A plastic package for a MEMS pressure sensor and a method of manufacture

Abstract

This invention relates to the field of microelectronic packaging technology, specifically to a plastic housing for MEMS pressure sensor packaging and its manufacturing method. The proposed plastic housing design and manufacturing method includes the lead / frame material and design structure, and the epoxy resin plastic housing material and design structure. The plastic housing structure design and manufacturing method proposed in this invention solves the problem of the influence of stress / strain on the sensor output performance when the bonding surface of the traditional pre-manufactured shell MEMS piezoresistive pressure sensor is epoxy resin plastic, under temperature and humidity changes, thus improving the output accuracy and stability of the MEMS piezoresistive pressure sensor.

Description

Technical Field

[0001] This invention relates to the field of microelectronic packaging technology, and in particular to a plastic housing for packaging MEMS pressure sensors and its manufacturing method. Background Technology

[0002] MEMS pressure sensors are miniature devices that convert pressure signals into electrical signals. Their core component is a micro-machined silicon thin film. When external pressure is applied to the silicon thin film, the silicon thin film undergoes a tiny deformation. An ASIC conditioning chip, which is packaged together with the MEMS pressure core, amplifies, filters, and tunes this tiny deformation before converting it into an electrical signal output. The MEMS pressure sensor mentioned in this specification specifically refers to a MEMS piezoresistive pressure sensor.

[0003] MEMS piezoresistive pressure sensors work by using the piezoresistive effect of a silicon thin film to sense minute deformations. However, stress / strain in the packaging can affect these minute deformations. Therefore, packaging determines the output reliability and manufacturing cost of MEMS piezoresistive pressure sensors, and is key to their practical application and commercialization.

[0004] MEMS piezoresistive pressure sensor packages are typically diverse in type. Based on manufacturing methods, they mainly include integral plastic encapsulation, pre-mold pre-manufactured housing, and others such as COB (Chip On Board) + dam or cap type.

[0005] Pre-Mold is the most common and widely used type of pre-manufactured housing. In pre-Mold housings, epoxy resin is usually the preferred housing material. The pre-manufacturing process involves wrapping the lead frame with epoxy resin to form a specific chamber structure. During encapsulation, adhesive materials are used to bond the MEMS piezoresistive pressure sensor and the conditioning ASIC chip together to the bonding surface of the pre-manufactured housing chamber. However, the epoxy resin material of the bonding surface of the pre-manufactured housing chamber will generate complex stress / strain when the temperature and humidity change. These complex stress / strains act directly on the pressure sensor, thereby affecting the signal output of the silicon thin-film piezoresistive bridge and causing sensor failure. Summary of the Invention

[0006] To address the aforementioned sensor failure issues, this invention proposes a novel Pre-Mold prefabricated plastic tubular shell structure design concept and manufacturing method. The plastic tubular shell designed and manufactured using this concept, after being packaged and calibrated as a MEMS piezoresistive pressure sensor, exhibits significantly improved sensor output accuracy and stability compared to MEMS piezoresistive pressure sensors packaged with traditional plastic tubular shell structures.

[0007] The failure of the aforementioned sensor was caused by a large discrepancy in the coefficient of thermal expansion (CTE) between the epoxy resin plastic and the silicon-based chip. The CTE of the silicon-based chip is 2.6–4.0 ppm / ℃, while the CTE of the epoxy resin plastic is 6–17 ppm / ℃ below the Tg value and 27–86 ppm / ℃ above the Tg value, where Tg is the glass transition temperature.

[0008] To address the issue of large discrepancies in the coefficient of thermal expansion (CTE) between epoxy resin plastics and silicon-based chips, the technical solution of this invention is as follows: A plastic housing for encapsulating a MEMS pressure sensor includes: a lead frame and an epoxy resin plastic housing, and its structural design concept is as follows: The lead frame is preferably made of Cu alloy, specifically a copper-iron-phosphorus alloy, with Fe content of 2.1% to 2.6% by weight, P content of 0.015% to 0.15% by weight, Zn content of 0.05% to 0.20% by weight, and the remainder being Cu. Sometimes, depending on the Cu alloy manufacturer, a small amount of Pb element may be added to the Cu alloy, with Pb content not exceeding 0.01% by weight.

