Brief analysis on the red glue process of the hott

With the rapid development of the electronic industry, SMT has become the mainstream in some large electronic processing enterprises, from the original Pearl River Delta to the Yangtze River Delta, and even now in Tianjin, Beijing and the Bohai rim. From the earliest hand patch to the current automatic equipment patch, most of them use SMT solder paste process, However, many companies' products have unavoidable plug-in device production (such as computer monitors and other products), so there are early red glue process and late through-hole wave soldering process. However, the production of red glue process has very strict requirements for the control of wave soldering and the manufacturability design of PCB. Below, I will only introduce a series of requirements for the design and process parameters of printing red glue process, which is the effective experience obtained from the one-year test conducted by Gulf company, which I now work in combination with the actual development of the industrial park. It is a reference for SMT industry

through the test of Gulf company's visual intercom products, from the packaging of devices, the packaging of jacks, the reasonable design of device layout, the technical requirements of template opening, and the reasonable adjustment of wave soldering parameters, the one-time qualification rate of circuit board welding has reached more than 92% (this product has six ICs with 203 and more than 48 pin clamping extension, and a 48 pin QFP). Only minor repair after wave soldering can achieve 100% qualification 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 peers and experts

for the thickness and opening requirements of the template

(1) template thickness: 0.2mm

(2) template opening requirements: the opening width of IC is 1/2 of the width of two pads, and multiple small round holes can be opened

layout requirements of devices

(1) the long axis of the chip element should be perpendicular to the direction of the conveyor belt of the wave soldering machine; The long axis of the integrated circuit device should be parallel to the direction of the conveyor belt of the wave soldering machine

(2) in order to avoid shadow effect, the ends of components with the same size are arranged in a straight line parallel to the solder wave direction; Components of different sizes shall be placed in a staggered manner; Small size components should be arranged in front of large components; Prevent the component body from blocking the welding ends and pins. When it is impossible to arrange according to the above requirements, 3~5mm spacing should be left between components

(3) the characteristic direction of components should be consistent. For example, the polarity of electrolytic capacitor, the positive pole of diode, the single pin end of triode should be perpendicular to the transmission direction, the first pin of integrated circuit, etc

component hole diameter and pad design

(1) the component hole must be arranged on the basic grid, 1/2 basic grid and 1/4 basic grid. The gap between the plug-in component pad hole and the pin diameter is until the solder can be well wetted

(2) when wiring high-density components, elliptical pad graphics should be used to reduce tin connection

basic requirements of wave soldering process for components and printed boards

(1) surface mount components with three-layer end structure should be selected, and the component body and welding end can withstand the temperature impact of wave soldering at 260 ℃ for more than two times. After welding, the device body will not be damaged or deformed, and the end of the chip component will not be decapped

(2) the base plate shall be able to withstand the heat resistance of 260 ℃/50s. The copper foil has good peel strength, and the solder mask still has enough adhesion at high temperature, and the solder mask does not wrinkle after welding

(3) the warpage of the circuit board is less than 0 0%

Design of wave soldering parameters

(1) flux system: the foaming air volume or flux injection pressure should be determined according to the condition that the flux contacts the bottom of PCB: the flux spraying amount requires a uniform and thin layer at the bottom of PCB, and the flux coating methods include brushing foaming and quantitative spraying

a the specific gravity of flux must be controlled by coating and foaming, and the specific gravity of flux is generally controlled at 0 83

b when the quantitative spraying method is adopted, the flux is sealed in the container, which will not volatilize, absorb moisture in the air, and will not be polluted, so the composition of the flux can remain unchanged. The key requirement is that the nozzle can control the amount of spray. The nozzle should be cleaned frequently and the spray hole should not be blocked

(2) preheating temperature: set according to the actual situation of the preheating area of the wave soldering machine (90-130 ℃). Preheating function: volatilize the solvent in the flux, which can reduce the gas produced during welding; Rosin and activator in the flux begin to decompose and activate, which can remove the oxide film and other pollutants on the surface of PCB pads, component ends and pins, and protect the metal surface from reoxidation at the same time; The printed boards and components are fully preheated to avoid the damage of printed boards and components caused by thermal stress caused by sharp temperature rise during welding

welding temperature and time: the welding process is a complex process of the interaction between the surface of welding metal, molten solder and air, etc., and the welding temperature and time must be well controlled. If the welding temperature is low, the viscosity of liquid solder is high, and it cannot be well wetted and diffused on the metal surface, it is easy to produce defects such as sharpening, bridging and rough welding surface. If the welding temperature is too high, it is easy to damage components, and it will also produce problems such as accelerated oxidation speed of solder joints, dark solder joints, and insufficient solder joints. The wave soldering temperature is determined according to the size and thickness of the printed board and the number and size of the components on the printed board. The wave soldering temperature is generally 250 ° C plus or minus 5 ° C. because the heat is a function of temperature and time, the heat of the solder joint and components increases at any time under a certain temperature. The welding time of wave soldering is controlled by adjusting the speed of the transmission belt, The speed of the transmission belt should be adjusted according to the length and width of wave crest welding of different models. The welding time is expressed by the time when each welding point contacts the wave crest. Generally, the welding time of the second wave crest is 2.5-4s. Board climbing angle and crest height: the climbing angle of printed board is generally 3-7 degrees, and it is recommended to be 5.5-6 degrees. It is beneficial to eliminate the residual gas generated by flux around the solder joint and components. For example, if there are few through holes in the design with SMD, the climbing angle (inclination angle) should be larger, and the appropriate climbing angle can avoid missing welding and play the role of exhaust; The peak height is generally controlled at 2/3 and 3/4 of the thickness of the printed board

(3) comprehensive adjustment of wave soldering process parameters: it is very important to improve the quality of wave soldering. Welding temperature and time are the primary conditions for forming a good solder joint. Welding temperature and time are related to preheating temperature, inclination angle and transmission speed. When comprehensively adjusting the process parameters, the welding temperature and time should be guaranteed first. The first wave peak of lead double wave soldering is generally about 220-230 ℃/1s, the second wave peak is generally about 230-240 ℃/3S, and the total time of the two wave peaks is controlled at about 4-7s. The copper content should not 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 fish copper once a month by wave soldering. The maintenance is to set the tin furnace at about 200 degrees and wait for 4-8 hours before fishing for copper impurities (cn6sn5) on the surface of the tin furnace

selection and use of red glue

(1) since it is not a dispensing process, but a printing red glue process, there are certain requirements for the thixotropic index and viscosity of red glue. If the thixotropic index and viscosity are not good, the formation after printing is not good, that is, collapse, so some IC bodies will not stick to red glue and be prepared. (reference thixotropic index: 4-5; reference viscosity: (8-10) x)

(2) the uncured glue on the bonding plate can be wiped off with acetone or propylene glycol ether or cleaned with special cleaning agent for red glue

(3) store the unopened products in a dry place at ℃ for 6 months (subject to the ex factory date outside the package). Before use, restore the product to room temperature for more than 24 hours

(4) it is better to choose a curing time of 90-120 seconds less and a curing temperature of about 150 ℃

(5) it should have good heat resistance, excellent electrical performance, extremely low moisture absorption and high stability

precautions for printer parameter adjustment

(1) the pressure is about 4.5 kg

(2) the dosage of red glue should make the red glue roll on the template best

(3) the distance in snap off is set to 0.05mm, and the speed is set to level 2

(4) set the automatic wiping frequency to 2

(5) the red glue on the template will no longer be used within 5 days after the completion of production, and waste treatment will be reported

(6) the red glue must be accurately printed in the middle of the two pads without offset

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