1.1 Form of heater
The main part of structure of the heater is the high-nickel complex hot line in a metal tube filled with calcium silicate material. It can quickly transfer inner heat to heat storage full metal plate and regional air out of the tube. Each plate has inner 6mm and outer 8mm hot air circulation holes in each 25mm pitch. It can blow down laminar flow of high temperature hot air from hole.
1.2 Heating method
Hot air from the plate holes come to PCB surface, a variety of components and solder paste (SMT adhesive).
Heat transfer to the PCB solder to ensure that temperature distribution in the machine and uniformity of the heating temperature on the work piece.
Hot air from the plate can be passed to the PCB, solder and components, so that:
- Be able to directly heat the solder under shaded shaped components
- Be able to directly transfer heat to pad and solder
- To prevent overheating of parts
- Make different solder components temperature equilibration
- Make different positions of the solder components the temperature equilibrium
- Can weld on different materials PCB, such as: soft flexible panels
1.4 Heating structure
Zone No.:Upper 2
Numbers of upper heater elements:2 sets
1.4.1 temperature control detection point
Each zone is equipped with a standard thermocouple detection point, which is a static temperature detection point,for testing represent temperature in that space. (The point is accurately tested before leaving the factory, don’t shift it without confirmation by factory).
Industrial control computer for temperature detection of each temperature zone, PID control and
high-power SSR driver.
- Temperature curve
Reflow machine is to heat the surface of the PCB with the paste components PADS bit, to make solder melt and slurry heated under reflux, to obtain the predetermined temperature to the solder paste , and without causing any damage to the PCB and components (for example, fuel combustion or dark burning, etc.). IPC standards welding temperature diagram:
Line A: Used for solder paste soldering.
General in 60 seconds, PCB pad heats up from room temperature to 120-150 ℃, the rate at 3 ℃ / s or less; from 60 -180 seconds to 90-150 seconds stabilize at about 150 ℃, less than solder paste melting point 183℃, so that the welding work piece reaches temperature equilibrium before the paste slurry liquefaction; It stays 30 seconds from 183 ℃ to 210-230 ℃ so that solder paste can be reflowed adequately.
Line B: used for fine pitch IC and tiny components (such as 1005) and other welding
techniques ,control temperature risen sharply in the preheating zone , postpone the flux in the solder paste to soften slightly, control the tiny solder paste tin powder to form a solder ball together .
Line C : Used for general SMD adhesive curing.
Stay at 150 ℃ for about 3-5 minutes to maintain temperature curing time.
In order to obtain maximum yield in highly automated SMT production environment, before starting machine must set heating temperature according to the paste heating chart. And, strict supervision in working time. Waste PCB is recommended to help to set up the digital temperature monitors of machine in use, in order to get the paste heating chart.
- Set temperature curve
To obtain a given temperature curve, it requires a process of seeking temperature profile, that refers to the temperature setting and speed of mesh. Changed Products, such as the board type, thickness, component type, the array density, the pad bit areas and types of solder paste, shape of printed tin, thickness, etc., will affect the temperature curves.The zone classification heated hot air reflow equipment heat products by gradually completing preheating, drying, melting and heating through progressively heating and solder paste. The first stage is a fast heating zone, in which, PCB gets rapid warm-up; The second stage is a long slow heating eating zone, finish drying Solder of PCBs in this stage; The third heating stage is for solder paste molten, after functional change in the second stage, solder paste is rapidly
heated and melted here, then get reflow; solder paste cooled rapidly through the cooling zone, the above process forms a complete temperature curve.
Now most of the machines (combined with hot air or full hot air) adopt hot air heated to reflux, that within the security of sensitive components, to avoid direct intense heating on components, top heating is relatively stable, through the hot air convection and heat conduction on PADS and tin slurry so that the surface components are welded stably. By this way, temperature curve is ease, the temperature difference between zones is relatively reduced, and the second functional areas set temperature is relatively increased.
Another change in control is the mesh speed. This will set the time that PCB board stays in oven. To match the soldering process of PCBs. Multi-layer PCB boards requires a little longer time in the machine,as it is thicker,relatively need a longer time to achieve a unified balanced heating.
Heaters on top and bottom are controlled independently, so that the board can be heated on top or bottom surfaces. Thus, if the element is sensitive to heat, you can choose to heat its backside.That means the bottom warming. About half of the total energy are used in the bottom preheat zone , this peak HF energy can pre-warm the PCB board evenly , thus reducing the surface impact to damage components, and reduce heat absorption of components. Small amount of energy is applied to the top to minimize warpage. If an insensitive component is to be heated, the preheat temperature can be set higher.
Long-wave Energy from top and bottom dryness area may heat the tin point slowly (and dryness). For hot air reflow in full hot fan, the top and bottom surfaces are mutual control and relatively stable balance. The top surface temperature is relatively higher. In the recirculation zone, full hot fan’s force micro hot air reflow
will be used to melt and reflow solder paste.
Temperature curve is established by:
First, classify the PCB boards to determine the heat absorption, component types ,density and degree of difficulty with the PCB production capacity in order to determine the top, bottom and reflux strategy determining a point, a set of components and machines with a thermocouple is conducive to promoting the
process of temperature curves. But this is not essential, the starting point as follows:
ZONE: 1 ……Upper preheating zone ZONE: 2 ……First upper dry zone
ZONE: 5 ……Lower preheating zone ZONE: 6 ……First lower dry zone
ZONE: 3 ……Second upper dry zone ZONE: 4 ……Upper reflow zone
ZONE: 7 ……Second lower dry zone ZONE: 8 ……Lower reflow zone
General machine completes cooling function with natural cooling curve.
Note: These are general starting point (in the basis of four eight reflow temperature, the other by
analogy, the 1st preheat zone includes zone 1and 6.The 2nd,3rd ,and 4th drying zone includes zone 2,zone7,zone 3,zone 8,zone 4,and zone 9. The 5th reflowing zone refers to zone 5 and zone 10), once the subject of a PCB board temperature curves for the End, similar to the PCB board is well on the PCB can be affected by temperature map (in increasing tape speeds) as a starting point.
- Temperature zone Description:
4.1 Preheating zone:
Preheating zone, also known as rapid heating zone, is used to preheat the PCB and solder paste to improve the boiling temperature of the flux. In the bottom of the heating strategy, the temperature zone is the key. Energy into the preheating zone, generally there is sufficient time to conduct or radiate to the PCB, so that the board can quickly reach thermal stable equilibrium point, to ensure sufficient time for the drying zone. As effect of components’ thermal contingency, heating rate should be within 3 ℃ / sec, otherwise it may damage relatively heat-sensitive components.