DE3837458A1 - Method for producing a fuse link by bonding, as well as a fuse link - Google Patents

Abstract

The invention relates to a method for producing an electrical connection between two contact surfaces with the aid of a wire, by bonding, in order to form a fuse link. Varying the parameters during bonding results in protective deformation of the bonded attachment and, in addition, the wire guidance is varied such that there is no need for bending up after the first bonded attachment and for bending down for the second bonded attachment. In a development, solutions are indicated as to how uninfluenced base material is used for the actual fuse path. The invention also relates to a suitably constructed fuse link.

Description

The invention relates to a method for producing a electrical connection between two contact surfaces with Help a wire by bonding to form a melt fuse, in which a bond head runs along the head under an end-side crimped wire first on the one contact surface and at the same time Deformed actuation of an ultrasound source and thus attached and then with simultaneous leakage let the wire out of the guide over the second contact leads and fixed the wire in the same way. The invention also relates to a fuse,  which essentially consist of two contact surfaces and one by fusing wire applied to the contact surfaces consists.

The bonding of fuse wires for fuses is in contrast to fixing the fuse wire through Soldering is attractive for two reasons. For one, one can appropriate security with the so-called SMD technology (Surface Mounted Device) for mounting on a Printed circuit board or on a chip heated to solder temperature be without the cohesion between each Kon tact area and the fuse wire is at risk or changes the connection must be feared.

For this reason is proposed in DE-OS 37 31 969, in the attachment area of a coated fuse element by bonding to apply a contact surface, the sheathing to eliminate the disadvantages of bonding. On the other hand aging is more manageable, which in particular applies to impulse loads. With soldered fusible wires This is because diffusion occurs with a high number of impulses pass between the solder and the fuse wire with which Consequence that a high impulse load the life of the Fuse greatly reduced.

The disadvantages that occur during bonding are in DE-OS 37 31 969 described in detail. Essentially acts micro cracks in the edge area of the bond fastenings, the usual uses of bonds for electrical Connection of chips and the like are of little importance, because of the low electrical load next to no effect. With a fuse however, thermal stress occurs regularly, which can cause the microcracks to propagate, so that after a certain number of starts a change in the electrical values. The tricky thing about micro cracks is that immediately after making one Bonding by means of electrical tests is practically non-existent are noticeable, but later adversely as a result noticeable reduction in cross-section.  

The use of coated fusible conductors for bonding, where the sheathing in the actual work area of the fuse wire is removed by etching after bonding has proven itself and above all has that hoped effect that possible micro cracks in the area of bond attachment not disadvantageous to the Life or function of a fuse Act.

However, the process is relatively complex because of a Covering the fastening area, a subsequent one Etching and cleaning included in the production process have to be. This creates interference, for the rest becomes a fuse in terms of cost charged. It is therefore an object of the invention to provide a method of the type mentioned in such a way that uncovered Fusible conductor fixed to the contact surfaces by bonding and still be the results of in the mounting area sheathed fusible conductors can be achieved.

To achieve this object, the invention proposes the following Features and steps ahead:

• a) Providing a one-piece metal body, the which contains two level contact surfaces,
• b) weak deformation of the wire during crimping,
• c) strong ultrasound dose, short duration,
• d) flat travel curve of the bond head from a contact surface to the other without significant bending of the Wire after the first attachment,
• e) connecting the two contact surfaces by a housing and  
• f) separating the metal body residues apart from one Soldering surface on every contact surface.

The usual procedure for a bond attachment consists of the Placing the bondhead on the contact surface, whereby between the bondhead and the contact surface of the enamel wire is guided. After deforming the fuse wire by squeezing while using an Ultra sound dose, which is usually applied from the bondhead the bond head is pulled vertically upwards, and by an amount that depends, among other things, on the distance to the next contact surface to be contacted becomes. After starting up, the second bond point is approached and by lowering the head the contact to the second contact surface.

During these steps, the one that is initially bonded The wire is bent upwards when the bond head is raised and then a fraction of the bend again bent back, namely when the bondhead the second loading has reached the consolidation point and is lowered. In the Er The result is that the bruises anyway strain-hardened zone immediately next to the bond attachment is deformed twice more, namely by opening bend and then flat bend the wire again. It has been shown that due to this bending point the strong and frequent deformation the formation of micro tearing is particularly likely. Here she creates Invention remedy.

First, through a particularly gentle deformation the degree of work hardening and thus the embrittlement severely restricted at the point of bond attachment. The slight deformation requires a relatively short, however high ultrasound dose, so a satisfactory one at all electrical contact between the fuse wire and the contact area comes about. The high supersonic dose causes e.g. the Use of metal bodies as carriers of the contact surfaces,  but also thin substrates made of aluminum oxide or other Ceramics can be used under certain conditions.

Another important feature is that the Bond head not vertical in the usual way from the contact surface is moved upwards, but in a flat area stretched curve towards the same level, second Contact surface, so that the explained two deformations by bending. Instead, the bondhead towards anyway due to the bond deformation easily upward wire section moved so that none Alternating bending stress of the wire immediately next to it the bond attachment occurs.

