Technology of reflow and matters needing attention

  Abstract:
Technology of reflow and matters needing attention
Far infrared reflow soldering 　
Far infrared eighty's use of the reflow soldering has the characteristics of fast heating, energy saving, stable running, but because the printed circuit board and various components for material, different color of radiant heat absorption rate has the very big difference, caused by a variety of different components on the circuit and the different parts of the nonuniform temperature, namely the local temperature difference.   For example, black plastic package integrated circuit will be due to radiation absorption rate is high and the overheating, and the welding part of the silver lead but low temperature to produce false welding.   In addition, the printed circuit board heat radiation is blocked sites, for example in large (high) welding pin or small components will components shadow parts due to the heating caused by lack of bad welding.  　　
1.2 full of hot air reflow soldering 　
The hot air reflow soldering is a tube to force air circulation fan mouth or heat by convection injection, so as to realize the welding method welding heating.   This kind of equipment began to rise in the 90's..   Because of the heating mode, the temperature of the gas heating area of printed circuit board and component temperature close to the given, completely overcome thermal infrared reflow soldering and shielding effect, therefore, widely used. Welding equipment in the hot air reflow, critical cyclic gas convection velocity.   In order to ensure the recycle gas in any region of the printed board, air flow must be fast enough.   This to a certain extent, easy to shift caused by the jitter and components of the printed board.   In addition, the heating heat exchange method, efficiency is poor, more power.  　　
1.3 infrared hot air reflow soldering 　
This kind of reflow furnace is coupled with the hot air in IR furnace based on the furnace temperature more uniform heating, is an ideal method.   The characteristics of such equipment makes full use of the infrared penetrating power, high thermal efficiency, energy saving, and effectively overcomes the thermoelectric infrared reflow soldering and shadowing effect and to compensate for the effect of hot air reflow on gas flow rate requirements too fast caused, so the IR+Hot reflow at present in the world is the most commonly used.  　　
With the advent of the assembly density increased, fine pitch assembly technology, also appeared in the reflow oven nitrogen protection.   Welding to prevent oxidation in nitrogen atmosphere, improve the wetting force, accelerate the speed of wetting, not pasted positive element correction force, bead decreased, more suitable for cleaning process.
The establishment of 2 temperature curves
The temperature curve refers to the SMA through a reflow oven, curve SMA a little temperature change with time.   The temperature curve provides an intuitive approach to analyze a temperature change in the reflow process conditions. This to get the best solderability, avoid because of overheating and damage to components, and ensure the welding quality are very useful.  　　
The following to analyse from the preheating section.  　　
2.1 preheating section
The objective is to room temperature PCB as soon as possible heating, in order to achieve the second target, but heating rate should be controlled in a proper range, if excessive, will produce thermal shock, circuit boards and components may be damaged; too slow, the solvent is not sufficient, the effects of welding quality.   Due to the heating speed, a larger temperature difference in the period of SMA within the temperature region.   In order to prevent thermal shock to the components of the damage, the general provisions of the maximum speed of 4 ℃ /s. However, usually rise rate is set to 1-3 ℃ /s.   Typical heating rate of 2 ℃ /s.  　　
The 2.2 insulating section 　　
Insulation section refers to the temperature from 120 ℃ -150 ℃ to solder melting region.   Its main purpose is to make each element of the SMA within the temperature stabilized, reduce the temperature difference.   Given enough time in this zone, so that a larger temperature to catch smaller components, and ensure the full volatilization flux paste.   The insulation end, pads and solder ball and components pins on the oxides are removed, the whole circuit board temperature equilibrium.   Should be noted that all elements of SMA should have the same temperature at the end of this section, or into the reflux section will because each part of the uneven temperature to produce all kinds of bad welding phenomena.
2.3 return period 　
The heater in the region of the temperature is set high, the components of the temperature rises rapidly to a peak temperature.   In return the peak temperature as the solder paste used vary, generally recommended for solder melting point temperature 20-40 ℃.   The melting point is 183 ℃ 63Sn/37Pb solder paste and melting point is 179 ℃ Sn62/Pb36/Ag2 solder paste, the peak temperature of 210-230 ℃, the reflow time not too long, to prevent adverse effects on the SMA.   The ideal temperature curve is covered more than solder melting "tip" area of the smallest. (this article from electronic engineering world: