1. Pressure welding method in micro connection technology
In the micro-joining technology, the pressure welding method is mainly used for the connection of the internal interconnection wiring of the solid-state circuit in the microelectronic device, that is, the connection between the chip (the surface electrode, the metallization layer material is mainly Al) and the lead frame. According to the inner lead form, it can be divided into wire bonding, beam lead technology, flip chip method and carrier tape automatic bonding technology.
1. Wire-bonding
This method combines common welding energy (hot pressing, ultrasonic or a combination of both) with a special tool and process (ball-squeezing method, wedge-wedge method) to form different bonding methods. A connection between a wire having a diameter of 10 to 200 μm and a chip electrode-metal film is achieved.
(1) Wire pressing thermobonding This is the earliest method for inner wire bonding. Hot press bonding is a typical plastic deformation of the joint zone by pressure and heat. Heat and pressure are applied to the bonding zone either directly or indirectly via a capillary or wedge shaped heating tool in a static or pulsed manner. This method requires a very high degree of cleanliness of the bonded metal surface and the bonding environment. Moreover, only the gold wire can ensure the bonding reliability, but for the Au-Al inner wire bonding system, the "purple spot" defect which causes the mechanical strength of the solder joint to be weakened is easily formed at the solder joint.
(2) Ultrasonic bonding of the wire ultrasonic bonding is to apply ultrasonic waves and pressure simultaneously on the bonding surface of the material, the ultrasonic vibration is parallel to the bonding surface, and the pressure is perpendicular to the bonding surface. The method generally adopts Al or Al alloy wire, which can avoid the "Ziban" defect of the Au wire thermocompression welding and solve the welding difficulty of the Al-Al system, and reduce the production cost. The disadvantage is that the tail wire is not well handled, which is not conducive to improving the integration of the device, and it is more difficult to achieve automation and the production efficiency is relatively low.
(3) The spheroidal wire is passed through the capillary of the hollow trowel, and then the projection is melted by arc discharge and spherical under the surface tension, and then the ball is pressure-welded to the electrode of the chip by a trowel. This method generally uses an Au wire. In recent years, the international market has been seeking to use Al wire or Cu wire instead of Al wire ball bonding. In the 1980s, foreign Cu wire ball welding has been applied in production. The Cu wire ball welding device developed in China adopts a controlled pulse discharge double-power ball system, and uses a microcomputer to control the number, frequency, bandwidth ratio and low-pressure dimension time of the high-pressure pulse of the ball, thereby realizing the energy of the spherical ball. Precise control and adjustment ensure the quality of the Cu filament ball under argon protection conditions.
2. Beam-lead
A beam composed of a plurality of layers of metal is prepared on a semiconductor wafer by a double deposition method, and the beam is used in place of a conventional inner lead to connect with an external circuit. This method is mainly applied in systems requiring long life and high reliability such as military and aerospace. The advantages are that the inner lead welding efficiency is improved and the high reliability connection is realized. The disadvantage is that the manufacturing process of the beam is complicated, the heat dissipation performance is poor, and the repair of the solder joint is not possible.
3. Flip-chip and tape automated bonding technology (TAB)
With the development of large-scale and very large scale integrated circuits, the number of leads in microelectronic devices has also increased. As a kind of spot welding technology, spheroidal welding technology can not meet the requirements of large-scale production, no matter the welding quality or welding efficiency, the group welding technology came into being.
The flip chip method was developed by IBM in the 1960s and is mainly used for thick film circuits. This method prefabricates the solder bumps at the electrodes on the silicon wafer, and simultaneously prints the solder paste onto the lead electrodes on one side of the substrate, and then inverts the silicon wafer to align the solder bumps on the silicon wafer. After heating, the needle materials of both sides are melted together to achieve connection. This method is suitable for the miniaturization and high-function requirements of microelectronic devices, but the solder bumps are complicated to manufacture, the post-weld appearance inspection is difficult, and pre-weld treatment and strict control of brazing specifications are required, so the application is limited.
The tape carrier automatic bonding technique is a flexible carrier tape bonding foil similar to 135 film, which is etched on the foil to make a lead frame pattern and then connected to the bumps on the chip. In the current TAB, the Au lead frame of Au plating and the Au bump on the chip are widely used for hot press welding. The advantage is high production efficiency, the disadvantage is that the process is also complicated, the cost is high, and the versatility of the chip is poor.
2. Soldering method in micro-joining technology
In micro-joining technology, soldering is mainly used for the connection of the outer leads of microelectronic devices. The outer lead connection refers to the connection between the signal terminal (outer lead) of the microelectronic device and the corresponding pad on the printed circuit board (PCB). Since 1962, Japan introduced the ceramic substrate ball grid array (CBGA). In 1966, RCA Corporation of the United States introduced chip resistors and capacitors. In 1971, Phlips officially proposed the surface mount (SMT) concept. By 1991, Motorola introduced the resin substrate ball grid array. (PBGA), making BGA technology practical, and the external lead connection technology of microelectronic devices has completed the historical leap from through-hole insertion technology (THT) to SMT, which greatly promoted the development of microelectronic technology.
1. Soft Needle Welding Process At present, the common PCB in the microelectronics industry is a plug-in hybrid method. The commonly used soldering process is wave soldering and reflow soldering.
(1) Wave soldering wave soldering is to use a solder pump to continuously and vertically melt the molten solder toward the narrow long exit to form a peak of 20 to 40 mm. In this way, the solder can be applied to the PCB at a certain speed and pressure, and fully penetrates between the device leads to be soldered and the circuit board, so that it is completely wetted and soldered. Due to the flexibility of the solder peaks, even if the PCB is not flat enough, as long as the warpage is below 3%, good soldering quality can be obtained.
