Pulse heat bonding, a method for interconnections with Anisotropic Conductive Adhesive Foil (ACAF) and Heat Seal Connectors (HSC).

Dipl. Ing Karl Lindner, Munich
Ing. Ton Reinders, Helmond
Paper presented during conference:
Adhesives in Electronics "94"
Berlin, November 1994

·     Purpose
·     Heat Sealing process for Anisotropic Conductive Adhesives
·     Heat Sealing process with pulse heating
     -   Process sequence and parameter.
     -   Systems requirements and application.
·     Advantages of pulse heating

Electrical conductive adhesive bonds between flexible and rigid circuit boards, glass panels and flex foils are performed by heat sealing. This paper describes the pulse heating method for the heat sealing process.
The essential characteristics of this process are: heating up and cooling off under pressure. This proves to be essential for the production of homogeneous, void-free adhesive bonds with anisotropic adhesives. Process, equipment, major conditiones and environment are described.

PURPOSE

Present developments of the new anisotropic conductive adhesives do require a heat seal system with improved process control. Heat seal systems using constant heat thermode do not offer similar process reliability and generate a lower yield, consequently.
Lifting the thermode in a hot condition and consequently cooling of the heat seal connection without pressure. may result in building up of unacceptable voids or even contacts being out of position.
Therefore the key to success for the heat sealing process is called pulse heating.
With the pulse heating method heating up and cooling off takes place under pressure. This is the only way to make sure that the parts to be joined remain under pressure till the adhesive has been cooled off to such a temperature level whereby the required strength has been reached.

HEAT SEALING PROCESS FOR ANISOTROPIC CONDUCTIVE ADHESIVES

Anisotropic conductive adhesives are filled with small conductive particles taking care of the electrical contact between the electrical conductive areas (pads) on a display, a printed circuit board (substrate) and a flexible foil with conductive tracks (Heat Seal Connector) (Fig. 1).
The adhesive can be applied as a foil with a protection film or as a paste by screen printing. Depending on the type of polymers used the adhesive can be a thermoplastic or thermosetting or a combination of both.
In order to bring the conductive particles, spread at random in the adhesive, in continuous contact with both electrical contact areas the adhesive must be heated under pressure. These filling particles will finally establish the electrical contact between the parts to be connected (Fig. 2)
During this process, a certain quantity of adhesives may be pushed aside in the spaces between the electrical contact areas during this process but without creating short circuits between these contacts. After cooling off and releasing the pressure all conditions remain the same.. This process of heating and pressing together in hot condition is called Heat Sealing. Configuration and dimensions of the particles in the adhesive are part of the characteristics of that adhesive and may vary depending on manufacturer and type of adhesive. In general particles do have a metal surface and a typical diameter of about 10-12 µm.
These electrical conductive particles may be divided in some categories. The most common are:
-        metal parts
-        carbon parts (graphite)
-        plastic parts metal coated
-        glass parts metal coated
-        combination of plastic and metal parts.
The composition of the adhesive is a trade secret.

HEAT SEALING PROCESS WITH PULSE HEATING

Process sequence and parameters

Pulse heating as process is known already from soldering techniques. For soldering a pulse heated tool (thermode) will transfer pressure and heat to the workpiece. Sending an electrical current directly through the thermode causes it to heat up.
The cross section of a thermode in the heating area should be as small as possible in order to achieve a fast process temperature rise and consequently a quick cooling off at the end of the soldering cycle. For that reason one chooses for a material combining a high electrical resistance with a good thermal conductivity e.g. Molybdenum or Nickel alloys.
The thermode is used in a temperature range between 250 - 450°C depending on the thermode material. dimensions and heat conductivity of the parts to be connected and the use of an interlayer or not. One of the most important conditions for the uniformity of a connection is the thermode flatness within a few µm when being heated up till the working temperature. Also, the temperature distribution over the contact area of the thermode should he as uniform as possible with just a few degrees deviation. In the first instance this is determined by the thermode configuration and the construction (Fig. 3).

