|
TECHNOLOGY Material technology of raw glass-ceramic | Slitting | Foil peeling | Via hole punching | Via filling | Printing | Stacking | Laminating | Co-firing | Laser trimming |
||||||||||||||||||||||||
| Material technology of raw glass-ceramic | ||||||||||||||||||||||||
|
The first step of manufacturing raw glass-ceramic is mixing various kinds of materials and it is homogenized by a ball mill. Base material is alumina dust, solvent, organic binder, dispersing agent and aggregate. The main component is alumina. The adjustment of viscosity is by using organic aggregates; dispersing agent (ground glass) does a part during firing. The aggregates:
The aggregates are usually oxides:
The suspension (slurry) made from these materials is poured onto polyester foil (Mylar, Kapton) or glass-plate. After it the thickness of the wet material is set by a blade. Another possibility is to let the suspension flow through between two reels, so thus it is plane. Figure below shows this process.
This is followed by multistage drying when the solvents evaporate. The raw green-sheet can be peeled from the foil. The consistence of the ceramic is similar to plasticine.
|
||||||||||||||||||||||||
| Slitting | back | |||||||||||||||||||||||
The raw flexible glass-ceramics are released in reels or in dimensioned sheets. The glass-ceramics are dimensioned to its appropriate size by machine, laser or hand-tool.
|
||||||||||||||||||||||||
| Foil peeling |
back | |||||||||||||||||||||||
|
Thin polyester foil (or polyimide) protects the raw glass-ceramic against damage. The glass-ceramic substrate is applied onto a thin (~30 µm) polyester foil. This polyester foil has an important function later, because this protects the substrate from splitting before utilizing, moreover the foil-side of raw glass-ceramic substrate is less rough and dirty than the other side. The reason of the less roughness is that the glass-ceramic mixture is ductile for about two weeks, thus the mixture completely fills the gaps between itself and the foil. In this technological step the foil is peeled from them by machine or hand.
|
||||||||||||||||||||||||
| Via hole punching | back | |||||||||||||||||||||||
|
Next important technological step is via-punching. Vias make connection between layers, connecting conductive network in distinct layers. Nowadays via-punching is realized by Nd:Yag laser in case of low quantity production. Forasmuch as the raw glass-ceramic contains organic materials too, it has to be eliminated before laser cutting with a thermal treatment (else it melts and makes a coating on the surface of cut, inhibiting the cohesion of via-filler). With optimal laser sets the depth of the holes of some 10 µm can be achieved, but it have to be reckon with that the diameter of the hole on the top side is not equal to the bottom side (arises from the focus of the laser). On the figure can be seen a 220 µm diameter via punched by Nd:Yag laser.
Beside laser there are alternative, faster and cheaper solutions to punch raw glass-ceramic. For instance the mechanical puncher with cutter stamps. Since there are hard alumina particles mixed into the LTCC green sheets, the abrasion of the punch have to be often checked up. When the sheets are punched with an abraded punch, the raw glass-ceramic can be easily chipped. Recently 100 µm diameter via holes are used in LTCC circuits. The dimensions of the sheet must not change during processing, the processing surface must be smooth, processing accuracy must be satisfactory, and process scraps must not stick to the sheet.
|
||||||||||||||||||||||||
| Via filling | back | |||||||||||||||||||||||
|
Vias can be filled with a conventional thick film stencil printer. Through the aperture according to circuit pattern, paste is applied above the via holes and it is drawn with a vacuum pump into the holes to fill them. The tape has to be placed on a sheet of non-spread paper which lays on a porous stone to avoid the leak of the paste. Figure below shows process of stencil printing.
Vias are filled with paste rarely with conductive powder. On the figure a) can be seen the via hole filled with powder. The metal powder can easily fall out of the hole overleaf if there is no lytic reaction and so on between the green sheet and the filling, adherence to green sheet is poor. Solution of the problem can be seen on the figure b) on which a cap is formed with paste at the bottom of the via hole before it is filled with powder.
