A low-stress packaging method for pressure-sensitive elements

Abstract

This invention provides a low-stress encapsulation method for pressure-sensitive elements, comprising the preparation of a pad body (1), physical and electrical connection between the pressure-sensitive element (5) and the pad body (1), and a molding process that releases the sensitive diaphragm area (5a) of the pressure-sensitive element. This invention features simple steps, convenient implementation, avoids direct contact and compression of the sensitive diaphragm by the molding compound, prevents molding compound sticking or contamination during demolding, significantly reduces thermal and mechanical stress transmission, is compatible with traditional molding production lines, is simple in steps, low in cost, suitable for mass production, significantly reduces encapsulation stress, has low zero-point drift, high precision, and excellent long-term stability.

Description

Technical Field

[0001] This invention relates to the field of sensor packaging technology, specifically a low-stress packaging method for pressure-sensitive elements. Background Technology

[0002] Microelectromechanical systems (MEMS) pressure sensors have become core components in the field of pressure detection due to their advantages of small size, excellent sensitivity, low power consumption, and ease of integration and mass production. Plastic encapsulation has become the most mainstream packaging form due to its low cost, mature technology, and suitability for automated mass production.

[0003] However, the traditional plastic encapsulation structure of pressure-sensitive elements has many unavoidable stress defects, which seriously affect the performance of the sensor. At present, most conventional pressure-sensitive elements adopt the traditional plastic encapsulation process. The chip is directly attached to the metal frame by adhesive, and an integrated encapsulation shell is used to complete the encapsulation.

[0004] This packaging structure has significant technical defects: First, the molding and adhesive curing processes generate irreversible residual stress. Simultaneously, temperature changes in the device's operating environment cause a mismatch in the thermal expansion coefficients between the packaging shell, adhesive, and silicon chip, continuously generating additional stress. This stress is directly transmitted to the pressure-sensitive membrane area, leading to static deformation, zero-point drift, and sensitivity shift, significantly reducing sensor measurement accuracy. Second, existing sensor membranes are mostly directly fixed structures without independent buffer protection, making them vulnerable to damage during equipment transportation and under vibration. Under dynamic and instantaneous impact loads, vibration and impact energy will directly act on the ultra-thin sensitive film, which can easily cause micro-damage and deformation failure of the sensitive film, resulting in poor anti-interference ability and environmental adaptability of the sensor. Third, existing packaging mostly adopts full-area bonding and fixation of the chip bottom, and the stress between the lead frame and the plastic package is unobstructed and can be directly transmitted to the chip force-sensitive film, resulting in significant stress concentration. Fourth, the mechanical stress generated by external vibration and impact has no buffering link and acts directly on the chip and bonding parts, which can easily lead to bonding point detachment and chip cracking, reducing the reliability and service life of the sensor.

[0005] The aforementioned stress issues severely limit the application of encapsulated pressure-sensitive elements in high-precision, wide-temperature-range, and harsh operating conditions. Therefore, developing a low-stress encapsulation structure for pressure-sensitive elements that can effectively isolate and release encapsulation stress while retaining the advantages of low cost and easy mass production is a technical challenge that urgently needs to be solved in this field. Summary of the Invention

[0006] The present invention aims to overcome the shortcomings of the prior art by providing a low-stress encapsulation method for pressure-sensitive elements.

