Laser grinding hole plugging method for Ka-band multi-layer micro-strip plate

Abstract

The invention relates to the technical field of millimeter wave multilayer boards, in particular to a laser grinding and hole plugging method for a Ka-band multilayer microstrip board, which is a lossless, rapid and high-precision resin hole plugging grinding method suitable for a Ka-band PTFE (polytetrafluoroethylene) system microstrip board and a resin hole plugging multilayer microstrip board which is laminated for multiple times and provided with blind holes, buried holes, back drilling holes and through holes. The method mainly comprises the steps of performing vacuum hole plugging on specified holes; curing the hole plugging resin; carrying out laser grinding on protruding resin redundant substances, and cleaning with a solution; according to the Ka-band six-layer microstrip board prepared by the technology, the flatness of a surface layer plug hole is reduced to 5-9 [mu] m from-25 [mu] m to + 25 [mu] m, the surface roughness Ra is reduced to 3 [mu] m from 7 [mu] m, the interlayer alignment precision is improved to + / -0.09 mm from + / -0.13 mm, the board thickness tolerance is reduced to + / -5% from + / -8%, and the yield is improved to 90% from 80%. And the repair rate of the surface-layer surface-mounted device is reduced.

Description

Technical Field

[0001] The present invention relates to the technical field of millimeter wave multilayer boards, and in particular to a laser grinding and plugging method for a Ka-band multilayer microstrip board. Background Art

[0002] With the rapid development of military electronic products toward lighter, thinner, smaller, higher-density, more multifunctional, and highly integrated designs, the size of microwave components and multilayer microstrip boards has shrunk exponentially, while assembly density has increased. As operating frequencies rise to millimeter-wave bands such as the Ka-band, the wiring density and pin pads of multilayer microstrip boards decrease to the hundreds of micrometer level. High-integration components like BGA and CCGA pin counts rise to over 1,000. To ensure long-term solder joint reliability, the ground and signal interconnect plated-through holes (PTHs) on the necessary pads are treated with resin plugging. Furthermore, while interconnect density increases, the number of microstrip board layers increases, but the number of lamination cycles cannot be increased indefinitely. Each lamination cycle negatively impacts inter-layer alignment accuracy and thickness and flatness consistency. Therefore, aerospace products require fewer than three press-fit cycles. To reduce the number of hot-press cycles, backdrilling becomes the only option for increasing interconnect density. After backdrilling, the non-metallized vias (PTHs) also require resin plugging to improve assembly and reliability. Therefore, resin plugging is a necessary process for high-frequency multi-layer microstrip boards, which has an important impact on the yield of multi-layer boards and the assembly, electrical performance and long-term reliability of subsequent board-level components.

[0003] Multilayer microstrip boards often use vacuum plugging for resin vias. To ensure via quality and avoid voids within the vias and gaps between layers, the resin often protrudes from the substrate surface after plugging. Therefore, excess resin must be polished to ensure via flatness. Traditional products require via flatness of ≤25μm for protrusions or depressions. However, for some multilayer microstrip boards used in aerospace and military applications, the requirement for via flatness is ≤5μm, or even 0μm.

[0004] There are four methods of plugging resin grinding currently used in the industry: abrasive belt grinding, ceramic grinding, non-woven fabric grinding, and manual grinding. Regardless of the grinding process, it has the following disadvantages: (1) When the resin plugging is uneven, especially when the resin protrusion is serious, the grinding plate will not be clean during the grinding process, and it usually needs to be re-grinded 1 to 3 times (plus the normal grinding times, a total of 4 to 6 times). In severe cases, it needs to be re-grinded more than 4 times (plus the normal grinding times, a total of more than 8 times); (2) The copper surface of the circuit and the raised substrate will be damaged during the grinding process. Severe cases may cause the copper thickness to be thinned by more than 10μm, which will bring greater process pressure to the consistency of the copper plating thickness and the consistency of the board thickness; (3) The resin after the plugging will be damaged during the grinding process, resulting in depression after the plugging, affecting the bonding effect of the inner layer and the welding quality of the surface device; (4) For multi-layer microstrip boards with PTFE substrates, grinding will affect the expansion and contraction of the substrate, and then affect the inter-layer alignment accuracy after lamination, as well as the matching with the steel mesh during the SMT process. Excessive inter-layer alignment accuracy may cause short circuit or short circuit, and the offset with the steel mesh will cause solder joint short circuit during the SMT process, resulting in batch scrapping of products.

