SMT Process

Reflow temperature profile

Electronics industry has been able to flourish, Surface Mount Technology (SMT, Surface Mount Technology) and sophisticated inventions have tremendous contribution. The reflow soldering (Reflow) Surface Mount Technology is the most important technologies. Here we try to explain some of the technical and settings reflow problems.

 

Reflow temperature profile includes a total of preheating, infiltration, reflow and cooling four parts, the following personal experiences finishing, if it is wrong please feel free advanced.

 

Preheating zone
 
A preheating zone is typically defined by the elevated temperature from room temperature to about 150 ℃ area in this region, the temperature ramp to facilitate some of the solvent in the paste and water in the air to evaporate in a timely manner, the IC components Electronic parts slowly warming , to meet behind the high temperatures. But PCB surface parts of different sizes, nor a degree heat off clothes in order to avoid a temperature unevenness in the preheating zone heating speed is usually controlled at 1.5 ℃ ~ 3 ℃ / sec. Uniform heating of the preheating zone Another purpose is to make moderate solvent evaporation and activate flux, because most of the activation temperature of the flux falls above 150 ℃.
 
Rapid heating will help to quickly reach the softening temperature of the flux, so you can quickly spread the flux and solder to cover the largest area, it may also make some of the activator into the actual alloy liquid. However, if the temperature is too fast, due to thermal stresses, may result in fine cracks in ceramic capacitors (micro crack), PCB uneven deformation of the heat (Warpage), voids or ic chip damage, and the solvent in the paste evaporation is too fast, the danger also cause collapse.
 
Slower temperature ramps allows more solvent evaporation or gas to escape, it also makes it more closely solder flux, decreased proliferation and possible collapse. But slow warming will lead to excessive oxidation and reduce the activity of the flux.
 
Preheat oven heating zone generally accounted 1/4-1/3 channel length, and its duration is calculated as follows: Let the ambient temperature is 25 ℃, if the ramp rate by 3 ℃ / sec calculation is (150-25) / 3, ie to 42sec, such as ramp rate in accordance with 1.5 ℃ / sec calculation is (150-25) / 1.5 is 85sec. Usually based on the degree of difference component size adjustment time to regulate the temperature gradient at 2 ℃ / sec The following is the best.
 
There are also several undesirable phenomena are associated with the warming of a relationship preheating zone, the following eleven Description:
 
1 collapse:
 
This mainly occurs in the solder paste before melting stage, the paste viscosity will decrease as the temperature rises, the temperature rise is due to the molecules makes the material by heat shock caused by a more intense; additional temperature that the solvent will rapidly increase (Solvent) no time to properly evaporate, resulting in more rapid decrease in viscosity. Correctly, the temperature rise causes evaporation of the solvent, and to increase viscosity, but the amount of solvent evaporation time and temperature are proportional, that is to a certain temperature, whichever is longer, the amount of solvent is more volatile. So slow warming warming faster than the paste viscosity of solder paste to the high viscosity of the solder paste shall also less prone to collapse.
 
Two tin beads:
 
Quickly out of the volatile gases out with even paste all together, in a small gap will form separate parts of solder paste blocks, when the separation of the solder paste reflow block will melt and emerge from the parts beneath tin beads formed .
 
3 solder ball:
 
Warming up too fast, solvent gas will rapidly evaporate from high school and put tin solder splashes caused. Slow the rate of warming can effectively control the production of solder balls. But slow warming will lead to excessive oxidation and reduce the activity of the flux.
 
4 wicks siphon phenomenon:
 
This phenomenon is solder wetting pin, the solder from the solder joint area along the pin upwards, so that the solder joints produced inadequate or air welding problems. It is probably caused solder paste melting stage, parts of the foot pad on the PCB temperature above the temperature due. Bottom of the PCB can be increased or extended temperature near the melting point solder paste at a time to improve, the best you can reach parts of the solder wetting front feet and pad temperature equilibrium. Once the solder has been wetting pad, the shape of the solder is difficult to change, by the temperature rise rate at this time is not in effect.
 
5 poor wetting:
 
Generally poor wetting is due to welding Guo Cheng Zhongxi powder caused by excessive oxidation can be reduced by preheating paste absorb too much heat to improve. Ideal reflow time should be as short as possible. If there are other factors that cause the heating time can not be shortened, and that proposal from room temperature to adopt linear temperature between the melting point solder, so when reflowed solder powder can reduce the possibility of oxidation.
 
