With the development of electronic products in the direction of miniaturization, portability, networking and high performance, higher requirements are put forward on circuit assembly technology and the number of I/O leads. Chips are getting smaller and smaller and there are more and more chip pins. , Which brings difficulties to production and repair.
In the original QFP (quad flat package) widely used in SMT, the limit size of the package spacing stays at 0.3mm. This spacing lead is easy to bend, deform or break. Accordingly, strict requirements are placed on the SMT assembly process, equipment accuracy, and welding materials. Even so, the defect rate of assembling small-pitch and fine-lead QFPs is still quite high, up to 6000ppm, which restricts a wide range of applications. In recent years, the BGA (Ball Grid Array packaged device), because the chip pins are not distributed around the chip but on the bottom of the package, actually turns the pins from the original four sides of the package shell substrate into an area array layout The pb/sn bump pins, which can accommodate more I/Os, and can replace the 0.4, 0.3mm of QFP with a larger pin pitch such as 1.5, 1.27mm, and it is easy to use SMT and PCB The wiring pins are soldered and interconnected, so that not only can the chip maintain more package capacity under the same package size as QFP, but also the I/O pin spacing is larger, which greatly improves the yield and defect rate of SMT assembly It is only 0.3-5ppm, which facilitates production and rework. Therefore, BGA packaging technology has been widely used in the field of electronic product production.
With the increase in the number of pins, it is difficult to repair defects such as bridging, missing solder, and missing solder during the assembly process of fine pins. It is difficult to repair with hand tools, and special repair equipment is required to complete it according to a certain repair process. .
According to the different packaging materials, there are mainly the following types of BGA components:
·PBGA (plastic BGA, plastic packaged BGA);
·CBGA (ceramic BGA, ceramic packaged BGA);
·CCBGA (ceramic column BGA, BGA in ceramic column package);
·TBGA (tape BGA, BGA with tape package);
· CSP (Chip Scale Package or mBGA).
PBGA is the most commonly used BGA at present. It uses 63Sn/37Pb solder balls, and the melting temperature of the solder is about 183°C. The diameter of the solder ball before soldering is 0.75mm, after reflow soldering, the height of the solder ball is reduced to 0.46~0.41mm. PBGA has the advantages of low cost and easy processing; but it should be noted that due to plastic packaging, it is easy to absorb moisture, so for ordinary components, generally should be used within 8 hours after opening, otherwise the rapid heating during welding will cause The moisture in the chip immediately vaporizes and causes the chip to be damaged. Some people call this the "popcorn" effect. According to JEDEC's recommendation, the period of use of the PBGA chip after unpacking is determined by the sensitivity level of the chip.
The composition of the CBGA solder ball is 90Pb/10Sn (the solder composition at the junction with the PCB is still 63Sn/37Pb). The height of the solder ball of CBGA is higher than that of PBGA, so its solder melting temperature is higher than that of PBGA, and it is not easy to absorb moisture than PBGA. , And the package is more reliable. The diameter of the solder joints on the bottom of the CBGA chip is larger than the pads on the PCB. After the CBGA chip is removed, the solder will not stick to the pads on the PCB.
The diameter of the CCBGA solder column is 0.51mm, the column height is 2.2mm, the solder column spacing is generally 1.27mm, and the composition of the solder column is 90Pb/10Sn.
The TBGA solder ball diameter is 0.76mm, and the ball pitch is 1.27mm. Compared with CBGA, TBGA requires strict control of the ambient temperature, because when the chip is heated, the thermal tension is concentrated at 4 corners, which is prone to defects during soldering.
The package size of the CSP chip is only slightly larger than the bare chip size (no more than 20%), which is the main difference between CSP and BGA. Compared with BGA, CSP has a shorter conductive path and lower reactance than BGA, and it is easier to reach the frequency range of 500-600MHz.
