The influence to the qualified rate of SMT and the credibility of the product is not weaker compared with the ESD, so it is the responsibility of the SMT that can't be shirked to know the importance of the MSD, understand its damage principle, study the related standard, norm the controlling process of the MSD and avoid the unqualified rate leaded by the damaged component to improve the credibility of the product.
The marking of the MSD.
The right marking of the MSD is the first thing that should be taken into consideration to control it. Most of the component manufacturers do many beneficial works in the sealing and packing of the MSD and the marking of the damp-proof bag but not all of them follow the IPC/JEDEC principle, some of them only handwrite "MSL" on the bag, some of them note the MSL with a form code, some of them don't mark, even some of them don't pack the component by damp-proof bag. It is very dangerous that the materials are thought as un-sensitive to humidity when receive them and find that they don't packed by damp-proof bag or there's no marking in the bag. The unique measure to avoid this is to build up a database including all MSD to insure the right wrapping and there's no other more convenient methods to get the information of humidity sensitivity besides observing the label of the original packing. SO it is a challenge work itself to build up and maintain a database of MSD.
The next, it is difficult to confirm the component sensitive to humidity again once they are taken out from the damp-proof bag. In order to acquire any possible controlling measure, it is very necessary to provide convenient and dependable methods for the person handling the component and the operator to get the code and related information, including the sensitive grade of the to humidity. The most of MSD are sealed in the plastic IC tray according to the JEDEC/EIAJ standard. But the IC tray has no enough space for sticking the label and people always stick papers or gum labels on the shelf, the machine sending material, damp-proof cabinet or the bag to distinguish each tray. For the related information, the original label should be reserved after different processes. The huge difficulty will come out in the process of following the sealing and packing of the material in the tray and the artificial mistake leaded by it and the person who experiences the SMT production line has the emotional stirrings deeply.
Furthermore, the MSD is divided into six types and the controlling method of each type also differs greatly according to the standard. At the same time, there are more than one thousand operators in a factory and everybody's cognition and knowledge level is different yet, so it really is a difficult thing to guarantee each operator understand the MSD and not make mistake.
The simple but practical marking method in the actual operation is as following: First, train and investigate all the operators continuously and guarantee that they understand the MSD at least. Next, establish the regulation and system of the MSD operation and clearly peg out the prize and punishment. Furthermore, build up the MSD database and assign the special person to response releasing the MSD form to the related section periodically. According to the actual production circumstance, most of the MSL of the MSD is level 3 and all MSD are operated with the method of level 3 except specially indicated in order to simplify the operation, thus the marking of the MSD becomes very simple. The managing person in the warehouse will stick a SAP label (includes the code, description and other information of the material) on the packing of each tray material at the time of store if adopt SAP system and the operator would note all the labels of the MSD with refreshing yellow labels and note the other material with white labels. The SAP label is unique and should be rightness to each material to guarantee the marking and following in the whole process. In order to insure the material is assembled in particular time, the assembling personnel may depend on the logistic completely to control it and its damage increases along with the variety of the manufacturing craft and the diversification of the product. Because the assembling personnel has not followed with the information of the safeguard and usage of the material, they also don't know how long the material is exposed and don't clear the comparison of the MSD that they have already exceeded the unpacking life.
How greatly is the damage of this way?! Illustrate with example as follows, suppose each finished product needs one BGA, taking out one tray of BGA packed as winding with the 4th humidity level and 72 hours of unpacking life. This means that the rate of the production line should bigger than 12 pieces per hour once the material is loaded on the sticking slice and it must continuously work for three days in order to complete the production before the material is expired. At the same time the exposed time leaded by the adjustment after loading material and other circumstances such as the variety of the production plan, the lack of material, the break-down of the machine and so on should be considered. It should also be taken into consideration that the material are changed many times for the variety of the production every day in most of the factories and the exposed time increases doubly because the same tray of material are loaded on or off the sticking slice many times. After considering the actual life of all aspects, you will find that the component exceeded the unpacked life before refluxing occupies the very big comparison.
The safeguard for the MSD
The materials are usually deposited in the dry environment such as dry box or sealing to pack with the drier after they are dismantled from the stick slice and before they are used again. Many assembling personnel think that it may stop to covariance the exposed time of the component after they are stored in the dry environment. In fact, the damp air will stay and seep in the sealed and packed component slowly to damage them after they are exposed in the air for a period of time (more than a hour).
Recently the investigation expresses that the time of the component in the dry environment is same important with its exposed time and Shook and Goodelle in LANGXUN SCIENCE & TECHNOLOGY CO. have announced a thesis discussing this accurately. It is certified that the water content of the PLCC component with the 5th level of humidity sensitivity only exposed for 16 hours has exceeded its fatal damp level after the PLCC is dried for 70 hours.
Some research enunciates that the floor life of the SMD exists certain function relation with the exterior environment since the SMD is taken out from the MBB. The safer method is to control the component strictly according to the standard of the J-STD-020 and the J-STD-033.
If the MSD component is not damped, the exposed time is very short (not more than 30 minutes) and the humidity degree in the exposed environment is not exceeding RH60%, then safeguard them continuously in the auto dry box or damp-proof bag. And the original drier can be used continuously if the exposed time is not more than 30 minutes when the damp-proof bag is used.
To the MSD with the 2nd to 4th level of the humidity sensitivity, it should take 5 times of the exposed time to dry them again if only its exposed time is not more than 12 hours. It may use the drier or the auto dry box and the humidity inside the auto dry cabinet should keep at the level lower than 10%RH.
Moreover, to the component with the 2nd, 2a or 3rd level of the humidity sensitivity, the exposed time doesn't include the time of that it is put in the auto dry box or damp-proof bag where the humidity is not more than 10%RH.
To the MSD with the 5th to 5a level of the humidity sensitivity, it should take 10 times of the exposed time to dry them again if only its exposed time is not more than 8 hours. It may use enough drier or the auto dry box at normal temperature for drying them and the humidity inside the auto dry cabinet should keep at the level lower than 5%RH. The exposed time restart counting from 0 after they are dried.
If the humidity inside the auto dry box keeps at the level not more than 5%RH, it equals to save the component in a intact MBB and the shelf life is free.
The packing of the MSD
Many companies choose to re-pack the MSD. According to the standard, the basic supplies for packing contain MBB, drier, HIC, etc. and the request of packing different grade MSD is different.
The component with the 2a to 5a level of the humidity sensitivity should be dried before they are sealed by MBB.
Because the containers such as tray, tube, reel, etc. will affect the level of the humidity sensitivity when they are put into the MBB with the component together, they should be dried too.
The fix of the MSD
The best way for dismantling the component on the main plank is to heat the part and control the surface temperature of the component not more than 200℃ to weaken the damage leaded by the damp air. If the temperature of some component is over 200℃ and exceeds their prescriptive floor life, it would bake the main plank before returning to use. The temperature that the component can bear is similar to the temperature for refluxing and welding.
Some SMD component and main planks can't bear baking with high temperature for long time. For example, some FR-4 materials can't bear baking at the temperature of 125℃ for 24 hours; Some batteries, electrolysis and electric capacities also are very sensitive to temperature. For the above reasons, it can choose the auto dry cabinet at normal temperature to safeguard the SMD component.
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