1. The problem leaded by not appropriate choosing and using the auto dry cabinet.
The damp air would enter into the IC through the sealed material and lead the internal electric circuit oxidize and decay, and the highly degree temperature within the welding process would make the damp air inside the IC inflate and create pressure, as a result that the plastics is taking off from the chip or the inner part, the chip is damaged, the internal crack and the crack extend to the component's surface is coming out, even the component are damaged and the articles constructed by them are discard. The more important is that the unseen and latent blemish conceals in the product and affects its credibility. Other IC electronics component also exists this kind of problem leaded by damp mostly.
The damage to the electronics component caused by damp come a very rigorous affair and it is becoming more and more serious. For this, the American IPC and JEDEC made and released the IPC-M-109, the standard and guide manual for the electronics component with high humidity sensitivity with common research. It puts forward the standard of damp-proof for electronics product and becomes the standard that the global electronics industry followed.
The IPC/JEDEC J-STD-033 standard in the IPC- M-109/ for the processing, packing, shipment and using of the SMD with humidity /reflux sensitivity draws up that place the IC 10 times of time that it is exposed in the damp air to recover its life. So many electronics industrial enterprises plan to purchase the auto dry cabinet which can control the inside humidity lower than RH10% at normal temperature. And how to choose a fit one is becoming very important. Because the auto dry box at normal temperature belongs to a new product, general customer does not understand it and is hard to choose an accommodate one but is easily induced by the superficial phenomenon such as price or appearance. In fact, many customers have already been not satisfied with the product after purchase it. And the problems are concentrated as following:
(1) The dehumidifying time is too long. It would take 8 to 12 hours or 3 to 5 days, even 10 days to half a month to lower the inside humidity from RH70-80% to RH10%. Even more, some products can't lower the inside humidity down.
(2) It is very inconvenient to customer that it should take a long time to lower the humidity down to RH10% again when open the door for taking or placing articles at the same time the damp air enters the box after the inside humidity is lowered down to RH10%. Because the auto dry box can't start to dehumidify immediately but should prepare for more than 2 hours and goes to preparing stage again after dehumidifying for a little time and thus repeats again and again. For this reason, some customers have to adjust the production to adapt the rhythm of the auto dry box, or only put the articles seldom used into the auto dry box, or purchase many dry boxes to solve this problem. But for the customers who want to take the auto dry box as tools, it is unable to attain the purpose.
(3) The product's life and dependability. Some products don't dehumidify after they has only used for short time as a few months or 2 to 3 years. But some products still work normally after they are already used for more than 10 years.
(4) The problem of anti- static electricity. Some products have no measures for anti-static electricity and the IC is damaged by static electricity. According to the above-mentioned problem, it is necessary to carry on analysis to the structure of auto dry box at normal temperature and provide science standard for the costumers to choose a fit product.
2. The structure and category of the auto dry cabinet at normal temperature
Currently, the dehumidifying principle of the auto dry cabinet at normal temperature be called "electronics dry cabinet", "electronics dry box" and " damp-proof box"...etc. have two ways as cooling type and physically resolving type. Among them, the cooling type uses a semi-conductor to condense and eject the water in the air. Because the cooling type can't lower the humidity down to RH10% provided in the IPC/JEDEC J-STD-033 standard, it does not belong to our discussion in this text. The physically resolving type is seen earliest as the patented product of Carteman (Chengdu) Electronics Co., Ltd. After several years some factories start to imitate and the imitators increase gradually along with the implement of the IPC/JEDEC J-STD-033 standard and the auto dry box is used extensively in electronics enterprises in recent years. The physically resolving type is a mechanical and electrical integrated machine. According to the driving way to open or close the drainage door for ejecting the inside damp air, it is divided as two major types of that driving by memory-alloy spring and driving by micro-motor. The drainage door can dependably open or close is the most important factor related to the life and credibility of the auto dry box and the safety of the articles. So-called memory-alloy spring is to increase some metal alloy in the metal material and pull it into spiral form to become spring. Put the memory-alloy into the aluminum tube with heating equip and connect the resetting spring and pull pole to be a driving device; Then switch on the heating device, the memory-alloy spring is shortened by heat and gradually create pull dint to open the drainage door for ejecting damp air after about few minutes to more than ten minutes. After ejecting the damp air, switch off the heating device to lower the temperature down and the memory-alloy spring cools off and is made longer to its original length to close the drainage door for the dehumidifier dry the inside air. Thus repeat again and again to attain dehumidifying purpose.