We can see the advantages of BGA from the following comparison of QFP and BGA chips, which are also 304 pins:
In summary, compared with QFP, the main characteristics of BGA are:
1. I/O lead spacing is large (such as 1.0, 1.27, 1.5mm), and the number of I/Os that can be accommodated is large (such as 1.27mm pitch BGA can accommodate 350 I/Os on an area of ​​25mm side length, and 0.5 The mm-pitch QFP only accommodates 304 I/Os in an area of ​​40mm side length).
2. The package has high reliability (will not damage the pins), low solder joint defect rate (<1ppm/ solder joint), and solid solder joints.
3. The centering of the QFP chip is usually observed by the operator with the naked eye. When the lead pitch is less than 0.4mm, the centering and welding are very difficult. The pin spacing of BGA chips is relatively large. With the aid of the centering magnification system, centering and welding are not difficult.
4. It is easy to process screen boards for large-size circuit boards.
5. The pin level identity is easier to ensure than QFP, because the solder ball can automatically compensate for the plane error between the chip and the PCB after melting.
6. During reflow soldering, the tension between the solder joints produces a good self-centering effect, allowing a 50% patch accuracy error.
7. It has good electrical characteristics. Due to the short lead, the self-inductance of the wires and the mutual inductance between the wires are very low, and the frequency characteristics are good.
8. It is compatible with the original SMT placement process and equipment, and the original screen printing machine, placement machine and reflow soldering equipment can all be used.
Of course, BGA also has shortcomings. The main reason is that X-ray inspection is required after the chip is soldered. In addition, because the pins are arranged in a spherical fence shape, multilayer circuit board wiring is required, which increases the manufacturing cost of the circuit board.
The heat-resistant temperature of most semiconductor devices is 240-2600°C. For BGA rework systems, the control of heating temperature and uniformity is very important. The hot air reflow soldering and rework system BGA-3592-G/CSP-3502-G of the United States OK Group and the MS series rework workstation of MS Engineering Co., Ltd. of Japan have solved this problem well.
This article takes the hot air reflow soldering and rework system BGA-3592-G of OK Group in the United States as an example to briefly explain the rework process of BGA:
The main purpose of preheating the circuit board and chip is to remove moisture. If the moisture in the circuit board and chip is very small (such as the chip has just been unpacked), this step can be avoided.
If the removed chip is not going to be reused, and the PCB can withstand high temperatures, a higher temperature (shorter heating cycle) can be used to remove the chip.
Cleaning the pad is mainly to remove the flux and solder paste left on the surface of the PCB after the chip is removed, and a cleaning agent that meets the requirements must be used. In order to ensure the reliability of BGA soldering, generally the old residual solder paste on the pad cannot be used, and it must be removed unless a BGA solder ball is re-formed on the chip. Due to the small size of BGA chips, especially CSP or mBGA, the chip size is smaller and it is difficult to clean the pads. Therefore, when reworking CSP chips, if the space around the CSP is small, no-clean flux is required.
Applying solder paste on the PCB has an important impact on the BGA rework results. By choosing a template that matches the chip, the solder paste can be easily coated on the circuit board. With OK Group's BGA-3592-G equipment micro-optical centering system, it is convenient to check whether the solder paste is evenly spread. For processing CSP chips, there are 3 types of solder paste to choose from: RMA solder paste, non-clean solder paste, and water flux solder paste. Use RMA solder paste, the reflow time can be slightly longer, use non-cleaning solder paste, the reflow temperature should be lower.
The main purpose of the patch is to align each solder ball on the BGA chip with each corresponding solder joint on the PCB. Since the solder joints of the BGA chip are located in parts that cannot be observed with the naked eye, special equipment must be used for centering. BGA-3592-G can be accurately aligned.
Hot air reflow soldering is the key to the entire rework process. Several issues are more important:
1. The reflow soldering curve for chip repair should be close to the original soldering curve of the chip. The hot-air reflow soldering curve can be divided into four sections: preheating zone, heating zone, reflow zone, and cooling zone. The temperature and time parameters of the four zones can be respectively. Setting, these programs can be stored and recalled at any time by connecting with the computer.