[0005] In view of the above-mentioned defects, the inventors of the present invention finally obtained the present invention after a long period of research and practice. Summary of the Invention

[0006] The purpose of the present invention is to solve the problem that the current conventional multi-layer microstrip board resin plugging process has many steps, inevitably causes damage to the substrate, and affects the lamination position accuracy. A laser grinding and plugging method for Ka-band multi-layer microstrip boards is provided.

[0007] To achieve the above objectives, the present invention discloses a laser grinding and plugging method for a Ka-band multilayer microstrip board, comprising the following steps:

[0008] S1, vacuum plugging the designated holes on the substrate;

[0009] S2, plugging resin curing;

[0010] S3, laser grinding of the protruding resin excess and solution cleaning;

[0011] S4, confirm the flatness and proceed with the subsequent plating process.

[0012] In step S1, the substrate material is a microwave copper-clad laminate with a PTFE base, and the adhesive material is a microwave prepreg with a dielectric constant ≤ 3. The substrate comprises at least a first microwave copper-clad laminate layer and a second microwave copper-clad laminate layer. The microwave copper-clad laminate layer comprises two layers of copper foil, upper and lower, and an insulating medium sandwiched between the copper foils. Each two layers of microwave copper-clad laminate layer are bonded together using an adhesive.

[0013] In step S1, the designated hole is either a plated hole or a non-plated hole, with a diameter of 0.1 mm to 1.0 mm and a thickness-to-diameter ratio of ≤16:1. A plated hole is a structure that penetrates at least two layers of copper foil and a layer of insulating dielectric, forming a circuit connection. The plated hole is filled with resin to provide structural reinforcement and functional support. After the resin cures, electroplating is performed on the resin and also on the copper foil at the opening of the plated hole.

[0014] The microwave copper clad laminate is one of the materials RS300B of Ruilong Company, CF294 and CFG294 of China Electronics Technology Group Corporation 46, GNC3004 of Guoneng Company, and SJ9300 of Shengyi Technology, or a compound thereof to form a multilayer board. The copper foil of the copper clad laminate is 18μm electroplated copper foil. The microwave semi-cured sheet is selected from any one of CFB278F of China Electronics Technology Group Corporation 46, RNP280 of Ruilong Company, SJ928B of Shengyi Technology, and CN300BP of Guoneng New Materials.

[0015] In step S1, the plugging method is screen printing assisted vacuum plugging, and the holes of the screen printing screen are opened by one of the mechanical drilling method and the laser drilling method. The hole diameter is consistent with the hole diameter on the substrate; the process parameters of screen printing are set as follows: the screen printing pressure is 70kgf~100kgf, the screen printing speed is 15mm / s~30mm / s, and the scraper pressure depth is 20mm~30mm.

[0016] The screen printing process is as follows: the screen and substrate are positioned by CCD and installed on the work surface of the vacuum plugging machine, the plugging resin is placed in front of the scraper, and printing is performed according to the set process parameters. During the printing process, the vacuum is turned on to ensure that the resin evenly fills the holes. After plugging, the screen is removed.

[0017] The plugging resin is a thermosetting material system, and is any one of San-Ei's PHP-900IR-6P, Perters' PP-2795 / PP-2794, Taiyo's THP-100DX, Bairou's PHP-9000-3F, and PHP-9000-4F. The mesh is any one of aluminum alloy, stainless steel, and epoxy bakelite, with a thickness of 0.1mm to 0.3mm.

[0018] In step S1, the overflow degree of the plugging resin in the microstrip board is controlled by controlling the plugging fixture and plugging parameters to meet the requirements of laser grinding.