[0007] This application provides the following technical solution: A low-stress packaging method for a pressure-sensitive element, characterized by comprising the following steps: Step S1: Take a copper alloy plate as the pad body. The pad body is provided with several packaging units. The packaging unit includes a first lead frame semi-etched groove on the bottom surface of the pad body in a ring shape. A pair of second lead frame semi-etched grooves are provided on the bottom surface of the pad body outside the lead frame semi-etched groove. The second lead frame semi-etched grooves correspond to the positions of the Pad points. The surface of the pad body is roughened. The first lead frame semi-etched grooves between adjacent packaging units are connected by semi-etched cantilever beams. Step S2: A pressure-sensitive element is connected to the inner side of the first lead frame half-etched groove on the front side of the pad body; Step S3: Use gold wire ball bonding to form an electrical connection between the pressure-sensitive element and the Pad point, and the bonding arc height of the gold wire leads is 170-180um; Step S4: The upper template is provided with protrusions that correspond to the pressure-sensitive elements in each packaging unit, and the protrusions are provided with relief grooves that correspond to the sensitive diaphragm areas of the pressure-sensitive elements. A release film layer is covered on the upper template, the protrusions and the relief grooves. Step S5: Place the protrusion on the pressure-sensitive element so that the relief groove covers the sensitive diaphragm area of ​​the pressure-sensitive element, and then perform molding encapsulation. The molding compound only covers the non-sensitive area of ​​the pressure-sensitive element. Due to the presence of the protrusion and the relief groove, the sensitive diaphragm area of ​​the pressure-sensitive element remains suspended after molding. Step S6: After molding and curing, remove the molded frame and perform post-curing and stress-relieving annealing at a curing temperature of 175℃ for 8 hours. Step S7: Slice, pick, and visually inspect several packaging units to obtain the finished low-stress pressure sensor.

[0008] Based on the above technical solution, the following further technical solutions are also possible: The release film layer is a high-temperature resistant polyimide release film.

[0009] The depth of the semi-etched groove of the lead frame is 30%-60% of the thickness of the pad body.

[0010] The molding compound is a stress-curing epoxy molding compound with a curing temperature of 175℃, a curing shrinkage rate of ≤0.3%, and an elastic modulus of ≤15GPa.

[0011] The height of the protrusion is ≥0.2mm.

[0012] The roughening process in step S1 is a single-sided roughening process on the front side of the pad body, which includes the following steps: a) Pretreatment: alkaline degreasing, overflow water washing, acid pickling activation, and pure water washing in sequence; b) Backside film protection: Dry film hot pressing and cooling are performed sequentially. The steps are required to ensure that the film is free of bubbles, has no curling edges, and fully covers the solder pad area. c) Front (i.e. plastic cover) roughening: roughening and multi-stage water washing are performed in sequence. The roughening process is any one of micro-etching, browning or organic acid ultra-roughening. After roughening, the roughness of the pad body is 0.10 to 0.35 μm. d) Stripping: Sequentially perform alkaline stripping solution to remove the dry film on the back side, overflow water washing, and pure water washing; e) Post-treatment: Perform antioxidant treatment and pure water washing in sequence; f) Drying: Dry with hot air at 80–120℃ for 10–30 minutes.

[0013] Advantages of the invention: This invention features simple steps and convenient implementation. After encapsulation, the sensitive area of ​​the pressure-sensitive element can avoid the molding compound, preventing direct contact and compression of the sensitive diaphragm, thus structurally eliminating stress sources. Release film isolation and protection prevent molding compound from sticking or contaminating during demolding, improving encapsulation yield and the consistency of the pressure-sensitive element. Low-stress frame pads significantly reduce thermal and mechanical stress transmission through semi-etching. The process is compatible with traditional molding production lines, is simple, low-cost, and suitable for mass production. Encapsulation stress is significantly reduced, resulting in small zero-point drift, high precision, and excellent long-term stability. Attached Figure Description

[0014] Figure 1 This is a schematic diagram of the bottom surface of the pad body in a single packaging unit: Figure 2 A schematic diagram of the structure when encapsulating a pressure-sensitive element; Figure 3 This is a schematic diagram of the packaged structure. Detailed Implementation

[0015] like Figure 1-3 As shown, a low-stress packaging method for a pressure-sensitive element is characterized by comprising the following steps: Step S1: Take a copper-nickel alloy plate as the pad body 1. The pad body 1 is provided with several packaging units. The packaging unit includes a first lead frame semi-etched groove 3 on the bottom surface of the pad body 1. The depth of the lead frame semi-etched groove 3 is 30% of the thickness of the pad body 1.

