Surface-mount ICs are found to have a short circuit in the pins during use. Based on this situation, this article analyzes and determines the cause of failure by means of visual inspection, X-ray analysis, and coating thickness inspection.
Overall Diagram:
Test Results:The overall appearance inspection of the tin connection position showed that there were non-wetting abnormalities in the 1-pin and 2-pin tin and 2pin pads, and there were also local non-wetting phenomena in other pin pads.
Device Side Test Points (Uncoated Solder Paste):
Other unmounted dots (with solder paste coated unmounted devices):
Test Results:The uneven thickness of the tin on the surface of the test point results in a significant color differenceAt the same time, it was found that there was a poor tin expansion state in other positions on the PCB.
Test Results:The analysis found that the 1pin and 2pin tin connection positions were at the root of the pin, showing an obvious tin lap state.
Plating thickness test
Test Results:The average tin plating thickness is 10077 m, low overall.
SEM analysis of short-circuit locations:
Test Results:The pin2 pad exhibits obvious non-wetting (rejection).
EDS analysis of the unwetted fraction:
Test Results:The main components of the unwetted part are Cu and Sn, and the content of Cu elements is nearly 40%, which is abnormally high, and the surface coating may be alloyed.
EDS analysis of the Lian Tin part:
Test Results:The solder composition of the tin connection part is SN and AG, and the terminal plating is SN, which indicates that the tin connection is caused by the printed tin.
Device-side test point SEM EDS analysis:
Test Results:The composition analysis showed that the main components of the test points were Cu and Sn, and the content of Cu elements was nearly 40%, and there may be surface coating alloying.
Slice 1 location illustration.
Metallographic analysis of section 1:
Test Results:There was no obvious abnormality on the surface of the pad.
Slice 1 section SEM analysis:
Test point 1: Test point 1
Test point 2 Test point 3Test Results:It is judged that the tin plating at the test point has been fully alloyed (Sn and Cu are completely combined to form an IMC layer), and the surface is basically tin-free adhesion, which will lead to a decrease in its solderability.
Slice 1 section EDS analysis:
Test Results:The coating has a high Cu content, mainly a Cu and Sn bonding layer (IMC layer).
Slice 2 position illustration.
Slice 2 section SEM analysis:
Pin2 solder joint status.
Pin2 solder joint status.
Pin position soldering status.
PCB pad position.
Test Results:
Pin2 solder joint status: pin soldering OK, tin state on PCB pad is poor;
Pin position soldering state: IMC layer is continuous and dense, thickness is about 3 m, soldering condition is good, pad surface plating is alloyed, IMC is exposed.
Slice 2 cross-section EDS analysis:
Test Results:The coating has a high Cu content, mainly the Cu and Sn bonding layer (IMC layer) (the intermediate filler is produced by filling during grinding).
Slice 3 position illustration.
Slice 3 section SEM analysis:
Integral section. Integral section.
Local sections. Local sections. Test Results:Grind until the pad is completely unwetted, and there is no obvious solder adhesion, so that the IMC layer is completely exposed.
Combined with the above analysis information, the failure analysis of the tin link is as follows:
1. The device 1pin and 2pin are connected to tin, and the 2pin extended pad has the phenomenon of non-wetting and bare plating
2. The thickness of PCB tin plating is low
3. There is obvious tin plating alloying in the 2pin unwetted pad, and the plating analysis of other untin positions also has this problem.
Based on the above analysis, it is judged that the cause of the device tin connection is that the IMC layer on the surface of the PCB pad is exposed, and there is no available tin to form an interconnection with the solder paste, which causes the solderability to be reduced, and the solder retracts and aggregates to form tin connection during reflow.
1. Strengthen the control of the quality of PCB tin spraying process.
2. Solderability management of PCB incoming materials
PCB tin plating thickness monitoring: not only pay attention to the tin thickness, but also pay attention to whether there is a problem of coating alloying;
Solderability test added: soldering or immersion tin method.
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