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With the rapid development of the electronics industry, some large-scale electronic processing enterprises, from the initial Pearl River Delta to the Yangtze River Delta, and now Jinjing and Bohai Rim, SMT has become the mainstream, from the earliest hand patch to the current automation Most of the equipment patches use SMT solder paste technology, but many companies' products have inevitable production of plug-in devices (such as computer monitors and other products), so the earlier red glue process and the later communication Hole wave soldering process. However, the production of the red glue process has very strict requirements for the control of wave soldering and the manufacturability design of the PCB. The following I only introduce a series of requirements for the design and process parameters of the printed red glue process. It is through the Gulf where I am now working. The effective experience obtained by the company after a year of testing has been referenced by the SMT industry!
Through the test of the Gulf intercom video intercom products, from the packaging of the device, the packaging of the jack, the reasonable design of the device layout, the technical requirements for the opening of the template, and the reasonable adjustment of the wave soldering parameters, the one-time pass rate of the circuit board welding reached 92 % Or more (the product has 6 ICs with more than 20pin and one QFP with 48pin). After wave soldering, only a small repair can achieve 100% pass rate, but the welding efficiency is more than 3 times higher than that of manual welding.
The following are some specific design requirements shared with SMT colleagues and experts.
For the thickness and opening requirements of the template (1) template thickness: 0.2mm
(2) Template opening requirements: The opening width of the IC is 1/2 of the width of the two pads, and multiple small round holes can be opened.
Device layout requirements
(1) The long axis of the Chip component should be perpendicular to the conveyor belt direction of the wave soldering machine; the long axis of the integrated circuit device should be parallel to the conveyor belt direction of the wave soldering machine.
(2) In order to avoid the shadow effect, the ends of components of the same size are aligned in a line parallel to the direction of the solder wave; components of different sizes should be staggered; components of small size should be arranged in front of large components; prevent components The body shields the soldering ends and pins. When it can not be arranged according to the above requirements, there should be 3 ~ 5mm spacing between the components.
(3) The characteristic directions of components should be consistent. For example: the polarity of the electrolytic capacitor, the anode of the diode, the single-pin end of the transistor should be perpendicular to the transmission direction, the first pin of the integrated circuit, etc.
Component aperture and pad design
(1) The component holes must be arranged on the basic grid, 1/2 basic grid, and 1/4 basic grid. The gap between the pad hole of the inserted component and the diameter of the pin is until the solder can be well moistened.
(2) When wiring high-density components, an elliptical pad pattern should be used to reduce tin connection.
The basic requirements of wave soldering process on components and printed boards
(1) The surface-mount components with three-layer terminal structure should be selected. The component body and the solder end can withstand the temperature shock of more than two wave solderings at 260 degrees Celsius. After soldering, the device body is not damaged or deformed, and the chip component ends are free from capping.
(2) The substrate should be able to withstand the heat resistance of 260 ° C / 50s. The copper foil has good peel strength, the solder mask still has sufficient adhesion at high temperatures, and the solder mask does not wrinkle after welding.
(3) The warpage of the circuit board is less than 0.8-1.0%
Design of wave soldering parameters
(1) Flux system: the foaming air volume or flux spray pressure should be determined according to the situation of the flux contacting the bottom surface of the PCB: the flux spraying amount requires a uniform and thin layer on the bottom of the printed board, and the flux coating method There are two kinds of brushing foam and quantitative spray.
A The specific gravity of flux must be controlled by coating and foaming methods. The specific gravity of flux is generally controlled between 0.8-0.83.
B When the quantitative spray method is used, the flux is sealed in the container, and will not evaporate, absorb moisture in the air, and will not be contaminated, so the flux composition can remain unchanged. The key requirement is that the spray head can control the spray volume, the spray head should be cleaned frequently, and the spray hole cannot be blocked.
