SMD component through wave soldering technics requirement
Nowadays, many electronic products are mixture production combination of plug-in and SMT technics. Most of the SMT are solved by the red glue technics. However, the production of red plastic technics has strict requirements for wave soldering controlling and the manufacturability design of PCB. The following only introduces the series requirements for design and technics parameters of printing red glue, which is obtained effective experience via trials of many countries’ customers.The following are some specific design requirements based on practice:
First,SMD component through wave soldering on the thickness and opening requirements of the red chip printing 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.
Second, SMD component through wave soldering layout requirements for chip components
(1) The long axis of the SMD component shall be perpendicular to the direction of the conveyor of the wave soldering machine; the long axis of the integrated circuit device shall be parallel to the direction of the conveyor of the wave soldering machine.
(2) In order to avoid the shadow effect, the ends of the same-sized components are aligned in parallel with the direction of the solder wave; components of different sizes should be staggered; small-sized components should be arranged in front of the large components; to prevent components blocking the soldered ends and pins.If it is not possible to arrange according to the above requirements, there should be a 3~5mm spacing between the components.
(3) The characteristic direction of the components should be consistent. Such as: electrolytic capacitor, two tube positive, three tube single pin end should be perpendicular to the transmission direction, the feet of the integrated circuit.
Third, SMD components through wave soldering on component aperture and pad design requirements
(1) The component holes must be arranged on the basic grid, 1/2 base grid, and 1/4 base grid. The gap between the plug hole and the pin diameter of the insert component is that the solder can be well wetted.
(2) Elliptical pad pattern should be used for high-density component wiring to reduce the connection of tin.
Fourth, SMD components through wave soldering on components and printed boards the basic requirements
(1) A three-layered end structure surface mount component should be selected. The component body and the solder joint can withstand temperature shocks of more than twice 260 degrees Celsius wave soldering. After soldering, the device body is not damaged or deformed, and the end of the chip component is uncapped.
(2) The substrate shall withstand heat resistance of 260 ° C / 50 s. The copper foil has good anti-peeling strength, and the solder resist layer still has sufficient adhesion at high temperatures, and the solder resist layer is not wrinkled after soldering.
(3) The warpage of the circuit board is less than 0.8-1.0%
Fifth,SMD components through wave soldering on wave soldering parameter design requirement
(1) Flux system: The foaming air volume or flux injection pressure should be determined according to the condition that the flux contacts the bottom surface of the PCB: the amount of flux sprayed requires uniform and thin layer at the bottom of the printed board, and the flux coating method are brushing foaming and quantitative spraying;
A. The coating and foaming method must control the specific gravity of the flux, and thegravity of the flux is generally controlled between 0.8 and 0.83;
B. When the quantitative sprayingmethod is used, the flux is sealed in the container, does not volatilize, does not absorb moisture in the air, and is not contaminated, so the flux composition can remain unchanged. The key requirements of the nozzle can control the amount of spraying, the nozzle should be cleaned frequently, and the spraying hole can not be blocked.
(2) Preheating temperature: setting according to the actual conditions of the wave soldering machine preheating zone (90-130 degrees Celsius).
The role of preheating: volatilizes the solvent in the flux, which reduces the generation of gas during soldering; the rosin and active agent in the flux begin to decompose and activate, can remove the printed circuit board pads, component ends and leads surface oxide film and other contaminants, at the same time protect the metal surface from re-oxidation; make the printed board and components fully preheated, to avoid sharp heating during welding to generate thermal stress damage to the printed board and components;
Welding temperature and time: The welding process is a complex process of welding metal surfaces, molten solder and air interaction, etc. The welding temperature and time must be controlled. If the welding temperature is too low, the viscosity of the liquid solder is strong, and it is not well wetted and diffused on the metal surface. It is easy to cause defects such as pulling, bridging connection, and rough welding surface. If the welding temperature is too high, the components are easily damaged. The problem like the accelerated oxidation rate of the solder joint, black solder joint , not full solder joint occur .
The wave soldering temperature is determined according to the size and thickness of the printed board, and the wave soldering temperature is normally 250 degrees Celsius plus or minus 5 degrees Celsius. Since the heat is a function of temperature and time. The heat of solder joints and components is increased along with time under constant temperature. The welding time of wave soldering is controlled by adjusting the speed of the conveyor belt. The speed of the conveyor belt is adjusted according to the length and peak width of different types of wave soldering, the welding time is expressed by the time when each solder joint contacts the peak, and the second peak welding time is 2.5-4 s.
Board climbing angle and peak height: The slope of the printed board is 3-7 degrees, and it is recommended to be 5.5-6 degrees. It is beneficial to eliminate the gas generated by the flux remaining 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, the appropriate climbing angle can avoid the leakage welding, and have the exhausting effect; the peak height is usually controlled in the 2/3 and 3/4 thickness of the printed board ;
(3) Comprehensive adjustment of wave soldering technical parameters: This is very important for improving the quality of wave soldering. Welding temperature and time are primary conditions for forming a good solder joint. The welding temperature and time are related to the preheating temperature, the tilt angle, and the transfer speed. When adjusting the technical parameters comprehensively, the welding temperature and time must be ensured firstly. The first peak of lead double wave soldering is about 220-230 degrees Celsius / 1s, the second peak is about 230-240 degrees Celsius / 3s, and the total time of the two peaks is controlled around 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 fill the copper for the wave soldering. The maintenance is to set the tin furnace at about 200 degrees and wait for 4-8H. Then, the copper-containing impurities (Cn6Sn5) on the surface of the tin furnace were removed.
Sixth, SMD component over wave soldering on the selection and usage requirements of the red glue
(1) Since it is not a dispensing but printing red glue technics, there is certain requirement for the thixotropic index and viscosity of red glue. If the thixotropic index and viscosity are not good, the forming after printing is not good, that is, collapse phenomenon, so some of the IC’s body will not stick to the red glue and be fell. (Reference thixotropic index: 4-5; reference viscosity: (2-6) x1000000);
(2) The uncured adhesive sticking to the circuit board can be wiped off with acetone or propylene glycol ether or washed with a special cleaning agent like red glue;
(3) Refrigerate the unopened product in a dry place at 2-10 °C for a storage period of 6 months (subject to the date of shipment outside the package). Before using, restore the product to room temperature and return to temperature for more than 24 hours;
(4) The curing time is selected to be: 90-120 seconds, and the curing temperature is preferably about 150 degrees;
(5) It has good heat resistance and excellent electrical properties, as well as low hygroscopicity and high stability.
Seventh, SMD components over wave soldering to parameters adjustment requirements of the printing machine
(1) The pressure is around 4.5 kg;
(2) The amount of red glue should be adjusted to make the red glue roll on the template;
(3) The distance between SNAP OFF is set to 0.05mm, and the speed is set to 2;
(4) The automatic wiping net frequency is set to 2;
(5) After the production is finished, the red glue on the template will be scrapped when it is no longer used within 5 days;
(6) Red glue must be accurately printed in the middle of the two pads and cannot be offset.
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