The porous ceramic ensures the optimal distribution of vacuum during stencil printing. The paper prevents to the via filler paste get across the porous ceramic and to clog up its gaps with paste. Mylar foil obstructs the way of vacuum above the unutilized part to focus the vacuum on the area under the substrate. If the vacuum is too strong the paste can be blurred overleaf the substrate. Sheer paper prevents the overflow of the paste furthermore it serves as a conveyance to the drying stove. It has to be taken into account, that solvent in the paste dissolves the raw glass-ceramic so that via holes can be deformed. The amount of solid ingredients in the paste is around 70%, the actual filling ratio of the conductor is limited. The more viscous is the past the larger is the filling ratio. Next figure is a filled via after firing, which electrical connects the top and bottom conductive network.
These are the technological prescriptions of stencil printing to fill vias suggested by the literature:
The filled vias are dried for 5 minutes at the temperature of 120 °C. There is another machinery, where the via filler paste is injected from a scroll into he holes.
|
||||||||||||||||||||||||
| Printing | back | |||||||||||||||||||||||
|
Screen printing technique is used for printing the RC pattern onto the raw glass-ceramic containing filled vias. During screen printing the sheet is pressed to the table with vacuum to arrest its shifting. The squeegee pushes the paste through the openings in the mask and the paste is in contact with the substrate according to the mask. After printing the substrate was dried for 5 minutes in the stove at 120ËšC temperature. The definition of printing is better than in case of alumina ceramic thick-film circuits, because of the smooth surface. The process parameters which influences the quality of screen printing:
Squeegee speed is decided by the viscosity of the paste. If printing speed is high, the viscosity of the paste falls, and its fluidity through the openings in the screen improves, because the move of the squeegee causes whirl in the paste and so the viscosity of the paste falls. This phenomenon is presented on the figure:
The squeegee press depends on the gap between the screen and substrate, and the amount of deformation of the bolting-cloth. High squeegee pressure causes bleeding readily, while conversely, low pressure may result in blurring.
DuPont suggests 325 mesh number stainless steel bolting-cloth. The least line width with DuPont conductive paste is 125µm. The thinner wiring network has to be applied by finer technology (photolithography, galvanization of laser activated surface, direct writing ( MAPLE-DW), etc).
Thickness of the printed layer is hardly influenced by the thickness of the mask. The figure shows a polyester bolting-cloth and the direct emulsion.
The thickness of the printed layer depends on the material of bolting-cloth (polyester, stainless steel), its mesh number and the emulsion. The figure shows the theoretical background of it.
This calculation can be affected by several factors, i.e. the viscosity of the paste, settings parameter of printer and the roughness of the substrate’s surface.
Figure below shows the imprints of the conductive and the resistive layer in the case of a cylindrical resistor. Because the viscosity and the content of the DuPont 2031 resistive paste (the black layer) differs from the DuPont 6145 conductive paste (the grey layer), consequently the printed layers differ too. The conductive paste distribution is uniform, whereas the resistive layer is not there. It can be seen in the enlarged parts: the resistive paste has ribbed margin, while the conductive paste has linear margin. During the printing process the paste with small viscosity produces blank regions at the crossings of the texture (this is the so-called pinhole effect).
Another important effect is the deformation of geometry during the printing process. It can be seen on the figure. In the course of printing the resistive layer, the surface of the substrate is not uniform due to the already printed conductive layer. The height of the bolting-cloth was adjusted to the substrate, but the conducting layer (because of its thickness) came nearer to the sieve. As a result of it, the resistive layer is defective above the conductive layer (1). As the squeegee passes over the conducting layer, it arrives at a cave, which can not be followed by the device neither and nor the bolting-cloth. In this place the paste fill the cave, but here the aperture do not contact the substrate, the filling will not be regular (2), the geometry get out of shape. The same thing happened when the squeegee approaches the conducting layer on the other side. The resistor width suited to the aperture can be carried out only by convenient distance (>300μm) from the ends of the resistor (3). The thickness of the layer resistors here will be suited to the adjustments also.
|
||||||||||||||||||||||||
| Stacking | back | |||||||||||||||||||||||
|
The purpose of aligning process is to align several layers of green sheet on which vias and wiring have been formed. An aligner device can be seen on the figure.
