How to detect the defects of ceramic substrates

How to detect the defects of ceramic substrates

In the process of electronic packaging, ceramic substrates are the key components. Reducing the defect rate of ceramic substrates is of great significance to improving the quality of electronic devices. However, there is no national or industry standard for ceramic substrate performance testing. Product promotion brings certain difficulties.

At present, the main performance includes substrate appearance, mechanical performance, thermal performance, electrical performance, packaging performance (working performance) and reliability.

(Image source: Forty-three Institute of China Electric Power)

Appearance inspection

The appearance inspection generally adopts visual inspection or a tool microscope to detect whether there are cracks and holes on the surface of the ceramic substrate, and whether there are any quality defects such as scratches, peeling, and stains on the surface of the metal layer. In addition, the size and thickness of the ceramic substrate, the thickness of the metal layer, the flatness (warpage) of the substrate surface, and the accuracy of the substrate surface pattern are all items that need to be inspected. Especially for flip-chip, high-density packaging, the surface flatness is generally required to be less than 3%, and the graphics accuracy is better than 30μm.

In recent years, with the continuous development of computer technology and image processing technology, enterprise labor costs have continued to rise, and enterprises have paid more and more attention to the application of artificial intelligence and machine vision in the transformation and upgrading of the manufacturing industry. Inspection methods and equipment based on machine vision have gradually become It is an important means to improve product quality and improve yield. Therefore, applying machine vision inspection methods to the inspection of ceramic substrates can improve inspection accuracy and reduce labor costs, and has good application value.

Mechanical performance testing

The mechanical properties of the planar ceramic substrate mainly refer to the bonding strength of the metal circuit layer, indicating the bonding strength between the metal layer and the ceramic substrate, and directly determine the quality of subsequent device packaging (bonding strength and reliability, etc.). The bonding strength of ceramic substrates prepared by different methods is quite different. The flat ceramic substrates (such as TPC, DBC, etc.) prepared by high-temperature processes are usually connected by chemical bonds between the metal layer and the ceramic substrate, and the bonding strength is relatively high. For ceramic substrates (such as DPC substrates), the metal layer and the ceramic substrate are mainly based on van der Waals force and mechanical bite force, and the bonding strength is low. Commonly used bonding strength testing methods include:

Schematic diagram of shear strength test/Schematic diagram of tensile strength test

(1) Tape method: Stick the tape close to the surface of the metal layer and roll it on it with a rubber roller to remove air bubbles in the bonding surface. After 10s, the tape is peeled off with a tensile force perpendicular to the metal layer to detect whether the metal layer is peeled from the substrate, which is a qualitative test method.

(2) Wire bonding method: select a metal wire with a diameter of 0.5mm or 1.0mm, weld it directly on the metal layer of the substrate through solder melting, and then measure the tensile force of the metal wire in the vertical direction with a tension meter.

(3) Peel strength method: the metal layer on the surface of the ceramic substrate is etched (scribed) into 5mm-10mm strips, and then peeled off in the vertical direction on a peel strength tester to test the peel strength. The peeling speed is required to be 50mm/min, and the measuring frequency is 10 times/s.

Thermal performance

The thermal properties of ceramic substrates mainly include thermal conductivity, heat resistance, thermal expansion coefficient and thermal resistance. Ceramic substrate mainly plays a role in heat dissipation in device packaging, so its thermal conductivity is an important technical indicator; heat resistance mainly tests whether the ceramic substrate is warped or deformed at high temperatures, and whether the surface metal circuit layer is oxidized, discolored, blistered, or peeled off. Layers, whether the internal vias are invalid, etc. Since the ceramic substrate is generally a multilayer structure, its thermal conductivity is not only related to the thermal conductivity of the ceramic substrate material (bulk thermal resistance), but also closely related to the bonding of the material interface (interface contact thermal resistance). Therefore, the use of a thermal resistance tester (which can measure the bulk thermal resistance and interface thermal resistance of the multilayer structure) can effectively evaluate the thermal conductivity of the ceramic substrate.

Electrical performance

The electrical performance of the ceramic substrate mainly refers to whether the metal layers on the front and back of the substrate are conductive (whether the quality of the internal through holes is good). Due to the small diameter of the through hole of the DPC ceramic substrate, defects such as unfilled and pores will appear during the electroplating and filling of the holes. Generally, X-ray testers (qualitative, fast) and flying probe testers (quantitative, cheap) can be used to evaluate ceramic substrates. Through hole quality.

Package performance

Ceramic substrate packaging performance mainly refers to solderability and air tightness (limited to three-dimensional ceramic substrates). In order to improve the wire bonding strength, generally electroplating or electroless plating a layer of metal with good welding performance such as Au or Ag on the surface of the ceramic substrate metal layer (especially the pad) to prevent oxidation and improve the quality of wire bonding. The weldability is generally measured with an aluminum wire welding machine and a tension meter.

The chip is mounted in the cavity of the three-dimensional ceramic substrate, and the cavity is sealed with a cover plate (metal or glass) to realize the hermetic packaging of the device. The airtightness of the dam material and the welding material directly determines the airtightness of the device package, and there are certain differences in the airtightness of the three-dimensional ceramic substrates prepared by different methods. The three-dimensional ceramic substrate is mainly used to test the air tightness of the dam material and structure, mainly including the fluorine oil bubble method and the helium mass spectrometer method.

Reliability testing and analysis

Reliability mainly tests the performance changes of ceramic substrates in a specific environment (high temperature, low temperature, high humidity, radiation, corrosion, high frequency vibration, etc.). The main contents include heat resistance, high temperature storage, high and low temperature cycles, thermal shock, and corrosion resistance. , Anti-corrosion, high-frequency vibration, etc. For failed samples, scanning electron microscope (SEM) and X-ray diffractometer (XRD) can be used for microscopic and component analysis respectively; scanning acoustic microscope (SAM) and X-ray detector are used for welding interface and defect analysis.

Reference source:

[1] Cheng Hao, Chen Mingxiang, etc. Ceramic substrate for electronic packaging

[2] Cheng Hao, Chen Mingxiang, etc. Ceramic substrate technology and application progress for power electronic packaging

[3] Huang Sibo. LED ceramic substrate defect detection system based on HALCON

(Edited by China Powder Network/Shanchuan)

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