2. In the process of reflow soldering, the heating temperature and time of each zone should be selected correctly, and the heating speed should be paid attention to. Generally, the maximum heating rate does not exceed 6°C/s before 100°C, and the maximum heating rate does not exceed 3°C/s after 100°C. In the cooling zone, the maximum cooling rate does not exceed 6°C/s. Because too high heating rate and cooling rate may damage PCB and chip, this kind of damage is sometimes invisible to the naked eye. Different chips, different solder pastes, should choose different heating temperature and time. If the reflow temperature of CBGA chip should be higher than that of PBGA, 90Pb/10Sn should be higher than 63Sn/37Pb solder paste. For no-clean solder paste, its activity is lower than that of non-no-clean solder paste. Therefore, the soldering temperature should not be too high and the soldering time should not be too long to prevent the oxidation of solder particles.
3. In hot air reflow soldering, the bottom of the PCB must be able to heat up. Heating has two purposes: to avoid warping and deformation due to heat on one side of the PCB; and to shorten the melting time of the solder paste. For large-size boards to rework BGA, bottom heating is especially important. There are two bottom heating methods for BGA-3592-G rework equipment, one is hot air heating and the other is infrared heating. The advantage of hot air heating is that it is evenly heated, and this type of heating is generally recommended for rework processes. The disadvantage of infrared heating is that the PCB is heated unevenly.
4. Choose a good hot air return nozzle. The hot air reflow nozzle is a non-contact heating, which relies on high-temperature air flow to melt the solder of each solder joint on the BGA chip at the same time. The OK Group in the United States first invented this nozzle, which seals the BGA components to ensure a stable temperature environment during the entire reflow process, and at the same time protects adjacent components from being damaged by convective hot air heating (as shown in Figure 1).
In the production of electronic products, especially computer and communication electronic products, semiconductor devices are developing towards miniaturization, multi-function, and green development. Various packaging technologies continue to emerge. BGA/CSP is the mainstream of today's packaging technology. Its advantage lies in further reducing the package size of semiconductor devices, thus improving the level of high-density mounting technology, which is very suitable for the development direction of light, thin, short, small and functional diversified electronic products.
In order to meet the rapidly growing demand for BGA packaging technology circuit board assembly and manufacturers' requirements for screen printing, centering patch and soldering process control accuracy, and to improve the quality of BGA assembly soldering and rework, it is necessary to choose a safer, faster, More convenient assembly and repair of equipment and processes.
The present invention provides a method for controlling the temperature of a flue-cured electronic cigarette and a flue-cured electronic cigarette. The flue-cured electronic cigarette includes an N-section heating body, where N is an integer greater than 1, and the heating body is used for heating tobacco. The method for controlling the temperature of the flue-cured electronic cigarette includes: the flue-cured electronic cigarette heats the i-th heating body, and i is an integer greater than 0 and less than N; after the first preset time, the flue-cured electronic cigarette pairs the i+ The first stage heating body is heated; after the second preset time, the flue-cured electronic cigarette stops heating the i-th stage heating body, and continues to heat the i+1th stage heating body. The technical solution solves the problem of unbalanced smoke output of flue-cured electronic cigarettes during multi-stage heating.The present invention provides a method for controlling the temperature of a flue-cured electronic cigarette and a flue-cured electronic cigarette. The flue-cured electronic cigarette includes an N-section heating body, where N is an integer greater than 1, and the heating body is used for heating tobacco. The method for controlling the temperature of the flue-cured electronic cigarette includes: the flue-cured electronic cigarette heats the i-th heating body, and i is an integer greater than 0 and less than N; after the first preset time, the flue-cured electronic cigarette pairs the i+ The first stage heating body is heated; after the second preset time, the flue-cured electronic cigarette stops heating the i-th stage heating body, and continues to heat the i+1th stage heating body. The technical solution solves the problem of unbalanced smoke output of flue-cured electronic cigarettes during multi-stage heating.
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