It is important that during the aligning process, the green glass-ceramic does not contact the material of the alignment device. Figure shows an aligned circuit.
|
||||||||||||||||||||||||
| Laminating | back | |||||||||||||||||||||||
|
The several layers of the raw glass-ceramic are made to a single substrate by pressure and heat.
There are two kinds of technology known to laminating:
After laminating process the substrate has deformation. The less divergence from the prescribed parameters can lead to delamination. There are five typical type of delamination defects after firing. Figure shows these forms.
Vertical splitting is delamination in which a crack forms from the middle of the edge of the substrate towards its center. Adhesion between layers is strong, and no delamination is observed between layers. During lamination, deforming forces are concentrated in the center of each edge of the green sheet. However, as the green sheets are enclosed in the mold, no distortion in the x and y axes is possible. The density of green sheets is higher at the center of the edges of the green sheet, the cracks form during firing.
This kind of delamination can be seen on this circuit.
Internal interlayer delamination is where delamination occurs at the interface between the internal conductor and the ceramic. The cause of problem is an area of poor adherence between the green sheet and conductive paste in the laminated body. When there are many layers, or the conductor on each layer is thick, there is a great difference in the thickness of the parts including the conductor and the parts with ceramic only. After firing these deformation can causes internal interlayer delamination. This phenomenon can be seen on this circuit.
Stepped interlayer delamination is caused by large conductor area of the surface of ground or power supply planes. The adherence between green sheet and conductive paste is less than the adherence between green sheet and green sheet. Besides this, the different thermal expansion is also cause of this phenomenon during firing.
Surface blistering is formed when undissolved matter in the binder and gas dissolved in the substrate. During firing it causes so high pressure that the surface geometry of the substrate is deformed by it. It is important that before lamination there is no gap and contaminant between the aligned green glass-ceramic substrates.
The reason of circular delamination is the conductor pattern is concentrated in the center of the green sheet, so that the total thickness of the middle part only is greater and is raised higher, uneven pressure is applied during lamination.
|
||||||||||||||||||||||||
| Co-firing | back | |||||||||||||||||||||||
|
The firing has two parts. The firing takes an hour in a 350 °C convection stove. The substrate is laid in a bin, and then it is put into the stove. After an hour 85% of the organic ingredients is burned out and the bins are taken to the last firing step. This process is in a general thick-film stove at the temperature of 850 °C. The conveyor speed is changed such way that about 30 minutes is the peak temperature. The firing process can be without pre-heating, if the efficiency of the vent and heat profile is controllable in the stove. This diagram shows the heat profile of Dupont 951 GreenTape.
The heat profile consists of four parts. At the first period the temperature rises sheer to the temperature of 400 °C. This is followed by a 20 minutes period, where the temperature is stabile, and the most part of materials is burned out, respectively deoxidizing. The sheer of the third period is 7.5 °C/minute till it reaches the peak temperature of 850 °C. Here is a requisite the presence of the oxygen. In the next, fourth part the temperature is kept on peak temperature for 30 minutes, and then the last, fifth period run down sheer to the ambient value.
During firing the reaction between the substrate and the setter (firing tool) is significantly effects the shrinkage rate. Such a setter has to be chosen which allows the gas in the atmosphere to flow underneath the substrate offering the additional merit of uniform firing. This figure shows this setter.
On the surface of LTCC substrate conductive and resistive network can be realized by common thick-film technology. These so-called top conductive and resistive layers demand the conventional thick-film firing technology.
