1, MSD and MSD damage caused by moisture.
If MSD is over the safety of its storing time specified by IPC-JEDC-033A and is continuously mounted once dry/vacuum packing opened and exposed under working environment, which may occur following failures?
- Corrosion damage:moisture through the packaging materials and components into the joints of the internal devices, this will result in internal oxidation corrosion circuit short-circuit.
- Pressure difference Injury:When MSD devices humidity absorption rate reach over 0.1% wt, in electronic components assembled in the high-temperature welding process will cause internal thermal expansion because of moisture generated enormous pressure on the plastic from the chip or lead frame. Separation (delaminating), line bundle injury, chip damage, and internal cracks is occurred. The most serious situation is bulging and bursting known as "popcorn", which lead to rework or even scrap the assembly. More importantly, latent defects might be founded later on final products. In order to avoid this, MSD should therefore be stored in low humidity storage cabinets (also known as ADH dries cabinets, drying oven, moisture-proof cabinets to keep moisture away before sending to reflow.
2, How is MSD kept in storage?
Materials from your suppliers have confirmed that the temperature and humidity storage conditions and floor-life. (Sometimes this information written on the packaging) or according to the material suppliers to provide the humidity level of MSD (see IPC-M-109 STD033A standard) to determine the storage conditions of MSD, and stored in low humidity corresponding to the device; (low humidity cabinets, drying cabinets, drying oven, moisture-proof cabinets
MSD In accordance with the standard IPC-M-109 STD033A, only below 5% RH relative humidity only be able to safely and effectively at all levels of storage MSD
3. MSD stored flow chart in PCBA factory
4. MSD after damp treatment
- In accordance with the standard IPC-M-109 STD033A; MSD in the damp, the way through baking to obtain a replacement floor life;
- Standards for different humidity levels of MSD, recommend to the baking conditions in a variety of baking
- The higher the baking temperature can improve the efficiency of baking, but the limited scope of application, at the same time will bring the chip pin oxidation potential danger
- A lower baking temperature (5%Rh /+40°C/+60°C) applies to a much wider range of chip pins at the same time to avoid oxidation
- Have special requirements for the device may also have recourse to the way Nitrogen or vacuum baking
5 .PCB materials stored in low humidity
a. PCB storage
PCB after welding process occur in the individual solder joints around the light green of the vesicles, but there would even be serious when the size of a nail cover bubble, not only affects the appearance of quality, serious when it will also affect the performance, are welding technology that often appear in one of the issues.
chemtronics causing air bubble is because of vapor/moisture between
chemtronics and substrate. When confronted with high temperatures, moistures goes expansion and results delamination on substrate. If welding, pad temperature is relatively higher, so bubbles first appear in the pad around.
If PCB pre-processing storage conditions is under high humidity and not running moistures-proofing process before welding。In addition, in the wave soldering process, frequent use of water flux, if the PCB preheating temperature is not enough flux of water vapor will pass along holes into the hole on wall to the internal PCB substrate, pad around the first to enter the water, after high temperature encountered in welding these circumstances will have a bubble.
Solutions are:
(1) Should be tightly controlled all aspects of the purchase should be tested for PCB storage. Usually the standard case, there should not be foaming phenomenon;
(2) PCB should be stored in dry environment (which can be considered stored in less than 10% RH in the humidity cabinet);
B.PP tablets dry storage
PCB production process mainly for material PP films, the damp will be used in the production process caused by PCB board delamination; blistering and other phenomena are caused by non-performing rate of main PCB board.
At present, the way to prevent the damp has:
1) Vacuum storage: disadvantage is the use of inconvenient, the effect of instability, high cost
2) CBI fast dry cabinets ADH Series (also known as drying cabinets, drying oven, moisture-proof cabinets)
* PP films after low melting point can not be used in damp heat wave in addition to baking the way. |