[0009] Furthermore, the lead frame is made of PMC194 copper, a copper-iron-phosphorus alloy. In addition to meeting the above-mentioned content requirements, its coefficient of thermal expansion is only 16.7 ppm / ℃, which is lower than the coefficient of thermal expansion of 17.6 ppm / ℃ of other copper alloys used in the manufacture of lead frames.

[0010] Furthermore, the lead frame has a thickness of 0.2mm ± 0.005mm.

[0011] Furthermore, the lead frame has a structure consisting of two parts: the first part includes a chip bonding platform; the second part includes inner pins, outer connecting ribs, and outer pins; the first part and the second part are connected by bending connecting ribs.

[0012] Furthermore, the inner pins of the lead frame are designed with small round holes at the junction of the inner pins and the epoxy resin plastic shell.

[0013] Furthermore, the lead frame is manufactured using a combination of etching and punching. After etching, the bending ribs connected to the first part of the chip bonding platform are then punched on the back to form an upward convex structure. At the same time, in order to ensure the flatness of the chip bonding platform, coing hardening treatment is performed at the bending points of the ribs.

[0014] Furthermore, after the aforementioned structure of the lead frame is manufactured, the inner tube foot is electroplated with Ag, with an Ag layer thickness of 2.5μm to 7.5μm. The other two sides, one end face, and the upper surface of the inner tube foot are not plated with Ag. After the outer connecting rib and the outer tube foot are manufactured, the entire surface is electroplated with pure Sn.

[0015] Furthermore, optionally, when the lead frame serves as the absolute / gauge pressure signal output carrier, no through hole is required in the center of the chip bonding area of ​​the first part of the convex structure chip bonding platform; when it serves as the differential pressure signal output, an through hole is required in the center of the chip bonding area of ​​the first part of the convex structure chip bonding platform as a pressure input path.

[0016] The epoxy resin plastic shell is made of low-stress epoxy resin plastic, which has the following characteristics compared to the epoxy resin plastic used in traditional plastic pipe shells.

[0017] Feature 1: The filler has a small sieve particle size, specifically 15-25 μm, while the sieve particle size of epoxy resin plastic filler used in traditional plastic pipe shells is 50-70 μm.

[0018] Feature 2: High glass transition temperature (Tg) value, specifically 145℃, while the glass transition temperature (Tg) value of epoxy resin plastic used in traditional plastic pipe shells is 125℃.

[0019] The third characteristic is that the filler content accounts for a small percentage of the total weight of the component, specifically, its filler content is <45%, while the filler content of epoxy resin plastic used in traditional plastic pipe shells is required to be >75%. A suitable filler ratio is a necessary condition for achieving low-stress molding resin.

[0020] Furthermore, the epoxy resin plastic shell has an upper surface feature and a lower surface feature.

[0021] Furthermore, optionally, when the epoxy resin plastic tube shell is used as an absolute pressure signal output carrier, its upper surface features are designed as a planar sealing structure. In this case, air inlet 1 and air inlet 2 do not need to be designed during the manufacturing of the epoxy resin plastic tube shell.

[0022] Furthermore, optionally, when the epoxy resin plastic tube shell is used as a differential pressure signal output carrier, its upper surface features are designed as a double boss and a groove that can be connected to the double air nozzle structure. After applying glue to the groove area, the plastic tube shell can be connected to the double air nozzle structure. In order to ensure the bonding direction of the double air nozzle structure, a chamfer mark is designed at one corner of the double boss on the upper surface features.

[0023] Furthermore, optionally, when the epoxy resin plastic tube shell is used as a pressure signal output carrier, its upper surface features are designed as a double boss and a groove that can be connected to the double air nozzle structure. After the groove area is glued, the plastic tube shell can be connected to the double air nozzle structure. In order to ensure the bonding direction of the double air nozzle structure, a chamfer mark is designed at one corner of the double boss on the upper surface features. A small hole is designed on the upper surface of the single air nozzle structure to communicate with the atmosphere.

