Structure and Temperature Uniformity Analysis of LTCC Furnace

Structure and Temperature Uniformity Analysis of LTCC Firing Furnace

Abstract:This article discusses the heating and atmosphere control system structure of convection intermittent lift LTCC sintering furnace. Combined with test analysis of the main factors affecting the sintering furnace temperature uniformity, parameter adjustment rules and methods during the process are described.

Key Words: LTCC sintering furnace, temperature uniformity adjustment test

1 .LTCC Introduction

LTCC (Low Temperature Co-fired Ceramic) technology is a remarkable integrated component technology developed in recent years. The basic steps of the typical LTCC substrate production process include: raw ceramic tape → molding → drilling → filling → screen printing → printing quality detection → lamination → lamination → sintering → post-processing → LTCC circuit detection.

With the rapid development of LTCC industry, production capacity and quality control put forward higher and higher requirements for the equipment. In the LTCC sintering process from the original 2 layers per furnace to the present 16 versions per furnace.However, in mass production, the equipment exposes many problems, such as firing the same furnace product, the shrinkage rate of materials placed in different areas of the furnace is very different, the consistency of products is very poor, which also causes more problems of product scrap and material waste.The chamber of the sintering furnace is 600x600x600mm, and the average temperature is 400x400x400m. Usually, we only put a few layers in the chamber, only a small part of constant temperature space is used, and only a few layers are added after the output is increased, which is far from exerting the maximum use efficiency of the sintering furnace.In this paper, the structure of LTCC sintering furnace with convective intermittent structure is analyzed by taking 43 sintering furnaces in use at present as an example, and the main factors affecting temperature uniformity control and adjustment methods are discussed.

2.LTCC sintering Temperature uniformity test

In order to find and solve the problems and improve the efficiency of the equipment, the author customized eight armored thermocouples and tested them with temperature patrol instrument. Eight thermocouples are evenly distributed in eight points in the furnace temperature area.

Fig. 1 Distribution of temperature test points

From the actual temperature test data in Table 1, it can be seen that the maximum deviation of the test point is 60 ℃ in the rising temperature section, and the maximum difference of the uniformity of sintering zone is 17 ℃ from 850 ℃ in the high temperature section, which obviously does not meet the requirements of process consistency.For LTCC sintering production, the uniformity control of temperature uniformity in the whole temperature curve of sintering furnace has a vital impact on the quality consistency and yield of LTCC substrate manufacturing.From the actual test results, the temperature uniformity index of the constant temperature zone of the sintering furnace used by us is very poor, and the real stable and consistent constant temperature zone is far less than the nominal index of 400x400x400mmn space, which can not meet the requirements of mass production, and can only be developed and produced in a small amount in the local stable space, which is also the main reason for restricting the consistency of product quality and the improvement of equipment capacity.In order to solve this problem thoroughly, we need to further analyze and adjust the sintering furnace, and fully tap the potential of the equipment.

Table 1 Temperature Test Data

Analysis of structure and temperature heat transfer field of sinter furnace

3.1 Structure of LTCC sintering furnace

In order to improve the temperature uniformity of sintering furnace, it is necessary to fully understand the structure of the equipment, especially the structure of heating and atmosphere control, which has important influence on temperature, and then analyze the main factors and adjustment methods that affect the change of temperature uniformity.What I use is a convective intermittent lifting LTCC sintering furnace, which is mainly composed of furnace heating system, atmosphere system, lifting loading system and electrical control system.The sinter furnace is box type, and the furnace body is designed as square, located at the upper part of the front side of the furnace. The insulation furnace lining is all made of multi-layer light ceramic fiber insulation plate, and four heating zones are arranged in the vertical direction according to the uniformity distribution of the heat field. Each heating zone is heated by four heating elements on the front, rear and right sides to ensure uniform heating in the furnace area. The structure is shown in Figure 2 below.

Fig. 2 Inner structure of sintering furnace

The atmosphere system mainly provides stable process gas and stable pressure control for the furnace, which is composed of air purifier, pressure reducing device, proportional regulating valve, flowmeter, gas heater and its temperature control system, pressurized ejector (air inlet box), collecting exhaust box and Venturi exhaust pipe.Corresponding to the temperature distribution in the furnace, the intake air is divided into four paths, each path is controlled by flow rate and temperature independently, the gas after preheating to a suitable temperature enters the furnace chamber evenly from the supercharged ejector on the right side of the heating chamber, the hot air flows evenly through the sintering bracket interlayer, then collects through the exhaust box, and then uses the Venturi injection effect of the exhaust pipe to maintain a certain micro negative pressure in the furnace through closed-loop control, so as to achieve the goal of making the gas flow in the furnace uniform.

3.2 Analysis of LTCC sintering furnace Temperature Field and Heat Transfer

The three-dimensional heat transfer phenomenon in LTCC sintering furnace is complex, which belongs to the unsteady heat transfer process with three heat transfer modes of heat conduction, convection and radiation in the closed space.

The convection heat transfer phenomenon in the furnace mainly includes the forced convection heat transfer between the air flow formed by the forced air inlet and LTC products, material shelves, burning plate, etc. The convection heat transfer in the furnace is the most important way of heat transfer in the furnace, it is the most important factor affecting the uniformity of temperature field in the furnace.The radiation heat transfer in sintering furnace is also the main way of heat transfer in furnace and the main factor affecting the temperature of furnace, but the influence on the uniformity of temperature field in furnace is not great because of the molding structure of sintering furnace, unless the oxidation aging degree of each heating plate and insulation layer of each part is inconsistent.