[0019] In step S2, the substrate is placed in a hot air oven for curing. Curing parameters follow the recommended process curve for the plugging resin, with a maximum curing temperature not exceeding 150°C and a curing time not exceeding 6 hours. After curing, the plugging resin has a volume shrinkage of ≤5%.

[0020] In step S3, the laser grinding equipment is an integrated automated CNC machine tool integrating light, machinery and electricity, and the laser grinding equipment includes a laser and its optical path system, a mechanical system, a computer and software control system, a dust collection system and a water cooling system; the laser grinding adopts a scanning mode, and any one of filling scanning, contour scanning and compound scanning is selected; the laser light source is one of 1064nm, 355nm and excimer laser, with a power of 10W-50W, a pulse width of 50ns-140ns, a spot radius at the focus of about 20μm-1000μm, and a power density of 3×10 4 W / cm 2 ~10×10 4 W / cm 2 The scanning speed is 10mm / s-100mm / s. If there is still a large protrusion after one laser grinding, repeat the laser grinding 1-2 times. By controlling the laser grinding parameters and the number of laser grinding times, the plug hole protrusion can be reduced to ≤0.02mm.

[0021] In step S3, after laser grinding, the residue is cleaned with a solvent and dried in an oven. The cleaning solvent is selected from either acid washing or alkaline washing. Afterwards, the residue is rinsed with deionized water. The drying temperature is 110° C.-130° C. and the drying time is 1 h-3 h.

[0022] In step S4, the plugged vias on the substrate are randomly inspected for flatness using a laser microscope with an accuracy of at least 10 nm. Plugged vias that meet the required flatness are then subjected to full-board electroplating or directly to subsequent processing steps according to design requirements. The resulting microstrip board includes a top-layer pad containing plated through-holes and plated blind vias after the plugged vias, which are used for interconnection with RF BGA components.

[0023] Compared with the prior art, the beneficial effects of the present invention are:

[0024] 1. The present invention is compatible with traditional PCB process flow. By changing a small process step, the performance of multilayer boards can be greatly improved. The prepared Ka-band six-layer microstrip board has excellent performance and high reliability.

[0025] 2. The plugging process of the present invention adopts selective laser grinding technology, which avoids the traditional contact method that requires the entire board to be ground, which easily causes the PTFE substrate to deform under stress, and eliminates expansion and contraction changes. It is particularly suitable for multi-layer microstrip boards with local plugging holes, solves the process problem of excessively large local board thickness differences, and also improves the inter-layer position accuracy of multi-layer boards;

[0026] 3. The selective laser grinding technology in the via plugging process of the present invention eliminates the damage to non-via plugging areas (patterns, substrates) caused by traditional whole-board grinding, improves the uniformity of the pattern layer and the thickness of the entire board, improves the welding reliability and yield rate of surface pads (via-in-pad plugging), and eliminates the impact of dimensional deformation caused by grinding on stencil matching;

[0027] 4. In addition to plug hole grinding, the laser grinding equipment in the present invention can also perform graphic production and shape processing. It is an environmentally friendly and multifunctional processing equipment, providing technical support for the flexible and reconfigurable production line of multi-layer microstrip boards. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 This is a schematic diagram of the structure of a six-layer Ka-band microstrip board according to an embodiment of the present invention;

[0029] Figure 2 This is a process flow chart of resin plugging according to an embodiment of the present invention;

[0030] Figures 3 to 19 Schematic diagram of the plug hole forming process according to an embodiment of the present invention, wherein:

[0031] Figure 3 Schematic diagram of substrate raw materials; Figure 4 Schematic diagram of drilling; Figure 5 Schematic diagram of hole metallization; Figure 6 Schematic diagram of resin plug hole; Figure 7 Schematic diagram of laser grinding; Figure 8 Schematic diagram of flatness and roughness test; Figure 9 It is a schematic diagram of plated clad copper; Figure 10 Bury holes for the core board; Figure 11 Schematic diagram of blind hole in upper board; Figure 12 Schematic diagram of blind hole in lower layer board; Figure 13 It is a semi-finished product of multi-layer board after lamination; Figure 14 It is a semi-finished product of multilayer board after plugging holes; Figure 15 This is a schematic diagram of a semi-finished multilayer board after laser grinding; Figure 16 It is a semi-finished multi-layer board coated with copper plating; Figure 17 This is a schematic diagram of a semi-finished multilayer board after back drilling; Figure 18 This is a schematic diagram of a semi-finished multilayer board after plugging, grinding, and plating; Figure 19 This is a schematic diagram of the finished multilayer board after graphic production, surface coating, and shape processing.

