Hot riveting with a laser - New possibilities in material processing
Empresa: LPKF LASER &ELECTRONICS SPAIN, S.L.
Introduction
Today, electronic printed circuit boards in large scale series utilisation are mostly accommodated in a protective housing for both technical and safe-ty-relevant reasons. In order to put this into effect, the printed circuit board is inserted into a plastic housing, is fas-tened in this housing and, wherever necessary, is provided with a protec-tive cover. For the stable joining of the printed circuit board with the housing, various techniques such as screwing, riveting, adhesive bonding etc. are utilised in production at present. Pref-
erence is given, above all, to the ultra-sonic and hot gas riveting processes. In the case of these joining methods, domes are attached to the injection moulding and protrude from the centre of the fastening boreholes of the printed circuit board. After or during simultaneous heating, a ram forms the top ends of these domes into rivet heads which fasten the printed circuit board after curing. The disadvantage of these processes is that the headforming ram must be brought into direct contact with the rivet dome. This results in the danger of material adhering to the ram which leads to a greater scope of cleaning in series production. Although this effect can be minimised by cooling the ram quickly after the forming process and by additional coating, it cannot be excluded altogether. Another disadvantage, in both ultrasonic and hot gas riveting, relates to the danger of damaging electronic assemblies in the immediate vicinity of the fastening position.
This greatly restricts the design freedom and, above all, the packing density in both traditional methods.
Process description
The process of hot riveting using a laser is intended to compensate for the specified disadvantages. Here, the plastified material does not have to be brought into contact with a ram.
Instead, an additional joining member is used as the rivet tip, the so-called rivet head. Hot riveting with a laser uses the easy-to-dose energy input of the laser and combines aspects of laser transmission welding in order to guarantee a reliable process for the fasten-
ing of printed circuit boards. Today, laser welding is a standard procedure for industrial material processing and is utilised successfully all over the world. It is characterised by low mechanical loads on the joining members, a small heat-affected zone as well as high reproducibility [2] and is utilised wherever material-locking joints between two thermoplastic components are demanded in a reliable process. Starting from the process principle of laser transmission welding, hot riveting with a laser may be described as follows.
The electromagnetic radiation of the laser at typical wavelengths from 808 nm to 980 nm passes through the rivet head which possesses sufficiently high transmission for these wavelengths.The dome surface which absorbs the specified wavelength range is heated.Thermal conduction serves to indirectly heat the bottom side of the rivet head as well until it has been plastified. A material locking joint between both joining members is manufactured with the action of a defined pressing-on force which is introduced into the joining zone by a suitable pressing-on jig. In this respect, precisely so much material of the dome is melted that the stable positive
locking fixing of the material to be fastened is guaranteed.
The course of the melted path as well as the laser power are portrayed over the time. Laser transparent and laser-absorbing (rivet dome) PBT with a glass fibre proportion of 30 % was used. At the beginning of the process, the clamping frame is located on the rivet head, the so-called starting point, at the pressure necessary for the process. Here, the laser energy is switched on after 0.07s. The plastification limit is reached at 0.28 s. Due to the clamping pressure, the rivet head moves towards the dome while dispelling the dome material.The welding path (0.37 mm) is reached at the point in time when the laser is switched off (0.49 s). If the clamping pressure continues to exist, the rivet head moves further downwards until the total joining path (0.4 mm) has been reached and, in a positive-locking form, is located on the element to be fastened.
Process-influencing variables
The following process-influencing variables are decisive for hot riveting with a laser: the laser power, the focus diameter and the pressing-on force between the two joining members. The external influencing variables such as the type and geometry of the thermoplastic material determine the process-influencing variables for a stable process.
The laser power must be chosen in such a way that the plastification limit of the material is reached without causing any irreversible material damage. The typical laser powers for hot riveting are approx. 5 to 25 W for relevant dome diameters of 1 to 3 mm.The chosen focus diameter is larger than the diameter of the dome. If the focus diameter is the same as or smaller than the diameter of the dome, the material dispelled by the clamping pressure cools down (material flash). This may prevent the continuation of the melting as far as the positive-locking joining of the element to be fastened.
In addition to the laser parameters, it is primarily the pressing-on force which plays a decisive role in determining the process course. The process can be accelerated or decelerated by increasing or decreasing the clamping pressure.
However, the deformation of the dome can be observed at an excessive pressure when the plastification has not been reached. An inadequate pressure may cause material damage to the rivet dome due to the poor thermal conduction contact to the rivet head.An ex-
perimentally established, convenient process window until the specified effects have been achieved guarantees that these can be excluded for a fabrication process.
The geometry of the dome is decisive for the melt flash. Two dome geometries are portrayed on Fig. 4, top. Diagram a1 shows a solid dome. In comparison with this, the dome on Diagram b1 has a hollow design. With this geometrical variant, the melt flash decreas-
es towards the material to be fastened since the cavity is filled up partially.The dimensions of the boreholes for the passage of the dome could be decreased in this case, as is evident when the caulked components on Diagrams a2 and b2 are compared. By varying the rivet head (as portrayed on Fig. 4, bottom), it is possible to push the melt flash into a cavity. A smaller borehole diameter can be chosen and the material to be fastened can be fixed in the horizontal position in addition [3].
Process monitoring
In contrast with traditional ultrasonic and hot gas riveting, hot riveting with a laser permits more effective process monitoring. Only process-influencing variables are monitored in the specified conventional processes.The crucial criteria are the frequency of the sonotrode in ultrasonic forming and the gas temperature in hot gas riveting.The joining process can be directly monitored and/or
regulated by applying hot gas riveting using a laser. The course of the joining path is a decisive criterion for this.
After an AD conversion, the established values of the path measuring system are evaluated with the aid of a software module. Not only the slope of the measuring curve (setting speed) but also the total path and the time needed for this are relevant in this respect.
System technology
The dimensions for a laser hot riveting module are in the same range as those of an industrial hand-held screwdriver for fabrication.The actual contact zone (pressing-on unit) is dimensioned as small as possible in order to guarantee maximum accessibility to the hot riveting zone. The module and process controller, including all the components, is located in a 19" withdrawable housing which is connected with the processing module by supply lines. Especially for the integration of the system into a fully automated cell, all the relevant input and output signals are made available at an external interface.The process influencing parameters such as the clamping pressure, the laser power or the melting path can be parametrised by the user at any time. The limits for the monitoring of the process can also be set directly on the laser hot riveting module. The results of the hot riveting
process can be read out for every single component via an RS232 interface and can be archived centrally. Fibre coupled diode laser systems which collimate with a suitable lens system after the fibre end and are then focused on the joining plane are utilised for the en-
ergy input. If a particular energy distribution is advantageous for the process, the power density distribution of the laser beam can also be adapted to the application using additional beam forming components.
In order to place the rivet head in an exact position, this is fed in an automated process and is fixed at the clamping ram by a partial vacuum until the welding operation has been completed. Via the moving clamping ram, especially adapted pneumatic cylinders apply that joining pressure towards the dome which is necessary for the process. Since some of the beam-forming components are located in the moving clamping ram, a constant focus diameter in the joining plane is guaranteed. Depending on the component geometry, a corresponding clamping ram is chosen in order to guarantee the maximum accessibility.
For the measurement of the movement of the clamping ram, an inductive path measuring sensor is integrated into the unit. The analog signal which exhibits behaviour proportional to the path course is monitored and evaluated electronically.
Prospects
With laser hot riveting, the user now has available a technology which yields decisive advantages over previous processes. In this respect, the robust and sophisticated basic technology of laser transmission welding offers certainty when this new process is utilised in fabrication. Areas of utilisation for this technology are today conceivable in extensive parts of automated fabrication. Even beyond the fastening of printed circuit boards, it is possible to imagine, for example, utilisation possibilities in which different materials are joined using riveting processes.
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