Nowadays science field researches the memory-alloy spring not enough. Although it has already been known that the temperature's variety will cause the metal produce a certain memory effect, its principle has not been made sure yet and it can't be repeated with science method. Therefore the memory-alloy has still been used as initial experiment and up to now all the countries in the world have no national or international standard for the memory-alloy spring. The memory-alloy material still exists a lot of shortages with the recent technical level. The mainly problem is that its characteristic is affected by time and environment much seriously and it often loses its remember function suddenly and can't shorten. If use it as the driving device of the drainage door of the auto dry box, it is possible that the drainage door can't open to eject the damp air or open with half travel as to mutual the inside and outside air and the inside humidity rises. This problem has not been overcome currently and it is also the basic reason of short life and low credibility of some products. The another problem of the auto dry box driving by memory-alloy spring is that the memory-alloy spring shortens or becomes long very slowly from few minutes to more than ten minutes controlled by the temperature. That's to say, it should takes from few minutes to more than ten minutes to open or close the drainage door and in this process the outside damp air will invade into the box and increase the inside humidity to aggravate the burden of the memory-alloy spring and lower the dehumidifying speed. The product driving by micro-motor is designed for making up the disadvantage of short life and low credibility of the auto dry box driving by memory-alloy. Suppose that the dehumidifier operates extremely for ever and everyday the drainage door opens and closes for 6 times, the micro-motor turns to move for one minute in a day, for one hour in a year and 10 years, 10 hours. But the life of micro-motor is about thousand hours, even high to ten thousands hours, so it is very normal that the auto dry box driving by micro-motor has been used for more than ten years with no break-down. As a result, its life is long and its credibility is very high. What's more, it dehumidifies much faster than that driving by memory-alloy spring because the drainage door opens or closes only within several seconds and the outside damp air can't invade into the box.<BR> The earlier product of Carteman (Chengdu) Electronics Co., Ltd. adopts memory-alloy as driving device and afterwards adopts micro-motor to avoid the disadvantage of memory-alloy spring. But since the recent year some "CARTEMAN" product uses memory-alloy spring again to meet the demand of the competing market with low price. Therefore, there are both auto dry boxes driving by memory-alloy spring and micro-motor in the market and the customer should distinguish carefully. The dehumidifying process of the physical resolving auto dry box is that start to dehumidify when the inside humidity is higher than the customer's demand examined by the humidity conductor. First, switch on the power supply to heat the electronics component in the dry box as well as to open the drainage door at the same time to evaporate the inside water and eject it out. After ejecting the damp air for more than ten minutes, the timer completes accounting and the power supply cuts off for lowering the temperature. The time for lowering the temperature is different just from more than ten minutes to more than an hour according to different models and different weather in summer or winter. The auto dry box doesn't dehumidify in the process of heating or lowering the temperature. It is called "preparing stage" and takes about from one and half an hour to two hours. The drainage door closed automatically when the temperature is lowered down and the inside humidity lowers down continuously when the damp air is ejected outside continuously. After a certain time the drainage door opens again, the heating device restarts to heat and the water is ejected. Thus go round and round, the auto dry box stops and remains on standby appearance until the inside humidity meets with the value pre-set by the customer and begins again when the inside humidity exceeds the pre-set value. It takes about 1 to 3 hours for drying according to different brand, different model and different demand. Comparing the "preparing stage" with the dehumidifying time, it can't dehumidify for about 1/3 to 1/2 of time within the whole process. So this kind of models belong to cleft dehumidifiers and its speed is slow. In addition, Carterman (Chengdu) Electronics Co., Ltd. releases a new model named "continuous dehumidifier" recently. It has no "preparing process" with very fast dehumidifying speed and is an exclusive product of Carteman (Chengdu) Electronics Co., Ltd.<BR>Now most of the auto dry boxes at normal temperature in the market belong to the type of "cleft dehumidifier" in addition to the ULD series of CARTEMAN products, that belongs to the type of "continuous dehumidifier".
