PCB News - SMT original classification and examination

SMT original classification and examination

1. Subject
The main purpose of this IPC/JEDEC J-STD-020C is to test and classify the moisture (moisture absorption) sensitivity of various non-hermetic SMT semiconductor components to remind the semiconductor industry to package, store, and follow-up. Attention should be paid to moisture prevention during use, in order to cope with the test of lead-free high temperature soldering or heavy work, and reduce the strong stress caused by instant vaporization due to moisture absorption. Such stress will cause the internal, interface or outer edge of the component to burst or crack, Which is the so-called "popcorn" phenomenon. As for the early DIP double row pin socket wave soldering package components, because the IC body does not directly face the heat source, but the pin soldering is performed away from the strong wave crest through the PCB, and this type of through-hole soldering is wave soldering IC is not under the jurisdiction of this specification.

Therefore, upstream IC component vendors should assess their moisture susceptibility and obtain the graded level after passing the two-stage examinations. According to the facts, they can inform downstream assembly and soldering companies, so that they can take necessary measures with goodwill in advance. Safe-haven actions to reduce the occurrence of disasters and financial losses. Since the body of the SMT component must directly face a strong heat source, usually the mounter is more prone to popcorn than the pin soldering person, so we have to take precautions to reduce the loss

2. Grading and Examination of Mounted Components
(1) Classification and test welding depth
Since various SMD-type packaged components (not necessarily ICs, in solder paste welding, their bodies must directly face the heat source and are prone to cracking. Because other components of different sizes need to be mounted on the board respectively, it is For the sake of considering that all large parts must be welded properly, the setting of the reflow (warm time) curve will inevitably cause overheating for small parts, which often makes small and thick parts more likely to burst.

Tolerance description of the asterisk in the table: The component supplier must guarantee the compatibility of the manufacturing process for the certified components when they reach the graded temperatures listed here.

(1) When the tolerance of the reflow curve set by the production line according to the variable capacity is ten 0 degree Celsius and one X degree Celsius, in order to obtain good control of the temperature curve, the process change should not exceed 5 degree Celsius. Suppliers must ensure the process compatibility of their products at peak temperatures.

(2) The so-called package volume here also includes the pins outside the body and various additional heat sinks (such as balls, bumps, pads, pins, etc.).

(3) The maximum temperature that the component can reach during reflow is related to the thickness and volume of the component. The use of air (or nitrogen) convection heating heat source can reduce the heat drop between the components, but due to the different heat capacity of each SMD, the heat difference between them is still difficult to completely eliminate.

(4) Anyone who wants to adopt the assembly process of lead-free soldering must comply with the lead-free grading temperature and the reflow temperature-time curve.

(2) Lead-free heavy industry (rework) capability
According to the above 4-2, the package components that can withstand lead-free soldering must be reworked below 260°C within 8 hours after leaving the dry box or oven.

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