CN103639558B - Thermal-ultrasonic-electromagnetic multi-field compound reflow soldering method - Google Patents

Abstract

The thermal-ultrasonic-electromagnetic multi-field composite reflow soldering method relates to a reflow soldering method. The steps of the method are as follows: place the printed circuit board at the middle position of the reflow soldering heating plate, place a “mountain” shaped magnet whose magnetic field lines are perpendicular to the surface of the flip-chip PCB assembly under the reflow soldering heating plate, and wind a durable magnet at the position of the middle magnetic core post. Strong current coil; the ultrasonic contact picks up the chip components with solder balls through the precision optical alignment device and aligns them with the pads on the PCB circuit board; turns on the reflow soldering heating plate, when the temperature of the reflow soldering heating plate reaches the heat preservation zone At this time, ultrasonic and magnetic fields are applied during the interconnection process to complete the reflow soldering process. The invention applies a directional magnetic field on the basis of the hot plate-ultrasonic reflow soldering technology, and realizes the orientation, preference, and rapid growth of IMCs at a lower temperature through the regulation of the magnetic field strength and direction, aiming at quickly obtaining mechanical properties at a lower temperature Excellent solder joints to improve electronic device reliability.

Description

Thermal-ultrasonic-electromagnetic multi-field compound reflow soldering method

technical field

The invention relates to a reflow soldering method, in particular to a reflow soldering method under the combined conditions of thermal-ultrasonic-magnetic field.

Background technique

In order to meet the development needs of electronic systems such as multi-functionality, high performance, low power loss, and small size, electronic circuit surface mount technology (SMT, Surface Mount Technology) has become one of the cores of electronic packaging technology, and is moving towards high density, three-dimensional, three-dimensional, Development in the direction of MEMS integration. With the increase of packaging density, the size of solder joints is decreasing day by day, and the proportion of intermetallic compounds (Intermetallic Compounds-IMCs) in solder joints is increasing, and the interface IMCs gradually become the main constituent units of solder joints. When the proportion of IMCs grains in micro-solder joints increases to a certain extent, the mechanical behavior of the solder joints under various loads will be significantly different from that of the original material and large solder joints of the same material, and the interface IMCs will become the dominant factor for the reliability of solder joints. The trend is becoming increasingly apparent. Many studies have shown that the IMCs at the solder-pad interface are closely related to solder joint reliability. Due to the anisotropy of the crystal mechanical properties of IMCs, solder joints with different preferred orientations of IMCs will exhibit different mechanical behaviors. Therefore, how to obtain IMCs with crystal orientation with excellent mechanical and physical properties is a challenge to reflow soldering technology.

Reflow soldering (also known as reflow soldering) is to apply an appropriate amount of solder on the printed circuit board (PCB, Printed Circuit Board) pad in advance, and place surface mount components, such as BGA (BallGridArray) devices. After curing, use an external heat source to use A process in which solder reflows to achieve interconnection purposes.

Traditional surface mount reflow soldering processes mainly include: hot plate conduction reflow soldering, infrared radiation heating reflow soldering, hot air convection reflow soldering, infrared hot air reflow soldering, and gas phase heating reflow soldering. Among them, hot plate conduction reflow soldering and hot air convection reflow soldering are the most commonly used. Although these reflow soldering processes can meet the basic needs of interconnection, they are far from meeting the requirements for the directional growth of intermetallic compound IMCs at the interface of solder joints. At present, the difference in reflow soldering process is mainly the difference in heating mode, heating position and heat conduction mode, but it is difficult to control the microstructure of solder joints, especially the grain orientation of intermetallic compound IMCs at the solder joint interface, as follows:

(1) Hot plate conduction reflow soldering. This method is the earliest reflow soldering method. Most of the heating devices are block-shaped boards. The circuit substrate is placed on the heating plate, and the heat is then transferred to the circuit substrate, and then transferred to the surface tile device by the solder paste. The solder paste is heated and melted to weld the device and the circuit substrate. This technology is mainly suitable for single-sided mounting of circuit substrates with good thermal conductivity.

(2) Hot air convection reflow soldering. Hot air convection uses heaters and fans to continuously heat and circulate the air in the furnace. This method has high production capacity and low operating cost, but the temperature is unstable.

(3) Thermo-ultrasonic reflow soldering. The method is to install an ultrasonic device on the basis of hot plate conduction reflow soldering. Ultrasonic vibration causes frictional heat generation between the pad and the interface, promotes the melting of solder joints, and can achieve bonding at a lower temperature, thus avoiding thermal damage to components caused by high temperature; ultrasonic is conducive to the diffusion of atoms, which can The intermetallic compound IMCs required to form reliable solder joints, shorten process time and improve production efficiency. However, the effect of ultrasound on the grain orientation inside the solder joint is not obvious, and needs to be further improved.

For this reason, many studies have shown that applying a directional magnetic field during reflow soldering can effectively change the morphology and growth mode of IMCs, making them directional and preferential growth, thereby improving the mechanical properties of micro-interconnection solder joints.