[0024] Furthermore, the lower surface of the epoxy resin plastic shell, viewed from bottom to top, has a three-step structure, namely step 1, step 2, and step 3.

[0025] Furthermore, when the epoxy resin plastic tube shell is used as the absolute pressure signal output carrier, the step 1 on the lower surface will serve as the bearing platform for the absolute pressure cover plate. After encapsulation, the absolute pressure cover plate will be designed with pores as a pressure entry passage.

[0026] Furthermore, the aforementioned lower surface feature step 2 serves as the bearing plane of the inner pin of the lead frame. The surface of step 2 is on the same plane as the lower surface of the inner pin. During injection molding, the upper surface and two sides of the inner pin are wrapped with epoxy resin molding compound, and only the Ag-plated area on the lower surface of the inner pin is exposed and distributed on the plane of step 2.

[0027] Furthermore, the lower surface feature step 3, the lower surface of the upper convex structure of the lead frame and the surface of step 3 are on the same plane. During packaging, the MEMS pressure core will be directly attached to the lower surface of the upper convex structure through adhesive material.

[0028] Furthermore, optionally, when the epoxy resin plastic tube shell is used as a differential pressure signal output carrier, the aforementioned lower surface feature step 3 has a through hole designed in a specific area of ​​step 3 as a pressure input passage. This through hole is concentric with the through hole of the lead frame and the air column hole of the upper double air nozzle structure.

[0029] A method for manufacturing a plastic housing for a MEMS pressure sensor package includes the following steps: S1. A semi-finished lead frame with copper strip etching, bending ribs, and small round holes for locking adhesive. S2, reverse stamping to form an upward convex structure and coing hardening of the bending connecting ribs; S3. Silver plating is applied only to the upper surface of the inner tube feet; S4. The semi-finished tube shell is injection molded using low-stress epoxy resin. S5. Shield and protect the inner pins, and plate the outer pins with nickel first, then tin, and anneal to obtain the finished product.

[0030] Furthermore, before injection molding in step S4, release adhesive is applied to the inner wall of the mold, and the injection pressure and holding time are controlled until the resin completely fills the locking holes to form a rivet locking structure.

[0031] According to the above-described technical solution of the present invention, its beneficial effects are at least manifested in the following aspects: (1) A small round hole is designed at the junction of the inner pin and the epoxy resin plastic shell. During the pre-manufacturing process, the molten epoxy resin molding material flowing into the small round hole will become small cylinders. These small cylinders will exhibit a rivet effect to firmly lock the lead frame, ensuring that there will be no interface gap between the lead frame and the epoxy resin plastic shell. (2) Epoxy resin molding compounds have a higher glass transition temperature and a wider range below Tg value, resulting in better stability when subjected to temperature and humidity changes. (3) The smaller sieve particle size of epoxy resin molding compound allows small particles to better fill the gaps between large particles, achieving "close packing", thereby increasing the filling amount, reducing the CTE thermal expansion coefficient, and enhancing thermal conductivity. (4) The lower surface of the convex structure of the lead frame and the surface of step 3 are on the same plane. During packaging, the MEMS pressure core and ASIC conditioning chip are bonded to the lower surface of the floating chip platform on the lead frame by the adhesive material. This structural design avoids the design drawback of directly bonding the MEMS piezoresistive pressure core to the epoxy resin molding compound. (5) The standard flatness of the bonding surface of the conventional Pre-Mold pre-manufactured plastic-sealed tube chamber is ±30μm, while the standard flatness of the PMC194 copper plate is ±3μm. After encapsulation and patching, the flatness of the silicon film of the MEMS pressure core is greatly improved, which further improves the overall output accuracy of the sensor. Attached Figure Description

[0032] Figure 1 A schematic diagram of a lead frame structure for traditional plastic tube shells; Figure 2 A three-dimensional view of the front of the differential pressure type plastic tube shell of the present invention; Figure 3 A three-dimensional structural diagram of the back of the differential pressure type plastic tube shell of the present invention; Figure 4 A plan view of the semi-finished product formed by etching the lead frame of this invention; Figure 5 The structural diagram of the lead frame after stamping, bending and hardening of this invention; Figure 6 The cross-sectional structure diagram of the injection-molded tube shell of this invention.

