LTCC Substrates On-off Failure Mechanism Analysis

LTCC Substrate On-off Failure Mechanism Analysis

Abstract: This paper introduces two modes of typical LTCC substrate failure: short circuit and open circuit, the failure mechanism and countermeasures are analyzed and discussed, the basic reason of failure is also explored. From the perspective of layout design and material selection, the solution is developed. The reliability of the sintering experiment and environmental experiment verification scheme is used to completely solve the problem of unsatisfactory continuity caused by silver diffusion and void de - lamination between layers, and can provide LTCC certainly guidance and help for t layout design.

Key words: short circuit, open circuit, layout design, silver diffusion, de - lamination

1 .LTCC Substrate Introduction

LTCC (Low Temperature Co-fired Ceramics) technology is a kind of multilayer substrate technology which appeared in the 1980s. LTCC substrate has the advantages of high wiring layers, low dielectric constant, low sintering temperature, and can realize the integration of passive components. Compared with other passive component integration technology (thin film integration, chip integration, etc.)LTCC technology is an ideal system integration technology to achieve high reliability, miniaturization and lightweight of electronic equipment. LTCC substrate is widely used in microwave components, radio frequency devices, filters and other industries, and its electrical performance is the key factor affecting the use of LTCC devices.

The processing of LTCC substrate is as shown in Fig. 1, each layer of raw porcelain tape is punched, the metal paste is pressed into the hole, the through hole is metallized and has conductive function, the circuit diagram is printed on the raw porcelain sheet with metal paste by screen printing method, the printed raw porcelain sheet is arranged in sequence, heating and applying pressure to realize lamination, the conductor metal and raw porcelain tape are sintered together, organic matter is discharged, and finally the ceramic substrate is obtained.

The on-off failure of LTCC substrate can be divided into open circuit (OPEN) and short circuit (SHORT), open circuit refers to the circuit state in which there is no current passing between two points in the circuit or the impedance value (or resistance value) is very large, which is characterized by the on-off resistance between two points is greater than the on-off threshold (K1); short circuit refers to the connection between two isolated networks in the circuit, which is characterized by the resistance between two networks is less than the insulation threshold (K2).

2.LTCC Substrate short circuit

2.1 failure mechanism analysis

The reason for the short circuit is the contact or connection of the insulated metal conductor. From the point of view of processing technology, the main reasons for the short circuit are: metal interconnection in the same layer, metal diffusion between layers, etc. The LTCC substrate network A and network B have short circuit, the failure rate is 20%, the network information is shown in Figure 2.Check the two networks in each layer of information, network A through TopLayer, MIDLayer4, MIDLayer5, MIDLayer7, network B through TopLayer, MIDLayer4, MIDLayer6, each layer of information as shown in Figure 3.Taking into account the printing, lamination process there are 0-50 of the contraposition deviation, so the different network between the adjacent pad safety spacing should be more than 0.2mm. Base plate length is 36.95mm, width is 26.3mm, from each layer layout, adjacent pad distance is far more than 0.2mm, so exclude the same layer metal conductor interconnection.

Taking into account the characteristics of LTCC substrate, raw ceramic tape in the sintering process, the binder will be fully decomposed, reduce carbon residue, in order to avoid the insertion loss. If the carbon residue is too high, it will seriously affect the breakdown voltage of the medium, and even affect the appearance of the substrate, density and bending strength.Silver ions are unstable. When the printed conductor is made of silver paste, the silver ions will diffuse along the gap, which makes the insulating raw ceramic tape conductive. As shown in Figure 4, from the thickness direction, the two networks have four different surfaces.As can be seen from FIG. 3, 1 occur between MIDLayer44 and MIDLayer5, 2 occur between MIDLayerS5 and MIDLaye6, and 3, 4 occur between MIDLayer4 and MIDLayer7. There are 3 layers of raw porcelain tape medium between MIDLayer4 and MIDLayer7, and that possibility of short circuit caused by silver diffusion is basically eliminated.

The paste used in each layer is shown in Table 1. FX30-025 is the ferroA6M raw porcelain tape matching gold conductor, suitable for the inner layer and surface layer, and CN33-398 is the ferroA6M raw porcelain tape matching silver conductor, suitable for the inner layer.

Table 1 Material use system

Map Layer Conductor Slurry

MIDLayer4 FX30-025

MIDLayers CN33-398

MIDLayer6 CN33-398

One is the interlayer interlacing between gold conductor and silver conductor, and the other is the interlayer interlacing between silver conductor and silver conductor. The EDS energy spectrum analysis method is used to analyze the cross sections of one and two places, and silver diffusion is found in two places, as shown in Figure 5, the white bright place in the figure is silver element, there is obvious silver diffusion between the two layers of conductors.

2.2 Response Measures and Experimental Verification

The reason for the short circuit between network A and network B is that there is a small gap in the raw porcelain belt after sintering, the silver ions are unstable and migrate, and the diffusion of silver ions leads to the interlayer interconnection.There are two solutions to the problem: one is to change the material use system, the other is to increase the insulation distance between the short circuit conductors in the two networks.

Option 1: Change the size used in MIDLayer 5 from CN33398 to FX30-025, and keep the others unchanged.