(2) Preheating temperature: set according to the actual situation of the wave soldering machine preheating zone (90-130 degrees Celsius). The role of preheating: evaporate the solvent in the flux, which can reduce the generation of gas during soldering; the rosin and active agent in the flux begin to decompose and activate, and the printed board pads, component ends and pins can be removed The oxide film and other contaminants on the surface also play a role in protecting the metal surface from re-oxidation; making the printed board and components fully preheated, to avoid the thermal stress caused by the rapid temperature rise during welding to damage the printed board and components.
Soldering temperature and time: The soldering process is a complex process of the interaction between the welding metal surface, molten solder and air, etc. The soldering temperature and time must be well controlled. If the soldering temperature is low, the viscosity of the liquid solder is large, and it cannot wet and diffuse well on the metal surface, which is prone to defects such as sharpening, bridging, and rough welding surface. If the soldering temperature is too high, it is easy to damage the components. There will be problems such as accelerated solder joint oxidation speed, blackened solder joints, and unsatisfactory solder joints. The wave soldering temperature is determined according to the size and thickness of the printed board, and the number and size of components on the printed board. The wave soldering temperature is generally 250 degrees Celsius plus or minus 5 degrees Celsius. Since the heat is a function of temperature and time, at a certain temperature The heat of solder joints and components increases with time. The welding time of wave soldering is controlled by adjusting the speed of the conveyor belt. The speed of the conveyor belt should be adjusted according to the length and wave width of different types of wave soldering. The time of the point contact wave peak represents the welding time. Generally, the second wave soldering time is 2.5-4s. Board climbing angle and wave crest height: The printed board climbing angle is generally 3-7 degrees, and 5.5-6 degrees is recommended. It is beneficial to eliminate the gas generated by the flux around the solder joints and components. For example: if there are fewer through holes in the design when there is SMD, the climbing angle (inclination angle) should be larger. Appropriate climbing angle can avoid leakage welding and play the role of exhaust; the peak height is generally controlled at the printed board thickness 2/3 and 3/4.
(3) Comprehensive adjustment of wave soldering process parameters: This is very important for improving the quality of wave soldering. Welding temperature and time are the primary conditions for forming good solder joints. The welding temperature and time are related to the preheating temperature, tilt angle, and transmission speed. When adjusting the process parameters comprehensively, we must first ensure the welding temperature and time. The first wave peak of leaded double wave soldering is generally around 220-230 degrees Celsius / 1s, the second wave peak is generally around 230-240 degrees Celsius / 3s, and the total time of the two wave peaks is controlled at about 4-7s. The copper content cannot exceed 1%. After the copper content increases, the surface tension of tin also increases, and the melting point is also high. Therefore, it is recommended to use wave soldering once a month for copper. The maintenance is to set the tin furnace at about 200 degrees and wait 4-8 After hours, the copper-containing impurities (Cn6Sn5) on the surface of the tin furnace are removed.
Red gum selection and use
(1) Since it is not a dispensing process, but a printing process of red glue, there are certain requirements for the thixotropic index and viscosity of red glue. If the thixotropic index and viscosity are not good, the molding after printing is not good, that is, the collapse phenomenon. Some of the IC's body may not be attached with red glue and prepared. (Reference thixotropy index: 4-5; Reference viscosity: (8-10) x1000000)
(2) The uncured glue sticking to the circuit board can be wiped off with acetone or propylene glycol ethers or cleaned with a special cleaner for red glue.
(3) Refrigerate the unopened product in a dry place at 2-10 ℃, the storage period is 6 months (subject to the date of delivery outside the package). Before use, restore the product to room temperature and return to temperature for more than 24 hours.
(4) Choose a curing time that is 90-120 seconds shorter, and a curing temperature of about 150 degrees is better.
(5) It must have good heat resistance and excellent electrical performance, as well as extremely low moisture absorption and high stability.
Notes on the adjustment of printing machine parameters
(1) The pressure is about 4.5 kg (2) The amount of red gum should be the best to roll the red gum on the template (3) The distance of SNAP OFF is set to 0.05mm, and the speed is set to: 2 grades.
(4) The frequency of automatic screen cleaning is set to 2.
(5) After the production is completed, the red glue on the template is discarded when it is no longer used within 5 days. (6) The red glue must be accurately printed between the two pads, and cannot be offset.
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