPTION

[0025] The technical solutions in the embodiments of the present invention will be described clearly and completely below in conjunction with the accompanying drawings in the embodiments of the present invention.

[0026] refer to Figure 2 The present invention provides a multi-layer flexible circuit board preparation process for avoiding pad PAD bumps, comprising the following steps:

[0027] S1 provides a double-sided substrate 10, and the double-sided substrate 10 is made of L1 layer circuit 11 and L2 layer circuit 12 on both sides;

[0028] S2. Laser blind vias from L2 layer circuit 12 to L1 layer circuit 11 and use plasma process to clean the inside of the vias;

[0029] S3. The electroplating process forms an upper copper blind hole 16 to connect the L2 layer circuit 12 and the L1 layer circuit 11;

[0030] S4. A layer of pure glue 13 is applied to the surface of the L2 layer circuit 12 and the upper blind hole copper plating 16;

[0031] S5. After pre-pressing, apply L3 layer of substrate and laminate;

[0032] S6. Making L3 layer circuit 14 on L3 layer substrate;

[0033] S7. Laser blind holes from L3 layer circuit 14 to L2 layer circuit 12 and use plasma process to clean the holes;

[0034] S8. The electroplating process forms a copper-plated blind hole 17 in the lower layer to connect the L3 layer circuit 14 and the L2 layer circuit 12;

[0035] S9. Repeat the above steps S4-S8 to form a multilayer structure;

[0036] S10. The last layer of circuits and blind holes are copper-plated on the surface with pure glue 13 and coated 15;

[0037] S11. Use the flat surface of the L1 layer circuit 11 to print solder paste 18 as a pad.

[0038] The present invention changes the design of the blind hole design of the upper and lower outer layers of the multilayer board from the traditional outer layer laser to the sub-outer layer to the sub-outer layer laser processing and manufacturing through the change of the prior art process flow. Therefore, after changing the product stacking structure and process, the SMT welding end will not be affected by the protrusions around the electroplated PAD conductive blind hole left by the graphic electroplating process, and the quality of the product after SMT is well guaranteed, avoiding the occurrence of device tilting, cold solder joints and other defects in SMT, and completely avoiding the quality risks of voids and cold solder joints caused by solder paste printing, thereby greatly improving the reliability of product welding.

[0039] The above description of the disclosed embodiments enables one skilled in the art to implement or use the present invention. Various modifications to the above embodiments will be apparent to one skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments shown herein, but rather to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A multi-layer flexible circuit board preparation process for avoiding pad PAD bumps, characterized in that: The steps include: S1. Providing a double-sided substrate (10), and making L1 layer circuits (11) and L2 layer circuits (12) on both sides of the double-sided substrate (10); S2. Laser blind via from L2 layer circuit (12) to L1 layer circuit (11); S3. The upper layer blind via is formed by electroplating process and copper is plated (16) to connect the L2 layer circuit (12) and the L1 layer circuit (11); S4. A layer of pure glue (13) is applied to the surface of the L2 layer circuit (12) and the upper copper-plated blind hole (16); S5. After pre-pressing, apply L3 layer of substrate and laminate; S6. Making L3 layer circuit (14) on L3 layer substrate; S7. Laser blind via from L3 layer circuit (14) to L2 layer circuit (12); S8. The electroplating process forms a copper-plated blind hole (17) in the lower layer to connect the L3 layer circuit (14) and the L2 layer circuit (12); S9. Repeat the above steps S4-S8 to form a multilayer structure; S10. The last layer of circuits and blind holes are copper-plated on the surface with pure glue (13) and coated (15); S11. Print solder paste (18) on the flat surface of the L1 layer circuit (11) as a solder pad.

2. A multilayer flexible circuit board preparation process for avoiding pad PAD bumps according to claim 1, characterized in that: After the laser blind hole in step S2, the inside of the hole is cleaned by a plasma process, and then the electroplating process in step S3 is performed.

3. A multilayer flexible circuit board preparation process for avoiding pad PAD bumps according to claim 1, characterized in that: After the laser blind hole is formed in step S7, the inside of the hole is cleaned by a plasma process, and then the electroplating process is performed in step S8.

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