Table 2 Material Use System

Before the change After the change

Map Layer Conductor Slurry Map Layer Conductor Slurry

MIDLayer4  FX30-025 MIDLayer4  FX30-025

MIDLayers  CN33-398 MIDLayers FX30-025

MIDLayer6  CN33-398 MIDLayer6  CN33-398

Scheme 2: The conductor of network B on MIDLaye6 is moved to MIDLayer7, so that the raw porcelain tape insulation layer between the two networks is increased from one layer to two layers, and the other remains unchanged.

After the two schemes are changed, the short circuit between the base plate network A and network B of this model is tracked, as shown in Table 3.

Table 3 Short circuit failure

Number of tests/only Shortcuts/only Short Circuit Proportion

Primitive 360 68 18.9%

Option 1 360 0 0

Option II 360 0 0

Through the sintering experiment, it is found that changing the material and adding the insulating layer can completely avoid the short-circuit problem caused by silver diffusion between layers. According to the scheme 1, MIDLayers4 and MIDLayer5 are "gold-gold" combination, MIDLaye5 and MIDLaye6 are "gold-silver" combination, and no short-circuit occurs.In order to verify the reliability of the experimental scheme, the substrate L1 … L5 processed by the initial process, the substrate M1 … M5 processed by the first scheme and the substrate N1 … N5 processed by the second scheme were selected to carry out environmental experiments. The high-temperature storage, low-temperature storage and temperature cycle were carried out in turn, and the resistance of each link was tested after the completion of each link. The test results are shown in Table 4.

From the table, it can be seen that after high temperature storage, the short-circuit substrate processed by the initial process becomes insulation state, the proportion is 20%, so the reliability of the substrate processed by the initial process is poor. The on-off condition of the substrate processed by the first and second schemes does not change after environmental test, and the insulation state is still maintained, so both schemes can effectively avoid the short-circuit phenomenon caused by silver diffusion. Gold paste is more expensive than silver paste, considering the production cost, it is suggested to adopt the second scheme to increase the number of insulation layers.

3 LTCC Substrate open circuit

3.1 Failure Mechanism Analysis

The reason of the circuit breaking of the substrate is that the conducting network is broken. From the point of view of the processing technology, the main reasons for the network breaking are the conductor defect, the hole is not filled, the hole is not fully overlapped with the conductor.The network information is shown in Fig. 7. The network passes through TOPLayer, MIDLayer & 8, MIDLayer9, wherein TOPLayer has pads P1, P2, MIDLayer8 has conductor L1, MIDLayer9 has conductor L2.Two holes at position 5 pass through TOPLayer, MIDLayerl..MIDLayer7, such that P is connected to L1; 6 holes pass through MIDLayer8, such that L1 is connected to L2; 7 holes pass through TOPLayer, MIDLayerl .. MIDLayera8, such that L2 is connected to P2.

The failure rate of the same network is as high as 82%, there must be some inherent factors, not due to the occasional plate events such as hole unfilled, poor printing quality. Check the screen printing screen of L1, L2, P1, P2, found that the screen is normal, printed conductor line without defect phenomenon; check the perforation data, found that there is no loss phenomenon, and perforated epigenetic porcelain belt hole information is complete. Check whether the hole and conductor overlap is sufficient, cut along 5, 67 grinding, found 5 in MIDLayer8 overlap.

3.2 Response Measures and Experimental Verification

The reason for delamination is that the hole and the conductor match bad, sintering process can not be combined with dense, from the MIDLayer8 layout, layered place, covering the hole on the conductor area is too large, and the conductor here only play a role in communication, to develop two solutions: one is to change the through-hole slurry; the other is to reduce the conductor area covered on the through-hole.

Scheme 1: The through-hole slurry is changed from CN33-407 to CN33-343, and the others are unchanged. CN33-407 and CN33-343 are both through silver hole slurry for FerroA6M.

Table 5 Material use system

Before the change After the change

OVERPORE SLURRY Conductor Slurry OVERPORE SLURRY Conductor Slurry

CN33-407 CN33-398 CN33-343 CN33-398

The second scheme is to reduce the conductor area covered by the through holes in the MIDLayer 8.

After making changes in both scenarios, trace the P1 and P2 connectivity, as shown in Table 6.

Table 6 Short circuit failure

Number of tests/only Number of open circuit/only Open _ circuit _ ratio

Primitive 96 75 78%

Option 1 96 10 10%

Option II 96 0 0

The experimental results show that the second scheme can completely solve the phenomenon of open circuit caused by delamination, the matching of through hole paste and conductor paste is particularly important when the coverage area of through hole pad is too large, comparing the matching of through hole paste and conductor paste CN33-398 found that CN33-343 is better than CN33-407, but still can not completely solve the problem of open circuit.

4 .Conclusion

(1) There are small voids in LTCC substrates after sintering, and there is short circuit risk if there are different planes interlaced in adjacent networks.

(2) The interlayer silver diffusion will not occur when the adjacent layer conductors adopt "gold paste-gold paste" or "gold paste-silver paste", and the interlayer silver diffusion risk exists when the adjacent layer conductors adopt "silver paste-silver paste".

(3) Increasing the number of insulation layers of the interlaced network can effectively avoid the short circuit caused by silver diffusion between layers.

(4) Through hole paste should be matched with conductor paste, and the area of through hole covering disk should not be too large, otherwise it will expose that the combination of through hole and paste is not dense, forming holes and causing delamination, resulting in network disconnection.

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