Research on LTCC Integral Substrate Packaging Process

Abstract: In this paper, the LTCC integral packaging process is introduced, focusing on parameters influence of packaging hermetic and welding void rate are studied. The research results show that integral packaging hermetic and void rate are mainly in-fluenced by packaging structure design, surface contamination and brazing pressure Based on the above parameters adjustment, finally the high hermetic and low void rate of integral packaging products are obtained

Key Words: LTCC, integral packaging, hermetic, void rate

1.LTCC substrate Introduction

LTCC substrate has the advantages of high wiring layer, low square resistance of wiring conductor, low dielectric constant, low sintering temperature, small thermal expansion coefficient, excellent high-frequency characteristics, suitable for high and low frequency mixing and digital-analog mixing design, and is the most effective technical way to further realize miniaturization, lightweight, multi-function and high reliability of military electronic equipment.LTCC integrated packaging is a new type of high-density packaging without metal tube shell, which directly weld the cutting frame on the surface of LTC multilayer substrate, and then parallel seam weld the cover plate, so that the multilayer substrate is not only used as the multilayer circuit interconnection substrate, but also as the base of the packaging shell, so as to realize the integrated gas-tight packaging of LTCC substrate and shell.With the increasing requirements of volume and weight for military electronic equipment, especially for airborne and spaceborne electronic equipment, the application of integrated packaging is becoming more and more extensive.

In this paper, an integrated packaging process for LTCC circuit board is introduced, and the structural design factors affecting the airtightness of integrated packaging are studied. At the same time, the related processes of high airtightness and bottom cavity rate of LTCC substrate and metal bottom plate and frame welding are emphatically studied.

Integrated structure and technology of 2LTCC

2.1 LTCC substrate Basic structure

As shown in FIG. 1, the basic structure of LTCC integrated package includes LTCC base plate, metal base plate, enclosure and cover plate. In the integrated package structure, the input and output interfaces of signals are designed at both ends of LTCC base plate. The installation position of components is designed on LTCC base plate.

Fig. 1 Schematic diagram of LTCC integrated package structure

In the structure, the LTCC base plate is connected with the metal base plate and the fender frame by a brazing, and the cover plate is sealed by a parallel seam welding process.

2.2 LTCC Integrated Packaging Process

The main process steps of LTCC integrated packaging are: (1) cleaning of the components to be welded: the LTCC substrate, bottom plate and enclosure are cleaned by gas phase, and the preformed Au80Sn20 welding sheet is cleaned by ultrasonic in anhydrous ethanol/acetone mixed solvent for 15 minutes; (2) mounting: the components to be welded are mounted in turn with a human graphite positioning fixture, and attention should be paid to keeping the components to be welded clean; (3)Integrated brazing: vacuum sintering furnace is used for brazing, the peak temperature is set at 340 ℃, and the welding time is 4 minutes; (4) chip assembly; (5) parallel seam welding: the welding is carried out under nitrogen protective atmosphere.

Fig. 2 Process flow chart of LTCC integrated package

Experimental results and discussion

3.1 LTCC substrate Integrated Package Failure Mode

(1) Basic requirements for integrated welding gastight packaging

The airtight package can isolate the electronic components installed in it from the external environment, prevent the harmful liquid and solid, especially the harmful gas from corroding or penetrating into it, and can effectively guarantee the long-term reliability of the product.

(2) The welding cavity rate between LTCC substrate and bottom plate is too high.

The hole formed by welding will increase the thermal resistance of the power components on the substrate, and easily lead to the failure or burning of the power components. The large hole under the substrate means that the signal can not pass through the common ground of the metal bottom plate, but can reach it by bypassing the hole. The position and size of the welding hole on the substrate are random, and there is no certain law. The larger the welding area between the substrate and the metal bottom plate, the more holes, and the larger the area of the single largest hole.For millimeter wave and other high-frequency circuits, the size of the hole may reach the order of high frequency signal wavelength, or even larger than the signal wavelength, which will affect the acceptance or emission performance of the circuit.For LTCC integrated packaging products, the general cavitation rate is less than 25%, and there is no welding hole under the microstrip transmission line whose size is more than twice the microstrip line width.

3.2 Effect of Integrated Structural Design on Airtightness of Packages

1) Influence of LTCC heat-conducting hole structure on the gas tightness of integrated package.

Heat conduction holes are usually set on the front of LTCC substrate to improve the heat dissipation performance of the substrate. The structure of the heat conduction through hole is shown in Figure 3. As the heat conduction through hole is sintered by gold paste with high metal content and low glass content, the material of the through hole can not be fully combined with the ceramic body during sintering, and the effective welding area between LTCC substrate and metal bottom plate is low, especially when there is a penetrating welding cavity in the heat conduction through hole (Figure 4).It is easy to form air leakage passage along the direction of welding hole and heat conduction hole. 5 products with through-type welding holes in the parts of heat conduction holes are selected for leak detection before sealing, 4 of the 5 products have unqualified leak rate, and the leak detection data are shown in Table 1. In order to avoid air leakage, the occurrence of through-type welding holes should be eliminated by optimizing the brazing process.

Table 1 Leak Detection Data of Penetrating Hollow Products

Fig. 3 Schematic diagram of heat conduction pore structure

(2) The influence of LTCC substrate inner conductor wiring structure on the gas tightness of integrated packaging

LTCC circuit substrate often have a large area of metal ground plane (such as gold conductor), as shown in figure 5A.Due to the less glass content in the gold conductor paste, it is difficult to form a dense connection between the gold conductor and the LTCC ceramic interface during sintering, and there is a small leakage channel, which is easy to cause the LT C circuit substrate itself to leak, so that partial grounding layer conductors can be removed during the wiring design of the LTCC substrate without affecting the circuit performance (as shown in FIG. 5B).In the sintering process, the dense connection between the upper and lower layers of raw porcelain is directly formed, thus avoiding the generation of air leakage channels.