[0016] A pair of second lead frame semi-etched grooves 4 are provided on the bottom surface of the pad body 1 outside the lead frame semi-etched groove 3. The front of the pad body 1 in the area of ​​the pad body 1 sandwiched by the two second lead frame semi-etched grooves 4 serves as the position of the Pad point.

[0017] The surface of the pad body 1 is roughened. The roughening process is a single-sided roughening process on the front side of the pad body 1. It includes the following steps: a) Pretreatment: Alkaline degreasing, overflow water washing, acid pickling activation, and pure water washing are performed in sequence.

[0018] b) Backside film protection: Dry film hot pressing is performed in sequence, and the film is left to cool and set at room temperature (22-28℃) in a clean room. The steps are as follows: no air bubbles, no curling edges, and full coverage of the solder pad area.

[0019] c) Front surface (i.e. plastic cover) roughening: roughening and multi-stage water washing are performed in sequence. The roughening process is micro-etching. After roughening, the roughness of the front surface of the pad body 1 is 0.20μm.

[0020] d) Stripping: The dry film on the back side is removed by alkaline stripping solution, followed by overflow rinsing and pure water rinsing.

[0021] e) Post-treatment: Anti-oxidation treatment is performed sequentially, that is, the bottom surface of the pad body 1 is immersed in BTA / TTA special passivation solution to generate a nano-organic protective film, which prevents oxidation and discoloration, maintains roughening adhesion, and maintains the solderability of the pad. Then, it is washed with pure water.

[0022] f) Drying: Dry with hot air at 90℃ for 20 minutes.

[0023] The first lead frame semi-etched grooves 3 between adjacent packaging units are connected by semi-etched cantilever beams 3a.

[0024] Step S2: A pressure-sensitive element 5 is attached to the inner side of the first lead frame semi-etched groove 3 on the front side of the pad body 1 by adhesive bonding. Low-stress conductive adhesive 2 is used for mounting, and the adhesive is cured at 125°C for 60 minutes.

[0025] Step S3: The pressure-sensitive element 5 is electrically connected to the pad body 1Pad by gold wire ball bonding, and the bonding arc height of the gold wire lead 10 is 170-180um.

[0026] Step S4: Extend protrusions 7 from the upper template 6, corresponding to the pressure-sensitive elements 5 within each encapsulation unit. The protrusions 7 are inverted trapezoidal structures, and their height is ≥0.2mm. Mill relief grooves 7a corresponding to the sensitive diaphragm areas 5a of the pressure-sensitive elements onto the lower surface of the protrusions 7. Cover the lower surface of the upper template 6, the protrusions 7, and the relief grooves 7a with a release film layer 8, which is a high-temperature resistant polyimide release film.

[0027] Step S5: Place the protrusion 7 on the pressure-sensitive element 5, so that the clearance groove 7a covers the sensitive diaphragm area 5a of the pressure-sensitive element. Then, perform molding encapsulation. The molding compound 9 only covers the non-sensitive area of ​​the pressure-sensitive element 5. Due to the presence of the protrusion 7 and the clearance groove 7a, the sensitive diaphragm area 5a of the pressure-sensitive element remains suspended after molding. The molding compound 9 is a stress-curing epoxy molding compound with a curing temperature of 175℃, a curing shrinkage rate ≤0.3%, and an elastic modulus ≤15GPa.

[0028] Step S6: After molding and curing, remove the upper template 6 and take out the molded frame. Then, perform post-curing and stress-relieving annealing at a curing temperature of 175℃ for 8 hours.

[0029] Step S7: Slice, pick, and visually inspect several packaging units to obtain the finished low-stress pressure sensor.

[0030] Tests have shown that the encapsulation stress in this embodiment is significantly reduced compared to traditional plastic encapsulation, the zero-point temperature drift is significantly improved, and the long-term stability meets the requirements of high-precision industrial pressure measurement.

