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Failure analysis of vacuum sintered die-bonding chip peeling off

In the production and processing of military electronic components, sometimes due to the abnormality of the chip back gold process, defects in the die bonding process during the packaging process may occur, resulting in abnormal chip bonding performance and reduced packaging yield. For a certain type of integrated circuit, because the thickness of the chip processing back gold did not meet the technological requirements, the chip fell off and failed after vacuum sintering. Starting from the molten state of the back gold and solder of the circuit chip, advanced physical analysis technology and equipment are used to analyze the failure. Finally, by enhancing the monitoring of the back gold process and reducing the process defects, the problem of abnormal bonding performance of the circuit chip is solved, and the packaging yield is improved.

Vacuum sintering refers to the use of alloys with a melting point lower than that of the material to be welded under a certain degree of vacuum. Interdiffusion and dissolution of the metals to be welded form intermetallic compounds, which are finally cooled to form highly reliable welds. After a certain type of integrated circuit adopts the vacuum sintering process to bond the chip package, the screening test and the quality consistency test are carried out. In the quality consistency inspection, the sample tube cap opening test was selected for the group test, and it was found that the sample tube chip was broken and separated from the base. In this regard, failure analysis and positioning and failure mechanism analysis are carried out, and corrective measures are formed to prevent failures from occurring again.

Mechanism analysis

A unique feature of the gold/nickel system is its better stability when exposed to higher temperatures during assembly and subsequent service life. If the coating is too thin or the uniformity of the coating is poor, the protection function of the coating on the surface of the chip will be reduced, resulting in poor wettability of the chip back gold and the brazing material when the chip is vacuum sintered and bonded, and there will be no residual brazing material on the back of the chip. Phenomenon.

In this batch of unpackaged wafers, 5 chips (numbered 1#~5#) were randomly sampled, and the chip back gold solderability test was used to simulate the vacuum sintering and bonding process, and the same packaging process but different 1 chip processed at the same time, and a comparative test was carried out.

Sampling the chip with the gold side up and placing the solder pads into the vacuum eutectic reflow soldering equipment. The process parameters are set according to the requirements of the vacuum sintering sticking process, and the sintering is carried out. After the sintering is completed, observe the wetting state of the chip back gold and the brazing material.

The molten solder spreads out on the backside gold surface of the chip and shrinks again, forming a rough and irregular surface with thicker solder bumps connected to the thin solder layer. Comparing the chip back gold and the brazing material to form a good wetting, it can be seen through visual inspection that a smooth, uniform and continuous brazing material coating is formed on the surface of the back gold, while the sample chip ( 1# ~ 5# ) Roughness, bumps, and pinholes appear. The brazing filler metal on the tube shell has a regular melting shape and a flat surface. There is no brazing filler metal residue on the back of the chip, showing a dark silver color. The width of the flow area where the brazing filler metal exceeds the edge of the chip is 1mm~2mm.

According to the above test analysis, the back gold quality of the circuit in question is quite different between chips and within chips. The thin nickel layer will affect the activity of the nickel layer, and the gold layer is thin or not dense enough, causing the air to pass through the gold layer to oxidize the surface of the nickel layer, resulting in poor chip solderability. After the vacuum sintering process is applied, the environmental and stress tests during the screening test will magnify the quality hidden danger of poor solderability, causing some circuit chips to fall off.

Selection of vacuum sintering equipment: The RVS vacuum sintering furnace provided by SIMUWU is a high-quality product for this kind of process. It has the characteristics of good temperature uniformity, high temperature control accuracy, and efficient glue removal system. SIMUWU specializes in the manufacture of vacuum furnaces, has more than ten years of relevant experience, and has a good reputation in the field of vacuum furnace manufacturing. The product line includes vacuum air quenching furnace, vacuum oil quenching furnace, vacuum brazing furnace, etc., which are widely sold in developed and developing countries. SIMUWU provides a professional team of engineers who can solve various problems encountered in the production process and are committed to giving customers the most convenient and efficient experience.