3.3 Analysis of Factors Affecting LTCC sintering Furnace Temperature Uniformity

According to the analysis of furnace thermal structure, the main factors affecting the temperature are as follows:

(1) The first is the structure of temperature field and the way of temperature control.

The temperature field of the furnace is uniformly arranged with four temperature zones along the height direction, four temperature control points, and a heater is arranged on the four sides of each temperature zone. The temperature control mode determines that the four-side heaters in the same temperature zone are heated or stopped synchronously in the thermal process. Because of the symmetrical structure design of the furnace, the comprehensive heat transfer of the furnace at any time is the same in the front and back direction. In the vertical direction, it is easier to be heated at the top than at the bottom due to the influence of the rising hot gas flow. This is the main factor affecting the uniformity of the furnace temperature in the vertical direction.

(2) The arrangement of air inlet and exhaust boxes and the mode of atmosphere flow field.

Due to the restriction of the arrangement of the exhaust chamber and the inlet chamber, the convection heat transfer in the left and right direction of the furnace is different, which becomes the direct factor affecting the uniformity of the furnace temperature. The asymmetry of the left and right structure of the temperature field and atmosphere flow field, the flow direction from right to left in the furnace, that is, the heat transfer trend driven by the convection heat transfer from right to left, will make the comprehensive heat transfer in the left and right direction of the furnace different at any time, which is the most important factor affecting the uniformity of the furnace temperature, but also the fundamental factor.

(3) Uncertain factors

43 that use sintering furnace has been for ten years, the oxidation degree of each heating plate and each part of the heat preservation layer is different, the heat radiation is not absolutely symmetrical, the uniformity of the atmosphere flow field in the furnace is uncertain due to the influence of the different mode, height and output of the sintering support in the furnace and the influence of the absorption of heat energy by the sintering support, so that the uniformity of the furnace temperature is also affected;

4  LTCC sintering temperature uniformity test adjustment

Analyze the data from the test (see Table 1)It can also be seen that the temperature uniformity is dynamic change, and there is a certain dynamic change law, the temperature of the upper A1, B1, C1, D1 test point is higher than the lower A2, B2, C2, D2 test point in the heating stage as a whole, this is because in the heating process, hot gas rises, heat upward transfer more and easier; B2, D2 on the right side is generally lower, because the right side is the air inlet end, the lower temperature of the atmosphere into pull low temperature.The temperature uniformity in the same height layer is the best. The temperature in the right side of the lower layer B2, D2 is obviously higher than that in the left side A2, C2, which can be analyzed in the direction of gas flow as shown in Figure 3. The gas flow from right to left and upward trend side by side, continuously absorbs the radiant heat energy of heating wire during the flow process in the furnace chamber. A2, C2 are located in the lower left side of the furnace chamber, which is relatively a dead zone of gas flow. The test data also verify the analysis of temperature distribution in the furnace chamber.

From the above test data and the analysis of the temperature field, we can draw a conclusion that the temperature in the left side of the furnace chamber is usually higher than that in the right side, and the temperature in the upper layer is higher than that in the lower layer; because of the endothermic effect of the bracket and the gas flow, the temperature difference in the air flow dead-angle region is very large at the initial temperature rise, and it will gradually decrease with the temperature rise; during the whole sintering process, the hot gas flow is the most fundamental reason that affects the temperature.

Fig. 3 Schematic diagram of hot gas flow in furnace chamber

The uniformity of conventional furnace temperature is measured after the temperature in the furnace chamber is constant, but in this process curve, the temperature is raised to 850 stable sintering only 10 minutes, the heating rate has a great influence on the product, so it is necessary to pay attention to the uniformity in the heating process, in this process, the maximum limit of radiation power of heating plate is particularly important to maintain the heat balance of four heating surfaces.

A. Output power of each ceramic fiber heating plate (16 in 4 temperature zones)

B. inlet flow rate;

C.4 gas preheating power and temperature;

Table 2 shows the internal parameters of the sintering furnace, which do not fully balance the influence of the internal hot gas flow rising, but partially limit the power in the left and right directions so that the temperature in the same horizontal layer is consistent, and do not fully consider the influence of hot gas flow in the vertical direction, and do not compensate for the parameters, which can be seen from the test data in Table 1.This is in line with the fact that when the equipment was first introduced into our Institute, the development volume was small, and the constant temperature space for a group of parameters with the same temperature, which was debugged by technical experts of the manufacturer for a small amount of development, was very small, but the temperature uniformity in a certain part of the same height layer was good.

Table 2 Original Equipment Control Parameters

According to the heat field distribution rule obtained from the above test and analysis, the adjustment scheme of equipment parameters is given as follows: considering the requirement of process atmosphere for discharging glue in sintering process, keeping gas flow rate unchanged, adjusting the internal control parameters of equipment, limiting power output by controlling each heating plate, limiting power in high temperature area and compensating power in low temperature area, so as to make the comprehensive heat transfer on the opposite side of furnace symmetrical at any time.The limited output power of the front and rear sides is the same, the limited output power of the heating plate in the air flow direction of the left and right sides is increased, and the power of the right air inlet is greater than that of the left side. Temperature compensation is used between the upper and lower layers to ensure the temperature consistency of each point in the constant temperature zone, so as to offset the factor of the rising of the hot air flow.After the initial parameters are determined, the sintering support is installed, repeated tests are carried out, the output power is appropriately increased near the lower temperature point, and local fine adjustment compensation is used to eliminate the temperature influence caused by the aging of the hot plate insulation layer and the influence of the sintering support on the gas flow, so as to optimize the most stable equipment state parameters.

5 Concluding remarks

The test data show that the furnace temperature field distribution law, through the power limit and temperature compensation can optimize the sintering furnace temperature uniformity, later we will further improve the process test and production verification.

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