[0032] The numbers in the figure represent:

[0033] 1-Interconnection hole between lines 12 and 7; 2-Interconnection hole between lines 4 and 7; 3-3-layer PTFE substrate; 4-Circuit; 5-Adhesive material between the first layer of microwave copper-clad laminate and the second layer of microwave copper-clad laminate; 6-Plug resin in the back-drilled hole; 7-Circuit; 8-Circuit; 9-Circuit; 10-Circuit; 11-Plug resin in the metallized hole; 12-Circuit; 13-Electroplated copper layer on the plug resin; 14-Interconnection hole between lines 10 and 9. DETAILED DESCRIPTION

[0034] The above and other technical features and advantages of the present invention are described in more detail below with reference to the accompanying drawings.

[0035] As attached Figure 1 As shown, the six-layer Ka-band microstrip board includes: 3 layers of PTFE substrate 3, 6 layers of circuits (4, 8, 10, 9, 12 and 7), plug-hole metallized buried vias (interconnection hole 14 between lines 10 and 9), plug-hole metallized blind vias (interconnection hole 1 between lines 4 and 8, interconnection hole 1 between lines 12 and 7), plug-hole metallized through-holes (interconnection hole 2 between lines 4 and 7) and plug-hole back-drilled holes 6. The board only needs to be pressed once, in which the interconnection between lines 4 and 9 is achieved by back-drilling 12 and 7 through the through-holes of lines 4 and 7. Therefore, the board contains three plug-hole productions (buried vias and blind vias are counted as one time for a single core board). The plug-hole process flow is as follows Figure 2 The various types of metallization formation processes of Examples 1-2 are shown in FIG. Figures 3 to 19 The finished metalized hole contains top layer copper, hole plugging resin, and hole wall copper.

[0036] The substrate in Examples 1-2 is Ruilong's RS300B, with each layer being 0.254mm thick and the double-sided copper foil being 18μm thick. The prepreg is Ruilong's RNP280, with a thickness of 0.1mm, resulting in a total board thickness of 1.05mm. The plugged vias have two diameters: 0.25mm and 0.5mm. Through-hole 2 and the blind vias formed by backdrilling serve as grounding and signal transmission holes on the BGA pads, requiring 183°C tin-lead solder for reliable soldering between the solder balls and pads.

[0037] Example 1

[0038] The plugging resin is PHP-900IR-6P from San-Ei. The process parameters for resin screen printing are set as follows: screen printing pressure of 70kgf, screen printing speed of 30mm / s, and scraper pressing depth of 30mm. The other steps are the same as the comparative example. In a hot air oven, 75℃@45min+110℃@45min+150℃@60min. The laser source uses a 355nmNd:YVO4 ultraviolet laser with an output power of 10W, and adopts a contour line scanning method. The pulse width is set to 70ns, the spot radius at the focus is about 50μm, and the power density range of laser grinding is 5×10 4 W / cm 2 , with a scanning speed of 30 mm / s. After laser polishing, the residue is cleaned with an acid wash solvent and then dried in an oven at 130°C for 1 hour. The vias are randomly inspected for flatness using a laser microscope with an accuracy of at least 10 nm.

[0039] The flatness of the surface plugged vias of the Ka-band six-layer microstrip board produced by this process decreased from -23μm to 9μm, the surface roughness Ra decreased from 6.5μm to 3μm, the interlayer alignment accuracy increased from ±0.11mm to ±0.09mm, the board thickness tolerance decreased from ±8% to ±5%, and the yield increased from 80% to 90%, reducing the rework rate of surface-mount components. After three 288°C (10s) tin pot thermal shocks and 350 cycles of high and low temperature cycling tests (25°C to 150°C), the plugged via multilayer board showed no short circuits or open circuits, and no cracks or delamination between layers or in the metallized vias.