On the other hand the thermode should he executed sturdy enough to convey the forces required for the heat sealing process. In fact the opposite of what is needed for fast heating up and coolingdown according to the informationgiven by the adhesive manufacturer. The required forces to be applied depend on the specific force per square unit and on the total surface contact area to be connected. Common and often used values, for most applications range between 200 and 1500N. This. force range determines in fact the requirement or specification for the sealing head: Force range from 100 to 1700 N. Play-and shock-free high precision gui-dance system but above all: the thermode must I run parallel to the sealing surface.
The heat sealing force will be built up after reaching a positive contact between thermode and parts to be connected heating may start either when the thermode starts moving downwards or after reaching the preset force The thermode will be heated up till the preset value and will stay on that temperature during the preset time.
The temperaturesetting chosen should be in such a range that the required sealing temperature of about 110 to 180°C will be achieved in the bonding area.
Every adhesive requires its own typical process settings and these are indicated and specified by the adhesive manufacturers. The typical temperature cycle must be kept during a certain time. This duration depends on how long that specific adhesive should be kept on a particular temperature. This sealing time varies between 3 and 20 seconds depending on the type of adhesive and manufacturer.
A plastic deformation of the adhesive takes place during the heating cycle. The opposite contact areas come so close to each other that the electrically conductive particles with the plastic core will be deformed elastically consequently forming an electric conductive connection. In order to secure that condition, achieved after a defined process time, the thermode heating must be shut off and the thermode must he cooled down.
The heat seal connection is kept under pressure by the thermode till a safe preset cooling off temperature, specified by the adhesive manufacturer, is reached.

C after having reached that temperature the thermode may go back into home position and the process cycle is completed. For a complete sequence of heating, keeping the temperature and cooling including the thermode movements one will find cycle times between 10 and 25 seconds depending on type of adhesive and manufacturer.
Such a process sequence is shown in the Fig. 4a and 4b. The temperature measured is the thermode temperature, not the temperature in the connection. The difference between the thermode temperature and the temperature required for the sealing process indicated by the adhesive manufacturer will be determined by the properties of the materials used, the thickness of the substrate, displays or the flex foils and if used the thickness of the interlayer out of polyimide or silicon rubber and of course by the thermode configuration.
This temperature difference can be up to 100-120K. When the pads on the board are lower it is possible to make a good bond using a double layer of this reinforced silicone. The glassfiber reinforced silicone is perfect when used on an LCD, because it enables the adhesive between the tracks in the foil to make an optimal bond to (lie glass-surface of the LCD A disadvantage of using an interlayer is the fact that the process parameters show wider spread which means an increase of the process window. Furthermore the loss of heat is increased due to thehigher thermode temperature.

The use of an interlayer between thermode and sealing area can have different reasons. It may be used as a protection for the thermode in order to avoid pollution of the thermode by adhesive escaping from the heat seal area in those cases where thermode and other parts are so close thus preventing adhesive running freely. The interlayer can also act as a buffer to absorb unevenness which may lead to force tolerances. Probably more important is, when using an interlayer, the fact that such an interlayer will force the fluid adhesive in the clearance between two adjacent contact areas which will increase the adhesive power of the connection (see photo 1 and table 1).

TABLE 2      Thermal material properties

	Silicon Rubber	Polyimid	Polyesther	Epoxy Glass	Glass	Al2O3 Ceramic
Thermal conductivity l    W/(K×m)	0,3(RT)	0,16(RT) 0,19(300°C)	0,15(RT)	0,23(RT)	0,46-0,92(RT)	10-28(RT)
Glass transition temperature (°C) per type of composition	>250	240-270	98-120	135-155	4651	1850-2000

Table 2 shows that the upper limit of the thermode temperature should be related to the glass temperature of the parts closest to the thermode The heat resistance of the adhesives in the flex foils should he taken into account as well.
The adhesives used are often based on epoxies with a heat resistance suitable for soft soldering applications. The higher the process temperature the shorter the heating time.
To avoid too much plastic deformation or thermal deterioration of the flex foil a lower process temperature with a longer duration may be considered.
Since the temperature at the thermode side of the bonding will be higher than that of ahesive side the glass transition temperature will determine also the maximum admissible thickness of the flex film.
At the same time it should be taken into account that, type of material thickness of the substrate, even the material of the fixturing determine the heat dissipation in the topside of the substrate where the heat sealing should take place.
The amount of heat drawn away from the sealing area should be replaced by the same amount in order to achieve the required process temperature profile specified by the adhesive manufacturer.
Not only the heat generated by the thermode in the contact area or the size of the contact area itself but also the thermal conductivity of the thermode material and the heat conduction to the heat seal area are responsible for the heat transport.
This conduction of the heat depends also on the thermal conductivity and thickness of the plastic parts (flex foil, interlayer used. Especially pulse heating offers to a high degree the conditions required for a weld defined generation and supply of this heat.
For the investigation of the process parameters the thermocouples can he placed between the parts to be heat sealed. These thermocouples should be as thin and flat as possible to avoid an undesired influence on the results.
Arithmetic determination of the temperature in the heat seal area requires a very complex and extensive mathematic model taking into account all the heat resistance values for the heat conduction and all the heat dissipation resistance values for the heat loss including the heat capacities for these resistances.

