Welding process of metal coating on chip electrodes and its influence

This article introduces the basic principles of gold wire ball welding technology and the methods for judging welding quality. It analyzes the influence of parameters such as ultrasonic power, welding pressure, application time, and welding temperature on welding quality, and identifies the debugging methods for key equipment parameters.

1 Introduction

Gold wire ball bonding is currently the most representative bonding technology in semiconductor device chip packaging and bonding processes. The bonding process is formed through the combined action of heat, ultrasound, and pressure, and the proper adjustment of process parameters directly affects the quality and reliability of the packaged devices. This article will study the various parameters of ball welding process through process experiments, and identify the debugging methods for key process parameters.

2. Gold wire ball welding process

(1) Gold wire ball welding process

The equipment used for the ball welding process in this article is the mainstream equipment K&S4524 gold wire ball welding machine in the current semiconductor device packaging production line. The ball welding machine is mainly composed of an ultrasonic power supply, a transducer, a wire feeding mechanism, a chopper, and a temperature controlled fixture. The welding process is as follows: during welding, the gold wire reaches the chopper tip through a hollow chopper, leaving a controllable length of tail wire. The ignition rod is activated, and the EFO system generates high voltage to discharge the tail wire, generating electric sparks. The tail wire end of the gold wire is melted instantly at high temperature. Due to the surface tension, the melted tail wire end quickly solidifies to form a gold ball. According to standard process recommendations and practical operating experience, the diameter of the gold ball is generally controlled to be 2-3 times the diameter of the gold wire; During ultrasonic welding, The Z-axis motion system will control the chopper to descend above the chip and press the gold ball onto the gold-plated pins and chip pads according to the pre-set pressure. At the same time, the ultrasonic power supply will be activated, and the electrical power will be converted into longitudinal or transverse mechanical vibration energy through the transducer. This high-frequency mechanical vibration will cause deformation and mutual diffusion of metal atoms between the gold ball or wire and the plated metal of the chip electrode. The first welding point will be completed at the end of the preset welding time, and the welding head can move to the position of the second welding point. Since there is no gold ball in the second welding point, wedge welding will be carried out. The pressure will be applied to the gold wire through the arc chamfer of the chopper tip to complete the second welding point in a wedge welding manner. Then, the chopper will rise, and the tail wire control system will control the wire clamp to complete the wire pulling process. The gold wire will be torn off at the second solder joint, and the chopper will return to its initial position, The gold wire will be precisely reserved to the preset tail wire length, and the system will activate the EFO system to ignite the gold wire tail wire into a ball, waiting for the next welding cycle process.

(2) Quality Assessment of Gold Wire Ball Welding Process

The important indicators for judging the quality of ball welding include: height of the solder ball, size of the solder ball, tensile testing, and shear testing of the first weld.

The so-called solder ball height refers to the height of the golden ball flattened on the solder pad; The size of the solder ball refers to the area of the solder pad occupied by the golden ball placed on it during welding. Generally, the size of the ball is required to be 3-3.5 times the wire diameter. These solder joint morphology parameters are mainly determined by visual inspection under a microscope. Through visual inspection under a microscope, various defects in the solder joints can be discovered to determine whether the welding quality meets the requirements.

Tensile testing mainly uses tensile testing equipment, such as the 4000PX Y-type tensile tester from the American company D A G E. This type of equipment has multiple statistical functions, including maximum value, minimum value, average value, high-low average value, median, standard deviation, overall standard deviation, Cpk value, etc. This test is a destructive test and an important test item for assessing the quality of wire bonding processes. It is also a commonly used evaluation item for packaging wire bonding processes. Test the maximum pulling force used to break the gold wire when it is hooked vertically and uniformly from the highest point of the wire arc.

The first welding shear force test also uses DAGE4000. The hook is replaced with a push knife, and the distance between the push knife and the bottom of the welding point is set to 3 μ m~5 μ m. The welding ball is pushed at a constant speed in the horizontal direction, and the force required to completely push off the welding ball will be recorded by the equipment as an important basis for judging the ability of the welding point to attach. The amount of gold residue on the pushed chip electrode is also required to be paid attention to. If there is no metal residue on the electrode surface after the push knife passes, it means that atomic diffusion has not been formed in this welding, and the welding quality cannot be guaranteed. The corresponding parameters must be adjusted to improve it. According to the distance between the push knife and the bottom of the welding point, the metal residue requirement is within 20%~25%. The thrust test data is judged to be valid. The solder joints are qualified.

3 Factors and Analysis Influencing the Gold Wire Ball Welding Process

During the welding process, improper parameter tuning can cause changes in hardware parameters such as cutting edge and temperature, which can easily lead to welding quality issues and affect the packaging and performance of the device. The factors that affect the wire bonding process have been summarized in production practice as follows:

(1) Factors affecting the first solder joint

Gold wire ball welding machine is a process of aligning the gold wire with a ball on the solder pad for welding. The process control is relatively easy, and the process requirements for the first welding point include: the gold ball is formed well and the size ratio is reasonable; The binding force with the electrode reaches its maximum; The size of the gold ball matches the size of the electrode; The neck condition of the golden ball is intact. In the actual production process, the main factors that affect the above process requirements are: the preset ultrasonic power, welding pressure, application time, and hot table temperature of the gold wire ball welding machine; The flatness, density, and surface cleanliness of the metal coating on the soldered chip; The selection of a cleaver requires a corresponding cleaver to match a certain diameter of gold wire.

(2) Factors affecting the second solder joint

The process requirements for the second solder joint include: complete and symmetrical shape of the groove; The bonding force with the solder pad reaches its maximum; The welding interface has a reasonable material thickness; The transition zone between the welding surface and the wire body is well formed. In the actual production process, the main factors that affect the above process requirements are: the preset ultrasonic power, welding pressure, application time, and hot table temperature of the gold wire ball welding machine; The flatness, density, and surface cleanliness of the metal coating on the soldered chip; The selection of a cleaver requires a corresponding cleaver to match a certain diameter of gold wire.

(3) Factors affecting the performance of the Golden Thread ball

The requirements for the playing technique include: the size and shape of the ball to be fired (too small balls can easily block the chopping knife, while too large balls can cause a short circuit between the gold wire and the firing rod during the firing process). The main factors affecting the performance of gold wire ball are: the length of the tail wire of the gold wire; The distance between the ignition rod and the tail wire; The setting of the diameter of the golden ball; The stability of EFO operation and the quality of gold wire.

The Influence of Four Key Factors on Welding Process

（1） Process parameters

The welding pressure, ultrasonic power, and ultrasonic action time have a significant impact on the ball welding process. By changing the process parameters and studying the weld qualification rate, the appropriate range of process parameters can be identified.

1 Welding pressure

Tightly contact the welding wire with the welding position through the action of pressure; Control the ball or line in a fixed position to prepare for energy transfer; Damage the pollution on the welding surface. Improper pressure setting can cause the following types of unqualified welding: setting pressure parameters too small or surface quality problems with the solder pads, which can result in weak solder joints and easy detachment during the welding process. The set pressure parameter is too large, which may cause damage to the gold wire during wire bonding, and the lead wire may be pulled off during the next welding point; Excessive welding pressure can cause deep weld marks and hinder the ultrasonic vibration of the chopper, resulting in poor transmission of ultrasonic energy and excessive loss, which affects the quality of welding.