This figure shows the polished section of vias in fired circuit, which keep the contact between conductive layer:
The buried components are put to multiple heats, like firing the laminates, firing the surface pattern, the melt of glass and soldering. These thermal shocks cause defects in buried layers. The explanation of it is that with every heat period the material of the substrate and the warming aggregates (conductive granules) can diffuse into the ceramic is surrounded. Because of the diffusion the original sheet resistance and the temperature coefficient is changed. In case of buried capacitors the diffusion reduces the distance between electrodes, and so it increases the capacitance. It increases the loss tangent of the capacitor. The main disadvantage of diffusion is that it is hard to hold, so thus it is hard to pre-compensate the effect at planning.
During firing the LTCC substrates shrink, the value of it in case of DuPont 951 raw glass-ceramic is between 12% and 16% (+-0.2%) in the x and y axes, and between 15% and 25% (+-0.5%) in the z axes (thickness). Figure below shows the shrinkage of the substrate.
There is a strong correlation between LTCC substrate density and its firing shrinkage rate. In order to confine firing shrinkage rate, low alkali glass with different melting points have to be mixed to the raw slurry. The correlation between density and firing shrinkage rate is shown on the diagram.
In order to achieve monolithic module, it is necessary simultaneously firing the conductor layer metal and dielectric layer ceramic. Figure shows the firing shrinkage behavior of ceramic material and conductive materials in the function of temperature:
The diagram above presents interfacial phenomenon caused by mismatch aligning. On the figure ΔT is the temperature difference in the initial phase of the shrinkage, and ΔS means the difference at the end of sintering. For example in case of ceramic this temperature where the sintering is perfect is 850 °C and in case of metal this temperature is 600 °C. The reason of ΔS is that there are gaps in the substrate and on the surface of the conductor, where the firing density is not equal. Consequently the ceramic substrate is distorted and it is also difficult to control the accuracy of the wiring dimension. The reason of the adherence defects at the interface of conductor/ceramic is ΔT. In order to have easier joint, it is necessary to optimalize the size of conductive granules, its content and the aggregates.
Consequently it is important that the optimal firing temperature of both materials nearly matches, and there is a match between the firing shrinkage behavior of both materials. As well as controlling the shrinkage behavior between both materials, it is essential to form good adherence between both materials with the aim of ensuring electrical and mechanical reliability of the LTCC itself by taking the conductor/dielectric interfacial phenomenon into consideration.
|
||||||||||||||||||||||||
| Laser trimming |
back | |||||||||||||||||||||||
|
While the actual value of the resistor cannot be measured during topological planning, the technological step trimming (after laminating and co-firing) makes possible a fine-tuning on the film resistance. It is necessary, because the tolerance of the thick film resistors is between +-20%, which is not permissible in many applications. The trimming is made by laser, which is a slow and expensive process.
The buried resistors are cut in by laser, thus the resistance increases. The cut can be in direction X, Y or in an L-shaped combination of these. The Y direction cut is more suitable than the X direction cut to achieve greater changes in the resistance. The change of the resistance can be estimated by the following figure:
The disadvantage of perpendicular trimming is that longer trimming causes extreme resistance sensitivity. The L-shaped cut combines the advantages of X and Y direction cuts: the fast, but sensitive Y cut and the slow, but controllable X cut the value of the resistor can be adjusted stably and precisely.
It is possible to measure the value of the resistor during trimming. If there are no trimming windows on the substrate, the insulator ceramic is cut through by laser, so the buried passive components become accessible. The resistor to be adjusted on the substrate has to be accessible to the laser beam.
If there are trimming windows on the substrate above the passive components, there is no need to cut through the insulator layer by laser, the component is directly accessible. The disadvantage of this method is that the trimming windows worsen the density of wiring and components on the surface.
Using laser trimming the value of the resistor can only be increased, the precision of the adjusted resistors is under +-0.25%. During reliability analysis it have to be considered that the laser beam damages the material and there will be a stricture in the current flow. The figure below shows the process and the faults that are caused by laser trimming:
|
||||||||||||||||||||||||
| back |