[0033] Among them, 101 is the outer pin; 102 is the inner pin; 103 is the outer connecting rib; 104 is the small round hole of the inner pin; 105 is the air inlet 1; 106 is the bending connecting rib; 107 is the upper convex bonding plane; 108 is the air inlet 2; 2 is the epoxy resin plastic shell; 201 is the boss 1; 202 is the boss 2; 203 is the groove; 204 is the chamfer mark; 205 is the air inlet 2; 206 is the lower surface feature step 1; 207 is the lower surface feature step 2; 208 is the lower surface feature step 3 through hole; 209 is the lower surface feature step 3; 210 is the air inlet passage 2; 211 is the air inlet passage 1. Detailed Implementation

[0034] It should be noted that when a component is referred to as "fixed to" or "set on" another component, it can be located directly or indirectly on the other component; when a component is referred to as "connected to" another component, it can be directly or indirectly connected to the other component.

[0035] To make the technical solutions and advantages of the present invention clearer, the present invention will be further described in detail below with reference to specific embodiments and the accompanying drawings. The same function is represented by the same number in the drawings. It should be noted that the terms "front," "rear," "left," "right," "up," and "down" used in the description of the present invention refer to directions in the accompanying drawings.

[0036] In the description of this application, it should be noted that the terms "first", "second", and "third" are used only to describe the purpose of the application and should not be construed as indicating or implying relative importance.

[0037] To address the issue of sensor failure caused by the MEMS pressure core being directly bonded to the epoxy resin plastic chamber during MEMS piezoresistive pressure sensor packaging, this invention proposes solutions from two aspects: the properties of epoxy resin materials and the design of the plastic shell structure.

[0038] This embodiment uses a differential pressure MEMS piezoresistive sensor encapsulated in a plastic tubing, such as... Figure 2 , Figure 3As shown, it consists of two main components: a lead frame and an epoxy resin plastic shell 2. The lead frame is divided into two areas: the first area is the chip bonding platform 107, and the second area consists of inner pins 102, outer connecting ribs 103, and outer pins 101. The two areas are connected by bending connecting ribs 106. The chip bonding platform 107 is formed into a convex structure by back-side punching. The MEMS pressure core and ASIC conditioning chip are bonded to the lower surface of the convex bonding plane 107 in reverse using adhesive material, completely detached from the epoxy resin shell, thus avoiding the direct impact of epoxy temperature stress on the silicon thin film. The epoxy resin plastic shell 2 covers the lead frame and has a three-step structure on the upper and lower surfaces. The lower surface of the housing consists of three-tiered steps: step 1 (206), step 2 (207), and step 3 (209). Step 2 (207) supports all the inner pins 102 and is flush with the lower surface of the inner pin. Step 3 (209) is coplanar with the lower surface of the upper convex bonding plane 107, providing a flat metal bearing surface for chip bonding. A small round hole 104 is opened at the joint position between the inner pin 102 and the epoxy resin housing 2. The injection-molded epoxy resin fills the small round hole to form a columnar rivet structure, realizing the locking of the lead frame and the housing and eliminating the interface delamination gap.