6 Weld or "pillow effect" (Head-In-Pillow):
 
The main reason is probably because Weld lights Rui siphon phenomenon caused or non-wetting. Lamp lights Rui Rui siphon siphon phenomenon can refer to the solution. If it is non-wetting problem, that is, the pillow effect, a phenomenon that has been dipped in solder parts foot, but did not form a real or wetting of gold, the problem can usually be improved by reducing oxidation, can refer to the wetting poor solution.
 
7 tombstone effect and skew:
 
This is because both ends of the parts caused by uneven wetting, wicks similar siphon, the solder paste can be extended by a time near the melting point to improve, or to reduce the rate of heating, the temperature of both ends of the part in the solder paste balance before melting. Another thing to note is that the PCB pad design, if there is obviously different sizes, asymmetry, or one with a grounding pad (ground) is not yet designed thermal resistance (thermal thief) and the other no grounding pad are appear likely to cause different temperatures at both ends of the pad, the pad when one first melted, due to surface tension pulls, parts will stand straight (tombstone) and pull the ramp.
 
8 holes (Voids):
 
Mainly because of the flux of solvent or water vapor rapidly oxidized, and did not live in the solder before curing escape due.
 
Infiltration area
 
Also known as the active region infiltrated in a constant temperature zone temperature is usually maintained at 150 ℃ ± 10 regions, when solder is melted on the eve of the volatiles in the solder paste is further removed, activators started, and the effective removal of weld surface oxides, PCB surface temperature by convection effects, different sizes, different textures component temperature can be kept uniform, the plate surface temperature difference △ T close to its minimum. Nearly horizontal curve shape, it is also the assessment of a reflow furnace process window, select the activity to maintain a flat temperature profile of the furnace will improve the welding effect, particularly to prevent tombstoning defects. Constant temperature zone of the furnace is usually 2,3 district, maintenance time is about 60 ~ ~ 120s, if the time is too long will lead solder oxidation problems that fly after welding bead increased.
 
Reflow zone
 
The highest temperature reflow area, often called the liquid over time (TAL, time above liquidous). In this case tin solder pad made ​​of copper or gold as the formation of intermetallic diffusion of compounds, tin-copper alloy, for example, when the solder melted, and quickly wetting the copper layer, a tin atom and Cu atom in its the interface penetrate each initial Sn-Cu alloy structure Cu6Sn5, with a thickness of 1-3μ, recirculation zone within the furnace critical stage, because the temperature gradient assembly must be minimized, TAL must remain in the solder paste manufacturer specified parameters within. Peak temperature of the product is achieved at this stage - the assembly of the furnace reaches the highest temperature. Care must be taken that the board should not exceed the highest of any temperature-sensitive component temperature and heating rate. For example, a typical lead-free processes tantalum capacitors have a maximum temperature of 260 ° C can only last up to 10 seconds. Ideally, the assembly of all the points should be at the same time, with the same rate reaches a peak temperature, to ensure that all parts in the furnace through the same environment. The recirculation zone, the product cooled and solidified solder, the assembly ready for the subsequent step. Controlling the cooling rate is critical, and cooling too quickly may damage the assembly, cooling slowly to increase TAL, may cause weak solder joints.
 
Reflow peak temperature, usually depends on the melting temperature of the solder and assemble parts can withstand temperatures. Generally the peak temperature should be less than the normal melting temperature of the solder paste to be higher than about 25 ~ 30 ° C, in order to successfully complete the welding operation. If less than this temperature, the most likely cause poor wetting of cold welding and shortcomings.
 
Cooling zone
 
Generally believed that rapid cooling of the cooling zone should the solder solidifies. Rapid cooling can also get finer grain structure together to improve solder joint strength and bright solder joints, and showed a continuous surface meniscus shape.
 
In contrast, in the slow cooling above the melting point easily result in excessive intermetallic compounds and greater co-crystal particles generated, reduce fatigue strength. Using relatively fast cooling rate can effectively deter the formation of intermetallic compounds.
 
In the accelerated cooling rate to be noted that while the ability of impact resistance parts, generally the maximum allowable capacitance of the cooling rate is approximately 4 ° C / min. The cooling rate is too fast may cause stress and cracking (Crack). Pad may also cause the pad to the PCB and solder joint or peeling, which is due to components, solder and solder each have different coefficients of thermal expansion and shrinkage results.

 

分享到:
Hits:  UpdateTime:2013-06-16 23:51:17  【Printing】  【Close