Contents of the invention

The purpose of the present invention is to provide a thermal-ultrasonic-electromagnetic multi-field composite reflow soldering method, which applies a directional magnetic field on the basis of the hot plate-ultrasonic reflow soldering technology, and realizes IMCs at lower temperatures by adjusting the magnetic field strength and direction The directional, selective, and rapid growth of the aim to quickly obtain solder joints with excellent mechanical properties at lower temperatures to improve the reliability of electronic devices.

The technical process of the heat-ultrasonic-electromagnetic multi-field composite reflow soldering method of the present invention is as follows:

The first step is to place the PCB circuit substrate:

Place the printed circuit board in the middle of the reflow soldering heating plate, place a "mountain"-shaped magnet whose magnetic field lines are perpendicular to the surface of the flip-chip PCB assembly under the reflow soldering heating plate, and wind a high-current-resistant coil at the middle magnetic core post;

The second step, picking up and aligning chip components:

The ultrasonic contact picks up the chip components with solder balls through the precise optical alignment device and aligns them with the pads on the PCB circuit board;

The third step, thermal-ultrasonic-magnetic field reflow soldering:

Turn on the reflow soldering heating plate, when the temperature of the reflow soldering heating plate reaches the holding zone, start to apply ultrasound and magnetic field during the interconnection process, the time for loading ultrasound is 1-5s, the frequency of ultrasound is 20-65kHz, and the time for loading magnetic field is 1 ～30s, the range of magnetic field strength is 0.1～40T, and the reflow soldering process is completed.

The thermal-ultrasonic-magnetic field composite reflow soldering technology provided by the present invention is based on the existing thermal-ultrasonic reflow soldering and applies a magnetic field to change the reflow soldering process time and improve the structure and performance of solder joints. The application of a magnetic field in the reflow process is mainly based on the following two principles: a strong magnetic field can transfer high-intensity energy to the atomic scale of the material without contact, change the behavior of atomic arrangement, matching and migration, and then change the organization and structure of the material and performance. The magnetic field has two main functions in the preparation of materials: one is the orientation of the magnetic field, and the other is that the magnetic field controls the flow of the fluid. It is possible to control the shape, size, distribution and orientation of crystal growth during the crystallization process of micro-solder joints by using a magnetic field, thereby controlling the structure of micro-solder joints, and finally obtaining solder joints with excellent mechanical and physical properties.

Description of drawings

Figure 1 is a schematic diagram of the distribution of magnetic field lines in a "mountain" shaped magnetic core energized spiral tube;

Figure 2 is a cross-sectional view of the reflow soldering heating plate;

Figure 3 is an overall schematic diagram of the thermal-ultrasonic-electromagnetic composite reflow process;

Figure 4 is a schematic diagram of a flip-chip PCB assembly under an ultrasonic contact;

Figure 5 is a cross-sectional view of the flip-chip PCB assembly under the ultrasonic contact;

In the figure, 1-infrared thermometer: monitor the temperature of the solder joint during the reflow soldering process, and transmit it to the computer through the data optical fiber; 2-reflow soldering heating plate: provide heat source for the reflow soldering process, and the preset program can be set Temperature curves of the three processes of hot zone, reflow zone, and cooling zone; 3-"mountain"-shaped magnet: guide the distribution of magnetic field lines; 4-ultrasonic contactor: provide different power ultrasound for reflow soldering process, through preset The program changes the frequency and vibration direction (horizontal and vertical) of the ultrasonic contact, the vacuum nozzle absorbs the chip components and aligns with the PCB board, and applies pressure through the ultrasonic indenter; 5-coil: wound in the middle of the "mountain" shaped magnet Core, forming a energized spiral tube to generate a vertical magnetic field, different coil turns can adjust the magnetic field strength; 6-Flip-chip PCB assembly: assemble components with BGA bumps on the PCB board; 7-Tesla Meter: An instrument for measuring magnetic induction intensity based on the Hall effect, which can measure the magnetic field intensity of flip-chip PCB components; 8-DC power supply: provide stable DC power in different directions for the coil; 9-wire: transmit current; 10- Chip components, 11-BGA solder balls, 12-welding pads, 13-PCB substrate; 14-thermal conduction substrate; 15-high temperature heating film; 16-heat insulation substrate.

Detailed ways

The technical solution of the present invention will be further described below in conjunction with the accompanying drawings, but it is not limited thereto. Any modification or equivalent replacement of the technical solution of the present invention without departing from the spirit and scope of the technical solution of the present invention should be covered by the present invention. within the scope of protection.

The technical process of the heat-ultrasonic-electromagnetic multi-field composite reflow soldering method of the present invention is as follows:

The first step is to place the PCB circuit substrate:

The PCB substrate 13 is placed in the middle of the reflow soldering heating plate 2, and the heating temperature range of the reflow soldering heating plate 2 is 50-400°C. The "mountain" shaped magnet 3 whose magnetic field lines are perpendicular to the surface of the flip-chip PCB assembly 6 is placed under the reflow soldering heating plate 2, and a coil 5 (such as a copper coil, a superconducting material coil) with high current resistance is wound at the middle magnetic core column position, so that Obtain a wide range of magnetic field strength, and the magnetic field strength ranges from 0.1 to 40T.

The second step, picking up and aligning chip components:

The ultrasonic contactor 4 picks up the chip component 10 with solder balls and aligns it with the pad 12 on the PCB substrate 13 through a precise optical alignment device (for example, an optical alignment system in flip-bonding). Ultrasonic frequencies range from 20 to 65 kHz. In addition, the ultrasonic contact 4 can also exert pressure on the assembled PCB components, and the range of 0-20N can meet the technical requirements of micro-solder joint interconnection.

The third step, thermal-ultrasonic-magnetic field reflow soldering:

First, turn on the reflow soldering heating plate 2 switch, and the reflow soldering heating plate 2 reaches the holding temperature after preheating (determined by the material of the lead-free solder ball, the temperature range is 50-400°C, and the holding time is 1-30s). When the temperature reaches the heat preservation zone, the ultrasonic and DC power supply 8 switches are turned on, and the ultrasonic and magnetic fields are applied during the interconnection process. The ultrasonic loading time is generally 1-5s, and the ultrasonic frequency is adjustable in the range of 20-65kHz; the magnetic field loading time is 1-30s, and the magnetic field strength ranges from 0.1-40T. Turn off the reflow soldering heating plate 2 switch, the ultrasonic device switch, and the DC power supply 8 switch to complete the reflow soldering process.

It needs special explanation: in the present invention, it is necessary to ensure that the magnetic field lines are perpendicular to the surface of the flip-chip PCB assembly 6, and the magnetic field intensity can satisfy the variation within the range of 0.1-40T. Therefore, on the one hand, the "mountain" shaped magnet 3 (Fig. 1) is designed, the reluctance is smaller than that of the "column" shaped core, the magnetic field lines are easily constrained, and a magnetic field of sufficient strength can be generated under a relatively small current ; On the other hand, the reflow soldering heating plate with sandwich structure (Figure 2) is used to control its thickness at 2-3mm. The top layer of the reflow heating plate is a smooth heat-conducting substrate 14, the thickness of which can be controlled within 1 mm; the middle layer is a high-temperature heating film (mica sheet heating film) 15, and the thickness can be controlled within 1 mm (temperature range is 30 ~ 500 ° C); The bottom layer is close to the magnetic core material, and the heat insulating substrate 16 with heat insulating material is selected, and the thickness is controlled within 1mm. The heating plate is simple to manufacture, has a fast heating rate, and can control the temperature, and can still meet the experimental requirements after being penetrated by the magnetic field lines.

Claims (9)

1. Thermal-ultrasonic-electromagnetic multi-field composite reflow soldering method, characterized in that the method steps are as follows: The first step is to place the PCB circuit substrate: Place the printed circuit board in the middle of the reflow soldering heating plate, place a "mountain"-shaped magnet whose magnetic field lines are perpendicular to the surface of the flip-chip PCB assembly under the reflow soldering heating plate, and wind a high-current-resistant coil at the middle magnetic core post; The second step, picking up and aligning chip components: The ultrasonic contact picks up the chip components with solder balls through the precise optical alignment device and aligns them with the pads on the PCB circuit board; The third step, thermal-ultrasonic-magnetic field reflow soldering: Turn on the reflow soldering heating plate, when the temperature of the reflow soldering heating plate reaches the holding zone, start to apply ultrasound and magnetic field during the interconnection process, the time for loading ultrasound is 1-5s, the frequency of ultrasound is 20-65kHz, and the time for loading magnetic field is 1 ~30s, the magnetic field strength is in the range of 0.1-40T, and the reflow soldering process is completed, the temperature range of the heat preservation zone of the reflow soldering heating plate is 50-400°C, and the heat preservation time is 1-30s. 2. The thermal-ultrasonic-electromagnetic multi-field composite reflow soldering method according to claim 1, characterized in that the coil is a copper coil or a superconducting material coil. 3. The thermal-ultrasonic-electromagnetic multi-field composite reflow soldering method according to claim 1, characterized in that the thickness of the reflow soldering heating plate is 2-3 mm. 4. The thermal-ultrasonic-electromagnetic multi-field composite reflow soldering method according to claim 1 or 3, characterized in that the reflow soldering heating plate has a three-layer structure, the top layer is a smooth thermally conductive substrate, and the middle layer is a high-temperature Heating film, the bottom layer is a thermal insulation substrate. 5. The thermal-ultrasonic-electromagnetic multi-field composite reflow soldering method according to claim 4, characterized in that the thickness of the top layer is within 1 mm. 6. The thermo-ultrasonic-electromagnetic multi-field composite reflow soldering method according to claim 4, characterized in that the thickness of the intermediate layer is within 1 mm. 7. The thermal-ultrasonic-electromagnetic multi-field composite reflow soldering method according to claim 4, characterized in that the high-temperature heating film is a mica sheet heating film. 8. The thermal-ultrasonic-electromagnetic multi-field composite reflow soldering method according to claim 4, characterized in that the temperature range of the high-temperature heating film is 30-500°C. 9. The thermal-ultrasonic-electromagnetic multi-field composite reflow soldering method according to claim 4, characterized in that the thickness of the bottom layer is within 1mm.