[0040] Example 2

[0041] The plugging resin is PP-2795 from Perters. The process parameters for resin screen printing are set as follows: screen printing pressure of 90kgf, screen printing speed of 15mm / s, and scraper pressing depth of 25mm. The other steps are the same as the comparative example. In a hot air oven, 60℃@30min+100℃@60min+150℃@60min. The laser source is a 1064nm Nd:YAG fiber laser with an output power of 30W, using a composite scanning method. The pulse width is set to 100ns, the spot radius at the focus is about 100μm, and the power density range of laser grinding is 10×10 4 W / cm 2 , with a scanning speed of 100 mm / s. After laser polishing, the residue is cleaned with an alkaline cleaning solvent and then dried in an oven at 110°C for 3 hours. The plugged vias are randomly inspected for flatness using a laser microscope with an accuracy of at least 10 nm.

[0042] The flatness of the surface plugged vias of the Ka-band six-layer microstrip board produced by this process decreased from 13μm to 9μm, the surface roughness Ra decreased from 7μm to 3μm, the interlayer alignment accuracy increased from ±0.12mm to ±0.09mm, the board thickness tolerance decreased from ±8% to ±5%, and the yield increased from 80% to 90%, reducing the rework rate of surface-mount components. After three 288°C (10s) tin pot thermal shocks and 350 cycles of high and low temperature cycling tests (25°C to 150°C), the plugged via multilayer board showed no short circuits or open circuits, and no cracks or delamination between layers or in the metallized vias.

[0043] Comparative Example

[0044] The microstrip board measures 320mm*320mm and is 3.0mm thick. 160 BGA components need to be soldered on the top layer, with pads measuring 0.5mm and a diagonal pad spacing of 425.5mm. The screen-printing stencil is mechanically drilled with apertures of 0.25mm and 0.5mm. Taiyo's THP-100DX is used for the via plugging resin. The resin screen printing process parameters are: 80kgf printing pressure, 20mm / s printing speed, and 20mm squeegee depth. The stencil and substrate are positioned using a CCD and mounted on the work surface of a vacuum via plugging machine. The via plugging resin is placed in front of the squeegee and printed according to the set process parameters. The vacuum is activated during printing to ensure that the resin fills the holes evenly. After plugging, the stencil is removed. The vias are machined in a hot air oven at 60°C for 100 minutes and 150°C for 60 minutes. The vias are polished using ceramic grinding, with a total of three passes, including the entire board. Repeated lapping caused irregular expansion and contraction of the microstrip board, resulting in dimensional changes. The pad position and stencil offset ranged from -0.1 to +0.15 mm, necessitating a redesign of the stencil. Five stencils were re-produced in increments of 0.05 mm. Before each run, the actual distance between diagonal pads was measured for stencil alignment. Because the expansion and contraction were irregularly distributed across the board, even with the new stencil, at least two BGA components per board still shifted after soldering, causing short circuits and requiring rework.

[0045] The above description is merely a preferred embodiment of the present invention and is intended to be illustrative rather than restrictive of the present invention. Those skilled in the art will appreciate that many changes, modifications, and even equivalents may be made to the present invention within the spirit and scope of the claims, all of which fall within the scope of protection of the present invention.

Claims

1. A laser grinding and plugging method for Ka-band multilayer microstrip boards, characterized in that: The following steps are involved: S1, vacuum plugging the designated holes on the substrate; S2, plugging resin curing; S3, laser grinding of the protruding resin excess and solution cleaning; S4, confirm the flatness and proceed with the subsequent plating process.

2. The laser grinding and plugging method for a Ka-band multilayer microstrip board according to claim 1, wherein: In step S1, the designated hole is a metallized hole or a non-metallized hole, the substrate material is a microwave copper-clad plate with a PTFE substrate, the adhesive material is a microwave prepreg with a dielectric constant of ≤3, the aperture of the designated hole is 0.1 mm to 1.0 mm, and the aspect ratio is ≤16:

1.

