SMT reflow welding four temperature zone function

776 Published by FASTPCBA 4月 12,2019

SMT reflow welding four temperature zone function

1.Basic knowledge of SMT reflow soldering

SMT reflow soldering technology is not strange in the field of electronics manufacturing, the various components on the board which used in computer is welded on the circuit board through this process, the device’s internal have a heating circuit, heat the air or nitrogen to a high enough temperature to blow onto the circuit board where the components are already mounted. Let the solder on both sides of the component melt and bond with the main board.The advantage of this process is that the temperature is easy to control, the welding process can also avoid oxidation, manufacturing costs are easier to control.

In the SMT mounting line process, after the mounting machine completes the mounting process, the next process is the soldering process. The SMT reflow process is the most important process in the entire SMT surface mount technology. Wave Welding, SMT reflow soldering and other equipment are normal welding devices.

Today I will discuss the functions of four temperature zones in SMT reflow soldering: the preheating zone, the constant temperature zone, the backwelding zone and the cooling zone. Each stage of the four temperature zones has its important significance.

 2.Reflow weldinginclude which temperature zones?What’s the difference for function?

SMT reflow preheating zone

The first step of reflow welding is preheating. Preheating is to activate solder paste and avoid bad welding caused by sharp high temperature heating during soldering. The normal temperature PCB board is uniformly heated to reach the target temperature. To control the heating rate in the heating process, too fast will produce thermal shock, may cause damage to the circuit board and components;Too slow solvent volatilization is not sufficient, affecting the quality of welding.

SMT reflow soldering insulation zone

The second stage – thermal insulation stage, the main purpose is to make PCB board  in the reflow furnace and each component temperature stability, so that the component temperature remains consistent.Due to the different sizes of components, large components need more heat, heating slow, small components heating fast, in the insulation area to give enough time to make the temperature of the larger components to catch up with the smaller components, so that the flux fully volatile out, avoid welding bubbles.At the end of the holding section, oxides on the pads, solder balls and component pins are removed under the action of the flux, and the temperature of the entire circuit board is balanced.Tip: all components should have the same temperature at the end of this section, otherwise there will be a variety of bad welding in the reflow section due to the uneven temperature of each part.

Reflow soldering back to soldering area

The temperature of the heater in the reflow soldering area rises to the highest, and the temperature of the component rapidly rises to the highest.Peak in the back street section, the peak welding temperature varies with the different paste used, peak temperature generally is 210-230 ℃, reflow time should not be too long, to prevent adverse effects on the components and PCB, may be the cause of scorched circuit boards etc.

SMT reflow machine

SMT reflow machine

Reflow cooling zone

In the final stage, the temperature is cooled below the solder paste freezing point to make the solder joint solidify.The faster the cooling rate, the better the welding effect.The slow cooling rate, will lead to excessive eutectic metal compounds, and the welding points easy to have the big grain structure, make the weld strength is low, cooling zone and cooling rate, generally at about 4 ℃ / S, cooled to 75 ℃.

3. Process flow

Reflow welding process for the surface of the board, the process is more complex, can be divided into two kinds: single-side mount, double-side mount.

Single face mounted on

Pre-coating solder paste →SMT(divided into manual mount and automatic machine mount)→ reflow welding →inspection and electrical testing.

Double-sided mounting

Surface A pre-coated solder paste→ mount (divided into manual mounting and automatic machine mounting) →SMT reflow welding →surface B pre-coated solder paste patch (divided into manual paste and automatic machine paste) →reflow welding →inspection and electrical testing.

4. What are the advantages of SMT reflow welding technology?

(1) reflow soldering technology does not require immersion of printed circuit board in molten solder, but uses local heating to complete the soldering task;Therefore, the soldered components have little thermal impact and will not be damaged by overheating.

(2) solder is only required to be applied at the welding site in the welding technology,  and local heating is used to complete the welding, so as to avoid welding defects such as bridge.

(3) solder is only secondary use, there is no re-use of the situation in SMT reflow solder technology,, so the solder is very clean, no impurities, to ensure the quality of the solder joint.

5.Reflow welding maintenance attentionmatters

In order to avoid improper cleaning furnace, resulting in combustion or explosion, it is strictly prohibited to use high-volatile solvents to clean the furnace inside and outside. If high-volatile solvents, such as alcohol and isopropanol, are not to be avoided by this method, please make sure that such substances are evaporated before using the equipment.all parts of the welding, dust, dirt or other foreign bodies must be cleaned before maintenance then add the oil!Special attention is that when we carry out routine maintenance of SMT reflow welding, if we find any machine fault, we cannot repair it without authorization, and we must inform the equipment management personnel to deal with it in time.At the same time must pay attention to safe operation in the maintenance process, Do not operate out of specification.

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