3. The important point for choosing the auto dry box at normal temperature meeting with the standard of IPC/JEDEC J-STD-033.
(1) Choose the auto dry box according to the usage. The cleft dehumidifier can lower the humidity down to RH10% and meet with the request of the IPC/JEDEC J-STD-033 standard but its speed is slowly. So it is fit for the customers who seldom open the box door and not fit for the customers who should open the box door to put in or take out articles everyday such as research laboratory. To the customers such as working-room who use the auto dry box as a tool and open the box door many times everyday, the cleft dehumidifier has the problem that it should take 8 to 10 hours or much more time to lower the inside humidity down because the water in the IC or other articles in the box is drawn out continuously to increase the inside humidity. So the box door can't be opened for putting in or taking out the articles again in a day and it should take 8 hours again to lower the inside humidity down because it enters the "preparing process" once the door is opened for taking the articles in or out. But according to the IPC/JEDEC J-STD-033 standard, the SMD component with 5-5a level of damp sensitivity should be put into the auto dry box with RH10% humidity for 200 minutes to recover its life if it is unpacked and exposed at the environment where the humidity is lower than or equal to RH30%-60% for 20 minutes. And the time to recover its life in the auto dry box should be much longer if it is unpacked and exposed in the environment where the humidity is higher than RH60%. Therefore, it should take more than ten hours, even more than twenty hours to recover the life of IC for one time from putting it in the auto dry box and start the "preparing process" to lower the humidity down to RH10%. Obviously, it can't meet with the demand for working-room. So the cleft dehumidifier is not the best choice for the customers who use it in working-room as a tool.
(2) Choose the auto dry box according to its credibility. The demand to the driving device for the drainage door of the auto dry box used for saving articles is lower than that of the auto dry box used as a tool because of seldom opening the door to take in or out the articles and it stops at once the inside humidity is lowered down to the pre-set value. If choose the auto dry box driving by memory-alloy spring, the memory-alloy spring is excessive tired to shorten its life so as to lead the auto dry box break down early. The after-sales fix is not guaranteed if the factory is closed because the memory-alloy spring is specially made by factory and can't be bought in the market. Therefore, to the customers who use the auto dry box as a tool or for saving important articles and need high credibility, the product drove by memory-alloy spring is not a proper choice. In addition, the circumstance of the manufacture such as the mature craft, stable quality, etc. should be taken into consideration in the process of choosing proper auto dry box.
(3) Choose the auto dry box according to the request of anti-electrostatic. It has already been known extensively that the static electricity is harmful to the electronics component and the environment humidity is lower, the damage is more serious. So it is necessary to purchase the stainless steel auto dry box or the auto dry box sprayed the anti-static electricity coating if put in or take out the IC in the environment with ultra-low humidity. In addition, the box body should install a pillar for exhausting the static electricity to the ground. The anti-static electricity coating contains the roast paint and resins. The roast paint with many kinds of color is more beautiful but its endure ability to rub is bad and is not proper for using at the working-room. The resin with simple color is not so beautiful but its endure ability to rub is good. Take the environment of working-room and long-time usage into consideration, the auto dry box sprayed the anti-static electricity resin is a proper choice.
(4) Choose the auto dry box according to the request of the control accuracy. A. The total numerical product controlled by calculator adopts electronics humidity conductor, digital hygrometer, CPU, saving device and so on. Its characteristic is that the customer pre-sets the inside humidity through the "+" "-" bottom and the initial value increases or decreases 1% when press once. This kind of product has very highly control credibility and accuracy. B. The semi-numerical product mainly controlled by imitate electric circuit adopts electronics humidity conductor and digital hygrometer without CPU and saving device. Its characteristic is that the customer pre-sets the inside humidity through pressing the light tube and the initial value increases or decreases 10%, not 1%, when press once and a light tube is lighting on. Its control credibility and accuracy is so high as that of the product controlled by the calculator.
How to choose auto dry cabinet for the electronics component with high humidity?
