1. Development of reflow welding equipment

In the electronics industry, a large number of surface-mounted components (SMA) are welded by reflow welding machines. The current heat transfer method of reflow welding has gone through three stages of far infrared-all hot air-infrared / hot air.

  1.1 Far infrared reflow welding
The far-infrared reflow soldering used in the 1980s has the characteristics of fast heating, energy saving, and stable operation. However, due to the different materials and colors of printed boards and various components, the absorption of radiant heat is very different, resulting in circuit The temperature of various components and parts is not uniform, that is, local temperature difference. For example, the black plastic package of an integrated circuit is overheated due to high radiation absorption, and the soldering site, the silver-white lead, has a low temperature, which causes false soldering. In addition, the parts on the printed board where heat radiation is blocked, such as solder pins or small components in the shadowed parts of large (high) components, will cause poor soldering due to insufficient heating.

1.2 Full hot air reflow welding
Full hot air reflow welding is a welding method in which convection jet nozzles or heat-resistant fans are used to force air circulation to achieve heating by the weldment. This type of equipment began to rise in the 1990s. Because this kind of heating method is adopted, the temperature of printed boards and components is close to the gas temperature in a given heating temperature zone, which completely overcomes the temperature difference and shielding effect of infrared reflow welding, so it is currently widely used. In full hot air reflow welding equipment, the convection speed of circulating gas is very important. To ensure that circulating gas acts on any area of ​​the printed board, the airflow must be fast enough. To a certain extent, it is easy to cause the jitter of the printed board and the displacement of the components. In addition, the use of this heating method is inefficient in terms of heat exchange and consumes more power.

1.3 Infrared hot air reflow welding
This type of reflow welding furnace is based on the IR furnace plus hot air to make the temperature in the furnace more uniform, which is currently an ideal heating method. This type of equipment makes full use of the characteristics of strong infrared penetrating power, high thermal efficiency and power saving. At the same time, it effectively overcomes the temperature difference and shielding effect of infrared reflow welding, and compensates for the too fast gas flow requirements caused by hot air reflow welding Influence, so this kind of IR + Hot reflow soldering is currently the most commonly used internationally. With the increase of assembly density and the emergence of fine-pitch assembly technology, a nitrogen-protected reflow furnace has also appeared. Welding under the protection of nitrogen can prevent oxidation, improve the welding wetting force, accelerate the wetting speed, have a large correction force for uncorrected components, reduce the number of welding beads, and is more suitable for no-clean processes.

2. Establishment of temperature curve
The temperature curve refers to the curve of the temperature of a point on the SMA as it passes through the reflow furnace. The temperature curve provides an intuitive method to analyze the temperature change of a component throughout the reflow process. This is very useful for obtaining the best solderability, avoiding damage to components due to over temperature, and ensuring the quality of soldering. The following is a brief analysis starting from the preheating section.

2.1 The purpose of this preheating section is to heat the room temperature PCB as quickly as possible to achieve the second specific goal, but the heating rate should be controlled within an appropriate range. If it is too fast, thermal shock will occur, and the circuit board and components may be affected. If it is too slow, the solvent volatilizes insufficiently, which affects the welding quality. Due to the fast heating speed, the temperature difference in the rear SMA of the temperature zone is large. In order to prevent damage to the components due to thermal shock, the maximum speed is generally specified as 4 ° C / s. However, the rise rate is usually set at 1-3 ° C / s. The typical heating rate is 2 ° C / s.

2.2 Insulation section Insulation section refers to the area where the temperature rises from 120 ° C to 150 ° C to the melting point of the solder paste. Its main purpose is to stabilize the temperature of each component in the SMA and minimize the temperature difference. Allow sufficient time in this area for the larger components to catch up with the smaller components and ensure that the flux in the solder paste is fully volatilized. At the end of the insulation section, the oxide on the pads, solder balls and component pins is removed, and the temperature of the entire circuit board reaches equilibrium. It should be noted that all components on the SMA should have the same temperature at the end of this section, otherwise entering the reflow section will cause various bad soldering phenomena due to uneven temperature of each part.

2.3 Reflow section In this area, the temperature of the heater is set to the highest, so that the temperature of the component rises rapidly to the peak temperature. The peak soldering temperature in the reflow section varies depending on the solder paste used. It is generally recommended that the melting point temperature of the solder paste be increased by 20-40 ° C. For 63Sn / 37Pb solder paste with a melting point of 183 ° C and Sn62 / Pb36 / Ag2 solder paste with a melting point of 179 ° C, the peak temperature is generally 210-230 ° C, and the reflow time should not be too long to prevent adverse effects on SMA. The ideal temperature profile is the smallest area covered by the “tip region” above the melting point of the solder.

 2.4 Cooling section The lead-tin powder in the solder paste in this section has melted and sufficiently wet the connected surfaces. It should be cooled as fast as possible. This will help to obtain bright solder joints and have good Form factor and low contact angle. Slow cooling will cause more decomposition of the circuit board into the tin, resulting in dark and rough solder joints. In extreme cases, it can cause poor soldering and weaken the bonding force of the solder joint. The cooling rate of the cooling section is generally 3-10 ℃ / s, and it can be cooled to 75 ℃. When measuring the reflow soldering temperature curve, a temperature curve tester (hereinafter referred to as a temperature meter) is used. The main body is a flat metal box, and one end of the socket is followed by several miniature thermocouple probes with thin wires. During the measurement, solder, adhesive, and high-temperature tape can be used to fix the test point. Turn on the switch on the thermometer. The thermometer enters the oven cavity along with the printed board to be measured, and automatically records and records according to the built-in time program. After the test is recorded, connect the tester to the printer to print out multiple temperature curves of various colors. Thermometers, as the eyes and tools of SMT craftsmen, have been used quite widely in foreign SMT industries.