Claims

1. A low-stress packaging method for a pressure-sensitive element, characterized in that: It includes the following steps: Step S1: Take a copper alloy plate as the pad body (1). The pad body (1) is provided with several packaging units. The packaging unit includes a first lead frame semi-etched groove (3) on the bottom surface of the pad body (1) in a ring. A pair of second lead frame semi-etched grooves (4) are provided on the bottom surface of the pad body (1) outside the lead frame semi-etched groove (3). The second lead frame semi-etched groove (4) corresponds to the position of the Pad point. The surface of the pad body (1) is roughened. The first lead frame semi-etched grooves (3) between adjacent packaging units are connected by a semi-etched cantilever beam (3a). Step S2: A pressure-sensitive element (5) is connected to the inner side of the first lead frame half-etched groove (3) on the front side of the pad body (1); Step S3: Use gold wire ball bonding to form an electrical connection between the pressure sensitive element (5) and the Pad point, and the bonding arc height of the gold wire leads is 170-180um; Step S4: The upper template (6) is provided with protrusions (7) corresponding to the pressure sensitive elements (5) in each encapsulation unit, and the protrusions (7) are provided with relief grooves (7a) corresponding to the sensitive diaphragm area (5a) of the pressure sensitive element. The upper template (6), protrusions (7) and relief grooves (7a) are covered with release film (8). Step S5: Place the protrusion (7) on the pressure-sensitive element (5) so that the relief groove (7a) covers the sensitive diaphragm area (5a) of the pressure-sensitive element, and then perform molding and encapsulation. The molding compound (9) only covers the non-sensitive area of ​​the pressure-sensitive element (5). Due to the presence of the protrusion (7) and the relief groove (7a), the sensitive diaphragm area (5a) of the pressure-sensitive element remains suspended after molding. Step S6: After molding and curing, remove the molded frame and perform post-curing and stress-relieving annealing at a curing temperature of 175℃ for 8 hours. Step S7: Slice, pick, and visually inspect several packaging units to obtain the finished low-stress pressure sensor.

2. The low-stress packaging method for a pressure-sensitive element according to claim 1, characterized in that: The release film layer (8) is a high-temperature resistant polyimide release film.

3. The low-stress packaging method for a pressure-sensitive element according to claim 1, characterized in that: The depth of the lead frame semi-etched groove (3) is 30%-60% of the thickness of the pad body (1).

4. The low-stress packaging method for a pressure-sensitive element according to claim 1, characterized in that: The molding compound (9) is a stress epoxy molding compound with a curing temperature of 175℃, a curing shrinkage rate of ≤0.3%, and an elastic modulus of ≤15GPa.

5. A low-stress packaging method for a pressure-sensitive element according to claim 1, characterized in that: The height of the protrusion (7) is ≥0.2mm.

6. The low-stress packaging method for a pressure-sensitive element according to claim 1, characterized in that: The roughening process in step S1 is a single-sided roughening process on the front side of the pad body (1), which includes the following steps: a) Pretreatment: alkaline degreasing, overflow water washing, acid pickling activation, and pure water washing in sequence; b) Backside film protection: Dry film hot pressing and cooling are performed sequentially. The steps are required to ensure that the film is free of bubbles, has no curling edges, and fully covers the solder pad area. c) Front (i.e. plastic cover) roughening: roughening and multi-stage water washing are performed in sequence. The roughening process is any one of micro-etching, browning or organic acid ultra-roughening. After roughening, the roughness of the pad body (1) is 0.10~0.35 μm. d) Stripping: Sequentially remove the dry film on the back side with alkaline stripping solution, rinse with overflow water, and rinse with pure water; e) Post-treatment: Perform antioxidant treatment and pure water washing in sequence; f) Drying: Dry with hot air at 80–120℃ for 10–30 minutes.

Images