Appropriate warm-up prior to reflow the PCB; rapid cooling after reflow solder joints. SMT rework for the success of the role play to help the two most critical process, but also the most prone to overlook two issues:
Because of these two fundamental processes are often overlooked as repair technicians, in fact, sometimes after repair than before the situation worse repair. Although some "repair" defects can sometimes be inspectors after the procedure found, but in most cases do not always see them, but immediately after the test circuit will be exposed.
Warm-up - the premise of a successful repair
Indeed, PCB for a long time at high temperature (315-426 ℃) under the process will bring a lot of potential problems. Thermal damage, such as pads and leads warping, delamination substrate, raw white or blistering, discoloration. Alice and burned plates usually cause attention inspector. However, it is because it would not "burn the board" does not mean that the "board not damaged." Temperature of the PCB "invisible" damage even more serious than the issues listed above. For decades, numerous tests repeatedly proven PCB and its components can be "through" inspection after rework and testing, the decay rate than the normal high-PCB board. This substrate warpage and its internal circuit elements decay and other "invisible" problem stems from the different expansion coefficients of different materials. Obviously, these problems will not self-exposure, even at the beginning of the circuit when testing undetected, but still lurking in the PCB assembly.
Although the "repair" after look good, but as is often said of the sentence: "The operation was successful, unfortunately, the patient may die." Heat stress causes a huge, room temperature (21 ℃) the sudden exposure to heat for the PCB assembly China is about 370 ℃ of the soldering iron to solder or hot air tools for local heating, and its components on the circuit board of approximately 349 ℃ temperature changes, resulting in "popcorn" phenomenon.
"Popcorn" phenomenon is present in an integrated circuit within the device or SMD rework process of moisture in the rapid heating, the moisture expansion, there micro burst or rupture of the phenomenon. Therefore, the semiconductor manufacturing industry and the circuit board in the reflow requirements of the production staff before, try to shorten the preheating time, quickly rose to reflow temperature. Indeed PCB assembly reflow process has included warm-up before the reflow stage. PCB assembly manufacturer is the use of either wave, vapor phase or infrared reflow convection, each method should be generally warm or insulated, the temperature usually in the 140-160 ℃. Prior to the implementation of reflow, the use of a simple short-term warm-up time of PCB rework can be solved many problems. This reflow process for several years of successful history. Therefore, PCB components before reflow are many benefits of warm-up.
As the board will reduce the reflow temperature is warm, so the wave, IR / vapor phase and convection reflow soldering can be around at about 260 ℃ under the welding.
There are many benefits of warm-up and integrated
First, warm up before starting and then stream or "thermal insulation" components contribute to activation of flux to remove oxides on metal surfaces to be welded and the surface film, and the flux itself volatile. Accordingly, prior to activation in the reflow solder wetting of this cleaning will enhance the effect. Preheat the entire assembly is heated to the melting point and lower than the solder reflow temperature. This can greatly reduce the substrate and the components of the risk of thermal shock. Rapid heating will increase the component or the temperature gradient and thermal shock. Components generated by the internal temperature gradient will form a large thermo-mechanical stress, causing the low thermal expansion rate of the material embrittlement, resulting in rupture and damage. SMT chip resistors and capacitors are particularly vulnerable to thermal shock damage.
In addition, if the components warm up, can reduce the reflow temperature and reflow time shortened. If there is no warm-up, the only way to be further increased reflow temperature, reflow time or extended, regardless of which methods are not appropriate and should be avoided.