When using the thermometer, pay attention to the following points:
  ① For measurement, a fully assembled board must be used. First of all, the thermal characteristics of printed circuit board components are analyzed. Due to the different heating performance of printed circuit boards, component sizes and material differences, the actual heating and temperature rises at each point are different. Find the hottest and coldest points and set thermoelectricity. Even the highest and lowest temperatures can be measured.
② Set up as many thermocouple test points as possible in order to fully reflect the true heating state of each part of the printed board. For example, the center of the printed board and the edge are not heated to the same degree. Large-capacity components and small components have different thermal capacities and heat-sensitive components must have test points.
③The shape of the thermocouple probe is small, and it must be fixed at the test position with the specified high temperature solder or adhesive, otherwise it will loosen due to heat and deviate from the predetermined test point, causing test errors.
  ④ The batteries used are lithium batteries and rechargeable nickel-cadmium batteries. Resonable test and timely charging in accordance with specific conditions to ensure the accuracy of test data.

3. Main factors affecting uneven heating of reflow welding
 There are three main reasons for the uneven heating of components in the SMT reflow welding process: the difference in the thermal capacity or absorbed heat of the reflow soldering components, the influence of the edge of the conveyor or the heater, and the load of the reflow welding product.
① Generally, the thermal capacity of PLCC and QFP is larger than that of a discrete chip component. It is more difficult to weld large-area components than small components. ② In the reflow soldering furnace, the conveyor belt is used to reflow the products for reflow soldering. It also becomes a heat dissipation system. In addition, the edges of the heating part have different heat dissipation conditions from the center, and the edges generally have a lower temperature. In addition to the different temperature requirements in the temperature zone, the temperature of the same load surface is also different.
③ Different effects of product loading. The adjustment of the temperature curve of reflow welding should take into account that good repeatability can be obtained under no-load, load and different load factors. The load factor is defined as: LF = L / (L + S); where L = length of the assembled substrate and S = interval of the assembled substrate. To obtain reproducible results in the reflow process, the larger the load factor, the more difficult it is. Usually the maximum load factor of the reflow furnace is in the range of 0.5 to 0.9. This depends on the product situation (component soldering density, different substrates) and different models of the reflow furnace. To get good welding results and repeatability, practical experience is very important.

4. Cause analysis of welding defects related to reflow welding

4.1 Solder slump also occurs during the bridge welding heating process. This situation occurs in preheating and main heating. When the preheating temperature is in the range of tens to one hundred degrees, the solvent as one of the components in the solder will be Reduce the viscosity and flow out. If the outflow tendency is very strong, the solder particles will be squeezed out of the gold-containing particles outside the soldering zone at the same time. If it cannot be returned to the soldering zone during melting, the remaining solder balls will be formed. In addition to the above factors, whether the terminal electrodes of the SMD component are smooth, whether the circuit board wiring design and the pad spacing are standardized, the choice of the solder resist coating method and its coating accuracy will cause the bridge.

4.2 Tombstone (Manhattan phenomenon) Warping of a chip component under rapid heating. This is because the rapid heating causes a temperature difference between the two ends of the component, and the solder on one side of the electrode is completely melted to obtain good wetting. One side of the solder is not completely melted, causing poor wetting, which promotes the component to stand up. Therefore, when heating, it is necessary to consider the time element to make the horizontal heating form a balanced temperature distribution to avoid the occurrence of rapid heat.
The main factors to prevent the component from standing are as follows:
① Selecting solder with strong adhesion, the printing accuracy of the solder and the placement accuracy of the components also need to be improved;
② The external electrodes of the component need to have good wettability and wet stability. Recommendation: The temperature should be below 40 ℃ and the humidity should be below 70% RH.
③ Use a small pad width to reduce the surface tension on the component end when the solder is melted. In addition, the printing thickness of the solder can be appropriately reduced, such as 100 μm;
④ The setting of welding temperature management conditions is also a factor for component uprighting. The usual goal is to make the heating uniform, especially before the welding fillet of the two connection ends of the component is formed, the equilibrium heating must not fluctuate.

4.3 Poor wetting Poor wetting refers to the solder and the pad (copper foil) of the circuit board or the external electrodes of the SMD during the soldering process. After wetting, no mutual reaction layer is formed, resulting in missing soldering or less soldering failure. . Most of the reasons are caused by contamination of the surface of the soldering area or adhesion of solder resist, or the formation of a metal compound layer on the surface of the joint. For example, sulfide on the surface of silver and oxide on the surface of tin can cause poor wetting. In addition, when the residual aluminum, zinc, cadmium, etc. in the solder exceeds 0.005% or more, the degree of activation is reduced due to the hygroscopic effect of the flux, and poor wetting may also occur. Therefore, anti-fouling measures must be taken on the surface of the soldering substrate and components. Select the appropriate solder and set a reasonable soldering temperature curve.

Reflow welding is a complex and critical process in the SMT process, involving a variety of sciences such as automatic control, materials, fluid mechanics, and metallurgy. To obtain excellent welding quality, we must thoroughly study all aspects of the welding process.