Systems requirements and applications

The machine set up or configuration will be defined by the required pulse heating process. The bonding head (foot pedal operated, pneumatically controlled or motorised) brings the thermode down to build up the specified force. Placement of the thermode must be parallel to the supporting surface and the thermode itself should have a perfectly flat contact area in the range of 5 to 10 mm.
In most cases the thermode is firmly mounted to the sealing head but pivot mounting is also possible in order to absorb, to a certain degree some surface condition tolerances.
Product fixtures are in principle customer specific and frequently placed on a sliding system, which enables a simple and easy assembly of the parts and offers enough field of view to carry out proper positioning outside the bonding area. If the adhesive is already present on one of the parts to be connected only two parts have to be aligned in relation to each other. If the adhesive comes as a foil a reel system is used. The adhesive film will be cut to length, placed on the substrate or flex foil and after pre-tacking the adhesive to the surface the protection film will be peeled off either in separate pieces or wound back on a reel
The alignment of the HSC to the position of the substrate or an LCD requires more carefulness when the pitches are becoming finer. A specific light source required to visualise the ITO (Indium Tin Oxide) on the LCD and a video system can help to carry out proper alignment. With a video mixer the image of the substrate contact area can be frozen. The image of the HSC-contact area can be easily aligned and properly positioned. By doing so both parts can be individually illuminated each with its own brightness.
Heat seal systems as used for several applications are shown on photos 2-3-4-5.
Photo 2 shows a manual station with a pneumatically actuated bond head and an ACAF transport system. Thermode and product fixture are custom made and product specific. The force range for such a system is 50 to 750N.
Photo 3 shows a system with a motorised bonding head and has force profiles from 5 to 500 N. Such a system makes it possible to work with temperature and force profiling with different levels in each profile. This enables a better process control and the process cycle time can be shortened.
Photo 4 shows in principle the same system as shown on photo 3 but this unit features an optical system consisting of a camera, special light source with lenses and monitor. With this light source the ITO's on the LCD panel are visualised in order to do a proper alignment of the HSC on top of the LCD display.
Photo 5 shows a production system with a two position turning table. One for loading and alignment and one for the actual heat sealing operation. The three part product fixture has two pneumatically controlled bonding positions. The force range for this system is up to 1750 N for a sealing length up to 180 mm.
In comparison with reflow soldering the heat sealing process is a slow process which makes working with turning- or sliding tables very attractive, because the "dead" machine time can be used for loading, positioning and unloading, which in turn leads to a substantial decrease of cycle time. consequently it increases the throughput of the system. It is quite obvious that with all these modular basic systems a further degree of mechanisation or automation can be achieved. Systems with a turning table with multiple positions enable the use of more sealing positions used in parallel which improves again the productivity of such a system.
For a proper heat sealing process the system should be of rugged construction and working precisely and properly, both in the low force range as well as in the sometimes very high force range required for those applications where very long sealing length with a thermode which should remain parallel to the bonding surface is necessary.
The substrate should be flat in the heat sealing area; the back should have enough clearance for local support.

ADVANTAGES OF PULSE HEATING. TRENDS.

	Defined process heat (temperature and time).
	Temperature controlled curing of the adhesive under pressure. This means fixation of the conductive particles in a prestressed position (very important for adhesives with thermoplastic properties).
	Prevention of bubble formation or voids caused by internal stress or contact failures of' the flex foil (common phenomenon when heat sealing with constant heat and long thermodes).
	Force-temperature profiling feature improving and optimising the process.
	Easy for automation.
	SPC-capabilities for a better process control and process data (ISO 9000). There is an increasing demand worldwide for flexible connections between Liquid Crystal Displays or Matrix Dot flat panels and printed circuit boards or flex foils. To improve the yield a reliable process with a professional system set up will be required.

The advantages of the currently available adhesives combined with all the advantages of pulse heating automatically leads to a better product quality, improved yield and to new applications.
It should be noticed that the adhesive manufacturers are working on new generations of adhesives with shorter curing times, consequently shorter process times which together with pulse heating form a must in order to rationalise production.

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