In order to overcome the shortcomings of double wave soldering, jet hollow peaks have been developed in recent years. The utility model adopts a special electromagnetic pump as a brazing material jet power pump, and uses the double action of the external magnetic field and the flowing current in the molten brazing material to force the brazing material to flow in a direction determined by the left hand rule and to eject a hollow wave. According to the principle of fluid mechanics, the solder can be fully wetted by the PCB assembly to achieve a firm soldering. The contact between the hollow wave and the PCB is only 10 to 20 mm, and the contact time is only 1 to 2 s, so that thermal shock can be reduced.
(2) Reflow soldering The material used for reflow soldering is a solder paste. The solder paste is applied to the PCB pad by printing or dripping, and the SMD is placed on the special device (mounting machine). Then, the heating melts the solder, that is, flows again, thereby achieving the connection. The difference between the various reflow methods is that the heat source and the heating method are different.
1) Infrared reflow soldering. The infrared radiant heating method generally adopts a tunnel heating furnace, and the heat source is mainly composed of infrared radiation, and is suitable for mass production of the assembly line. And the equipment cost is low, which is the most common reflow soldering method in Japan. The disadvantage is that SMD has different heat absorption due to the depth of the surface color, the difference of materials and the distance from the heat source; the large SMD will cause shadow to the small SMD, which will reduce the welding quality due to insufficient heat; temperature Setting is difficult to consider.
2) Vapor phase reflow soldering. The heat source is derived from the latent heat of vaporization of a saturated vapor of a chlorofluorocarbon solvent (typically FC-70). The PCB is placed in an atmosphere filled with saturated steam, and the vapor condenses upon contact with the SMD and emits latent heat of vaporization to melt and reflow the solder paste. The most widely used vapor phase reflow soldering is the United States. The advantage is that the solvent vapor can reach every corner, the heat conduction is uniform, and the high-quality welding irrelevant to the product geometry can be completed; the welding temperature is accurate, (215 ± 3) °C, and no overheating occurs. The disadvantage is that the solvent is expensive and the production cost is high; if the operation is improper, the solvent is decomposed by heating to generate toxic hydrogen fluoride and isobutylene gas.
3) Laser reflow soldering. The heat source is from CO2 or YAG laser beam. It is based on laser with excellent directionality and high power density. It is characterized by high localization of heating process; no effect force, heat sensitive device will not be subjected to thermal shock; welding time is short, welding The point microstructure is refined and the thermal fatigue resistance is improved. The disadvantage is that as the only spot welding technology for reflow soldering, the production efficiency is low. At present, laser wave soldering is mainly used for high-density assembly with lead pitches below 0.65-0.5m. For example, the "Laser Number PLM1" developed by Philips has assembled a dedicated IC after wave soldering or reflow soldering, and the lead pitch is 0.2 mm and will not be bridged.
The laser reflow process is highly automated and provides process control. The domestically developed laser reflow soldering equipment uses a microcomputer to control the laser power supply to achieve accurate output of heating energy. The positioning of the solder joints adopts a human rights teaching method, and the welding process is automatically completed under the control of the microcomputer.
2. Soldering material
In the micro-joining technology, the solder materials used are mostly conventional solder Sn-Pb alloys and organic solders, which are generally cleaned after soldering. In recent years, with the increasing attention of the international community on environmental protection issues and fierce market competition, no-clean flux and no Pb solder have been developed abroad and have been commercialized.
(1) No-clean flux Fluor (CFC) solvent is the main solvent for cleaning flux residue during soldering. The international convention for the complete ban on CFC in 2000 has been signed internationally. The use of no-clean flux removes the cleaning steps, reduces the resulting production equipment and space; saves on materials, energy, waste disposal, etc. associated with the cleaning process; microelectronic devices that are incompatible with existing cleaning solvents are widely available application.
A typical no-clean flux contains 2% to 5% by mass of the solid component, which is reduced by increasing the amount of solvent and reducing the amount of rosin and active agent. The active agent system is specifically chosen to be relatively sensible to minimize its long-term corrosivity. The carrier is generally a synthetic rosin, a resin or a modified rosin, and since the low slag flux containing the resin still contains a halide, artificial rosin is considered to be the best choice. Since the injection contains a large amount of ethanol, it is necessary to add other solvents or foaming agents to promote foaming to ensure that the flux can be applied to the printed circuit board. However, excessive blowing agent will seriously affect the corrosiveness of the flux.
The technical difficulties of no-clean flux are as follows:
1) Optimization of flux concentration to produce sufficient wetting and minimize residue;
2) In order to determine the amount of solder for a given process, the density must be determined. However, because the ethanol in its composition has a high vapor pressure, the density is difficult to control;
3) Since the ethanol has water absorption, the no-clean flux has obvious water absorption. Excessive moisture in the flux will inhibit the flux activity and cause oxidation of the metallization layer;
4) Due to the large amount of ethanol, low slag flux is difficult to foam (an important method for applying flux in wave soldering);
5) In order to complete high quality solder assembly, the process parameter window required for no-clean flux is narrower.
(2) Pb-free solder The conventional solder is a Sn-Pb alloy, and Pb is toxic. The key to the development of Pb-free solder is to find one or more alloying elements that have an approximate melting point with the alloy of Sn, so that instant replacement can be achieved without changing the existing process conditions, and the wetting properties should be compatible with Sn. -Pb alloy is equivalent. Major foreign companies have successively launched their own patented Pb-free soldering compounds, but they have not been widely used in actual production.
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