3. The laser grinding and plugging method for a Ka-band multilayer microstrip board according to claim 2, wherein: The microwave copper clad laminate is one of the materials RS300B of Ruilong Company, CF294 and CFG294 of China Electronics Technology Group Corporation 46, GNC3004 of Guoneng Company, and SJ9300 of Shengyi Technology, or a compound thereof to form a multilayer board. The copper foil of the copper clad laminate is 18μm electroplated copper foil. The microwave semi-cured sheet is selected from any one of CFB278F of China Electronics Technology Group Corporation 46, RNP280 of Ruilong Company, SJ928B of Shengyi Technology, and CN300BP of Guoneng New Materials.

4. The laser grinding and plugging method for a Ka-band multilayer microstrip board according to claim 1, wherein: In step S1, the plugging method is screen printing assisted vacuum plugging, and the holes of the screen printing screen are opened by one of the mechanical drilling method and the laser drilling method. The hole diameter is consistent with the hole diameter on the substrate; the process parameters of screen printing are set as follows: the screen printing pressure is 70kgf~100kgf, the screen printing speed is 15mm / s~30mm / s, and the scraper pressure depth is 20mm~30mm.

5. The laser grinding and plugging method for a Ka-band multilayer microstrip board according to claim 4, characterized in that: The screen printing process is as follows: the screen and substrate are positioned by CCD and installed on the work surface of the vacuum plugging machine, the plugging resin is placed in front of the scraper, and printing is performed according to the set process parameters. During the printing process, the vacuum is turned on to ensure that the resin evenly fills the holes. After plugging, the screen is removed.

6. The laser grinding and plugging method for a Ka-band multilayer microstrip board according to claim 5, characterized in that: The plugging resin is a thermosetting material system, and uses any one of San-Ei's PHP-900IR-6P, Perters' PP-2795 / PP-2794, and Taiyo's THP-100DX. The screen uses any one of aluminum alloy, stainless steel, and epoxy bakelite, with a thickness of 0.1mm to 0.3mm.

7. The laser grinding and plugging method for a Ka-band multilayer microstrip board according to claim 1, wherein: In step S2, the curing process is to place the substrate in a hot air oven, and the curing parameters are in accordance with the recommended process curve of the resin for plugging the vias, with the maximum curing temperature not exceeding 150° C. and the curing time not exceeding 3 hours.

8. The laser grinding and plugging method for a Ka-band multilayer microstrip board according to claim 1, wherein: In step S3, the laser grinding equipment is an integrated automated CNC machine tool integrating light, machinery and electricity, and the laser grinding equipment includes a laser and its optical path system, a mechanical system, a computer and software control system, a dust collection system and a water cooling system; the laser grinding adopts a scanning mode, and any one of filling scanning, contour scanning and compound scanning is selected; the laser light source is one of 1064nm, 355nm and excimer laser, the power is 10W-50W, the pulse width is 50ns-140ns, the spot radius at the focus is about 20μm-1000μm, and the power density of the laser grinding is 3×10 4 W / cm 2 ~10×10 4 W / cm 2 , scanning speed is 10mm / s-100mm / s.

9. The laser grinding and plugging method for a Ka-band multilayer microstrip board according to claim 1, wherein: In step S3, after laser grinding, the residue is cleaned with a solvent and dried in an oven. The cleaning solvent is selected from either acid washing or alkaline washing. Afterwards, the residue is rinsed with deionized water. The drying temperature is 110° C.-130° C. and the drying time is 1 h-3 h.

10. The laser grinding and plugging method for a Ka-band multilayer microstrip board according to claim 1, wherein: In step S4, the plug holes on the substrate are randomly inspected for flatness using a laser microscope with an accuracy of more than 10 nm. For plug holes that meet the flatness requirements, full-board electroplating is performed according to design requirements or subsequent processing steps are directly performed.

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