Fig. 5 Schematic diagram of barrier layer design

Take the two wiring schemes (12 LTCC substrates) for integrated brazing test, after the test, leak detection before sealing, the leak detection results are shown in Table 2, after setting the barrier layer, the qualified rate of gas tightness of LTCC integrated packaging products has been significantly improved.

Table 2 Leak Detection Data of Products with or without Barrier Layer

(3) The effect of enclosure height on the airtightness of the integrated package

The cover plate of LTCC integrated packaging product is gas-tight welded by parallel seam welding process. When parallel seam welding is carried out, the instantaneous high temperature 3 up to 1500 ℃ will be produced, and the huge heat will be transferred to the welding interface between the enclosure and LTCC substrate along the retractable enclosure, which will cause thermal shock to the interface, and when the thermal shock stress exceeds the strength of LTCC substrate at the interface, the LTCC substrate will crack, thus leading to gas leakage of LTCC substrate itself.In order to avoid this phenomenon, besides optimizing the parameters of parallel seam welding and reducing the heat generated by parallel seam welding, the height of enclosure (≥ 1.5mm) should be increased at the beginning of the design of integrated packaging structure, which can make part of the heat generated by parallel seam welding dissipate before reaching the welding interface, thus reducing the thermal impact on LTCC substrate in parallel seam welding process.

3.3 Effect of Brazing Process on Weld Cavity Ratio

(1) The influence of welding surface contamination

The existence of organic/inorganic pollutants on the surface of the welded assembly will make it difficult for the gold-tin solder to infiltrate on the welding surface, resulting in the formation of welding voids, especially when there are pollutants on the welding interface between the frame and LTCC substrate, there will be holes in this area after welding, resulting in gas leakage of the product.Before welding, all parts must be cleaned strictly, different parts should be cleaned in different ways, LTCC base plate, metal bottom plate and enclosure should be cleaned in vapor phase, gold solder should be cleaned in anhydrous ethanol/acetone mixed solvent by ultrasonic wave for 15 minutes.

Fig. 6 Radiographs of welding before and after cleaning (a: before cleaning; B: after cleaning)

(2) The influence of welding interface pressure.

When carrying out LTCC integrated package welding with metal bottom plate, pressing a certain pressure on the welding surface can effectively discharge the residual gas between the LTCC substrate and the welding parts, thereby reducing welding voids. The pressure applied on the welding interface should be moderate in size. If the pressure is too small, the effect of discharging gas is not obvious; if the pressure is too large, it is easy to cause a large amount of molten solder to be extruded and overflow to the non-welded parts of the product during the welding process, thus affecting the welding yield.The experiment shows that the pressure of 0.2 N/cm2 is appropriate for LTCC substrate welding. Fig. 7 shows the X-ray pictures of LTCC substrate and metal substrate at the welding pressure of 0.1 N/cm2 and 0.2 Ncm2 respectively.

Fig. 7 X-ray photographs of welding under different pressures


It should be noted that when the LTCC substrate is welded with the metal bottom plate and the foving enclosure frame, the welding area between the LTCC substrate and the foving enclosure frame is much smaller than that between the LTCC substrate and the metal bottom plate, and the two welding surfaces should be pressed separately, otherwise, the solder between the LTCC substrate and the enclosure frame will be extruded in large quantities, resulting in insufficient solder at the welding interface, thus leading to the failure of gas-tight packaging.

Fig. 8 X-ray photographs of different thickness welding pieces


(3) The influence of the thickness of the welding piece

The thickness of the welding sheet should be appropriate, the welding sheet is too thin, the solder is not enough to fill the welding interface gap, resulting in welding voids. The welding sheet is too thick, not only increases the welding cost, but also easy to appear solder overflow phenomenon. The pure gold LTCC substrate general warpage control requirement is ≤ 3 ‰, with the increase of the size of the substrate, its warpage height is also increasing.

In order to ensure good welding, the larger the size of the substrate, the greater the thickness of solder required. Figs. 8A and B are products welded with 0.05mm and 0.1mm Au80Sn20 solders under the same conditions, respectively. It can be seen that when 0.05mm solders are used, the solder can not completely fill the interface gap between the base plate and LTCC substrate, resulting in large-size welding voids, with a welding void rate of more than 25%. When 0.1mm solders are used, the interface gap is almost completely filled with solder, with a weld void rate of less than 10%.

5 Conclusion

(1) The heat conduction hole structure of LTCC substrate will affect the gas tightness of integrated packaging products, such as the penetration hole in the welding interface between metal bottom plate and LTCC substrate, which will cause gas leakage of integrated packaging products.

(2) When wiring the inner conductor of LTCC substrate, the gas leakage of integrated packaging products can be effectively prevented by designing a barrier layer.

(3) When the height of felled enclosure is insufficient, the parallel seam welding process is easy to cause LCC substrate cracking, which can be avoided by increasing the height of enclosure.

(4) To clean the welding component, the Au80Sn20 welding sheet with thickness of 0.1 mm is selected, and the pressure of 0.2 N/cm2 is applied to the integrated product to achieve good welding effect. The leakage rate of the final product before sealing is ≤ 1 × 10-pa · cm3/s (He), and that after sealing is ≤ 1 × 10-2pa · cm3/s (He), and the welding cavity rate is < 10%.

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