[0039] like Figure 4 As shown, the leadframe substrate is made of copper-iron-phosphorus PMC194 copper alloy, with the following composition by weight percentage: Fe 2.1%~2.6%, P 0.015%~0.15%, Zn 0.05%~0.20%, balance Cu; impurities Pb≤0.01%; the plate thickness is strictly controlled at 0.2mm±0.005mm; the coefficient of thermal expansion of this alloy is 16.7ppm / ℃, lower than the conventional leadframe copper alloy of 17.6ppm / ℃, thus reducing the CTE difference with the silicon core. It is prepared using an etching + back-side punching composite process: 1) After cleaning and drying the copper strip, photosensitive emulsion is coated, followed by exposure, development, etching, and removal of the photosensitive emulsion to obtain... Figure 4 1) The semi-finished product outline is etched to form the outer pin 101, inner pin 102, outer connecting rib 103, bending connecting rib 106, and locking small round hole 104; 2) The etched semi-finished product is placed in reverse on a press, and a punch is used to press the back of the bending connecting rib 106 area, lifting it to form an upward convex bonding plane 107; at the same time, the bending connecting rib 106 is subjected to coing extrusion hardening treatment to eliminate bending stress and ensure that the flatness of the upward convex bonding plane reaches ±3μm, which is far superior to the ±30μm of traditional plastic-sealed cavities. After bending and forming, the lead frame is only electroplated with silver on one side of the lower surface of the inner pin 102, with a silver plating layer thickness of 2.5μm~7.5μm; the upper surface, two sides, and end face of the inner pin are not exposed with silver layer, which is suitable for the dual requirements of upper bonding and bottom bonding after reverse mounting. After injection molding, the inner pins are protected by a shielding fixture. The outer connecting ribs 103 and the outer pins 101 are then electroplated with a nickel barrier layer and a pure tin plating layer. After tin plating, the fixture is removed and the tin layer is annealed to suppress copper-tin interdiffusion, avoid tin whiskers, and improve the reliability of external soldering.

[0040] The shell 2 is made of customized low-stress epoxy resin, which is different from traditional molding compound. 1) The filler particle size is 15-25μm (traditional 50-70μm), and the fine particles are densely packed to reduce the overall thermal expansion; 2) The glass transition temperature Tg=145℃ (traditional 125℃), which widens the low-temperature to medium-temperature stable range; 3) The weight ratio of inorganic filler is <45% (traditional >75%), which reduces the rigidity of the resin and reduces the thermal shrinkage stress. The upper surface is provided with boss 1 (201) and boss 2 (202), and an adhesive groove 203 is opened between the two bosses for bonding the double air nozzles; a chamfered positioning mark 204 is set on one corner of the boss to prevent fooling. The shell is provided with air inlet 1 (105) and air inlet 2 (205), which are connected to air inlet passage 1 (211) and air inlet passage 2 (210) respectively.

[0041] The complete manufacturing process of the plastic tube shell is as follows: S1. After cleaning and drying the copper strip, a photosensitive material is coated on its surface. The photoresist is exposed using a mask, developed, etched, and excess photosensitive material is removed. The strip is then cleaned and dried again to obtain the final product. Figure 4 Semi-finished lead frame with bending connecting ribs, small round holes for locking the inner tube pins, and chip bonding platform; S2. Place the pre-manufactured lead frame upside down on the press and stamp out the upward convex structure using a stamping punch. Perform coking hardening treatment at the bending ribs. This yields a lead frame with an upward convex structure. Figure 5 The structure; S3. Clean the stamped lead frame and electroplate a silver layer with a thickness of 2.5μm to 7.5μm only on the inner tube foot surface, and cover the rest of the area without silver layer. S4. The inner wall of the injection mold is pre-coated with release adhesive evenly; the silver-plated lead frame is fixed to the mold cavity and the mold is closed; the low-stress epoxy resin is heated and melted, and injected into the cavity. The injection pressure and holding time are controlled to ensure that the molten resin completely fills all the locking small round holes 104. After cooling, the resin forms cylindrical rivets in the holes to lock the lead frame and the shell; after the resin has cooled sufficiently, the plastic tube shell semi-finished product is removed. S5. Use a shielding fixture to protect the inner pins of the housing. First, plate a nickel barrier layer on the exposed outer pins of the lead frame, then electroplat a pure tin layer. After electroplating, remove the fixture and anneal the tin layer to obtain the finished plastic housing.

