How to control and protect the MSD component effectively!
The component with high humidity sensitivity
According to the standard, the MSD is mainly annoyed the non- hermetic SMD
component. It includes the plastic sealing and packing material and other
polymer such as wreath oxygen, the organic resin and so on. The IC, the chip,
the electrolysis and electric capacity, the LED all belongs to the non-hermetic
SMD.
The grade of the humidity sensitivity of the MSD is divided into 6 major
types according to the J-STD-020 standard. The main difference among them is the
floor life, the volume and the refluxing temperature of the welding surface. The
main factors affecting the MSL are the Die attach material/process, Number of
pins, Encapsulation (the mold compound or glob top) material/ process, Die pad
area and shape, Body size, Passivation/die coating, Leadframe/substrate/and/or
heat spreader design/ material/finish, Die size/thickness, Wafer fabrication
technology/process, Interconnect, the Lead lock taping size/location as well as
material.
The engineering research shows that the temperature of the smaller SMD component
is higher than that of the big ones when they pass by the welding cooker with
the same pre-set temperature curve.
The "volume" here means that the length multiply the breadth multiply the
height and these sizes don't include the exterior pin, the "temperature" is
pointed the surface temperature of the component.
The product whose grade of humidity sensitivity is grade 1 does not belong to
the SMD component.
The damage principle to the credibility of the product leaded by the humidity
sensitivity.
The water in the air will spread and permeate into the packing material of the
MSD in the process of its exposition in the atmosphere. The component will
reflux into the welding stove for welding after it is stick on the PCB and it
should stop in the stove where the temperature is higher than 183 degrees, even
the highest to 210 degrees to 235 degrees for 30 to 90 seconds. Its electronics
function will be affected or damaged because of its layering or bursting leaded
by that the water in the component inflates quickly and the conjunction between
various materials loses regulation under the high temperature in the refluxing
area. Like the breakage of the ESD, this damage can't be seen by naked eye under
most circumstances and the MSD doesn't be expired completely in the test
process.
Added: Tuesday, November 28, 2006
Submitter: meilysha | meilysha@126.com
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Related Link: Article from Autodry.net
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