[0042] The above description is only one of the most commonly used embodiments of the present invention. This embodiment has 10 pins, which can meet the needs of most MEMS pressure sensors for analog or digital output bonding. The above differential pressure signal output embodiment is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A plastic tubular shell for encapsulating a MEMS pressure sensor, characterized in that, The product includes a lead frame and an epoxy resin plastic shell; the lead frame is provided with a chip bonding platform and inner pins and outer pins, and the chip bonding platform and the inner pins are connected by bending ribs; the epoxy resin plastic shell covers the lead frame and forms an upper surface and a lower surface structure, characterized in that: The chip bonding platform is punched to form an upward convex structure. The MEMS pressure core and the ASIC conditioning chip are bonded in reverse to the lower surface of the upward convex structure. The lower surface of the housing is provided with three-level stepped steps: step 1, step 2, and step 3. Step 2 supports the inner pins, and step 3 is coplanar with the lower surface of the upward convex structure. A small round hole for locking glue is opened at the junction of the inner pins of the lead frame and the epoxy resin plastic housing. During injection molding, the molten epoxy resin fills the small round hole to form a rivet locking glue structure.

2. The plastic tubing shell for MEMS pressure sensor packaging according to claim 1, characterized in that, The lead frame is made of a copper-iron-phosphorus alloy, with the following weight percentages: Fe 2.1%~2.6%, P 0.015%~0.15%, Zn 0.05%~0.20%, and the balance being Cu; the Pb content in the alloy is ≤0.01%; the overall thickness of the lead frame is 0.2mm±0.005mm.

3. The plastic tubing shell for MEMS pressure sensor packaging according to claim 2, characterized in that, The lead frame is formed by a composite process of etching and back-side punching; the bending and connecting ribs are hardened by Coing to ensure the flatness of the chip bonding platform; the lead frame is made of PMC194 copper with a coefficient of thermal expansion of 16.7ppm / ℃.

4. The plastic tubing shell for MEMS pressure sensor packaging according to claim 1, characterized in that, The inner pins are only electroplated with a silver layer of 2.5μm to 7.5μm thickness on the lower surface, and the other surfaces of the inner pins are not plated with silver. After injection molding, the outer connecting ribs of the lead frame and the outer pins are plated with a nickel barrier layer and a pure tin plating layer in sequence, and then annealed after the tin layer is completed.

5. The plastic tubing housing for MEMS pressure sensor packaging according to claim 1, characterized in that, The epoxy resin plastic shell is made of low-stress epoxy resin, with filler particle size of 15-25 μm, Tg=145℃, and filler weight ratio <45%.

6. The plastic tubing housing for MEMS pressure sensor packaging according to claim 1, characterized in that, The upper surface of the housing is divided into three types of structures: the absolute pressure type is a flat seal without air inlet; the differential pressure and gauge pressure types are equipped with double bosses, adhesive grooves and anti-foolproof chamfers, the differential pressure type is equipped with double air inlets, and the gauge pressure type has a single hole to connect to the atmosphere.

7. The plastic housing for MEMS pressure sensor packaging according to claim 6, characterized in that, Step 1 is the bearing platform of the absolute pressure cover plate; Step 2 is flush with the lower surface of the inner tube foot, with only the silver-plated lower surface of the inner tube foot exposed on Step 2; Step 3 of the differential pressure type is provided with a through hole, which is coaxial and concentric with the through hole of the lead frame and the air nozzle hole on the upper surface.

8. The plastic housing for MEMS pressure sensor packaging according to claim 4, characterized in that, When adapting to absolute pressure and gauge pressure sensors, there are no through holes in the bonding area of ​​the upper convex structure; when adapting to differential pressure sensors, a pressure input through hole is opened in the center of the upper convex structure.

9. A method for manufacturing a plastic housing for encapsulating a MEMS pressure sensor according to any one of claims 1 to 8, characterized in that, Includes the following steps: S1. A semi-finished lead frame with copper strip etching, bending ribs, and small round holes for locking adhesive. S2, reverse stamping to form an upward convex structure and coing hardening of the bending connecting ribs; S3. Silver plating is applied only to the upper surface of the inner tube feet; S4. The semi-finished pipe shell is injection molded using the low-stress epoxy resin as defined in claim 5. S5. Shield and protect the inner pins, and plate the outer pins with nickel first, then tin, and anneal to obtain the finished product.

10. The manufacturing method according to claim 9, characterized in that, In step S4, before injection molding, release adhesive is applied to the inner wall of the mold. The injection pressure and holding time are controlled until the resin completely fills the small round holes to form a rivet locking structure.

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