After bonding it is important that the two contact no longer surface despite removing the metal body remnants are moved towards each other or almost no longer are moved so that the fuse wire and the Bond fastenings are not damaged. Here offers the use of the housing covering the contact areas surrounds later, as a stability aid. Of course can also be used other devices that have a purely auxiliary function if after bonding in the use of the housing is not yet the assembly process it is your turn or for other reasons on a case conventional type is dispensed with. It just comes insist that the contact surfaces after bonding to each other take the same relative position and finally clamped, welded, glued or cast.

In particular come as wire material for the fuse element Gold and aluminum or their alloys in question. If a metal body made of copper must be selected in the bond area gold plating. So that this doesn't diffused into the copper must act as a diffusion barrier a layer of nickel between the copper and the gold the copper can be applied to which then the gold plating for example in a thickness of 0.2 to 2 µm  will be carried. For wires made of aluminum or aluminum Alloys require a copper coated with aluminum sheet as a metal body, the respective coatings galvanically, by roll cladding or in any other way can be applied. The metal bodies are preferred wisely manufactured by punching. It is important to ensure that the inevitable degree of punching when bonding the bondhead is facing so that the metal body is full on it Underlay and so the sound energy of the ultrasound dose actually reached the joint.

In common bonding machines there is to be separated of the wire behind the bondhead a brace that follows the last bond is closed and a traction on exercises the bond position. Because of the cross-sectional weakening and embrittlement in the area immediately breaks the wire behind the bond attachment. It goes without saying the section between the tear-off point and the bracket then pre-stretched, i.e. work-hardened, because it is stretched has been subjected to, almost always, the plastic Deformation area reached. If no countermeasures this stretched section forms part of the next fuse wire that is in a series manufacturing following the previous on two contact surfaces is applied. It was already explained at the beginning that a pre-stretched wire is more brittle than the pure exit material and therefore more sensitive to microcracks when bonding is. Added to this is the change in the crystal structure, see above that the electrical resistance is disadvantageous or undefined is changed.

In a development of the invention it is therefore proposed that the first bond attachment a further, zero bond fastening upstream and the second another third Bond attachment is subordinated, and that the further Bond attachments to the others through the same wire are directly connected. This measure will still stretched section contained in the bondhead as an empty sheet attached to one contact surface, the length This blank should be chosen so that it is the pre-stretched one  Section safely used up. For the actual melting pot rich of the fuse element then becomes unstretched output material used, which consequently provides the best conditions for a perfect function.

After completion of the second bond attachment on That would end of the actual fuse element section described cutting off the wire by tearing off a Bring with it danger for this second bond attachment, because their parameters are only with regard to the electrical conductivity and not with regard to one high strength are designed. So for security another blank sheet and a third one Bond attachment completed. This in turn can finally carried out according to strength considerations with a relatively strong deformation and a relatively weak ultrasound dose with a long on duration of action. The wire is then removed in the usual way cracked, due to the decoupling by the second Blank sheet influences the actual fuse element stretch is no longer to be feared. A corresponding one designed fuse is in claim 9 under protection posed. It goes without saying that the blank sheets can be shaped almost arbitrarily. Even if later due to vibrations or other stresses on a the blank sheets or both blank sheets at any point being cut has no effect on the function the fuse as long as the first and second bond attachment are fine on both sides of the melting section.

Exemplary embodiments of the invention are which are shown in more detail in the drawing, explained in more detail; show in the drawing:

Fig. 1 is a schematic cross-sectional view of a preferred embodiment of a fuse according to the invention, prepared according to the method presented under protective ge,

Fig. 2 is a cross-sectional view of a so-called SMD fuse, manufactured according to the protected method and

Fig. 3 to 5 schematic plan views of metal body configurations for the realization of mass production according to the protected process.

In FIG. 1, the basic structure is schematically represented a fuse according to the invention. At the opposite ends of two sheet metal strips 1 , the base material of which is made of copper, for example, there are two contact surfaces 2 , a fuse element 3 being applied to bridge them to form a fuse. The fusible conductor consists, for example, of a gold wire a few hundredths of a millimeter thick, in which case the sheet metal strips are coated with a nickel layer and in the contact area area with a wafer-thin gold layer.

The fuse element 3 is divided into three sections, namely in the actual melting section 5 and two blank sheets 6 and 7 on both sides of the melting section 5 . The following sequence can be observed when attaching the fuse element 3 :

First, with a conventional bonding head (not shown), a first bonding attachment 4/0 is applied to one contact surface 2 and the fuse wire fastened in this way is curved to form the first empty sheet 6 . Within the blank sheet 6 is the section which is more or less pre-stretched in the preceding separation process and that solely from the point of view of good electrical contact between the contact surface 2 and the fusible conductor 3 and from the point of view of protecting the wire, i.e. with a relatively small deformation without micro cracks.

Subsequently, the bond head is led flat over to the other contact surface 2 , so that there is no bending and subsequent bending down on the bond attachment 4/1 , but the position which is slightly upward anyway is continued in a natural way by the bond connection. Subsequently, the 4/2 bond attachment is made, again in accordance with the same criteria according to which the bond connection 4/1 has been made. Due to the consideration of the electrical concerns, the strength of the 4/2 bond attachment may be relatively low. so that another blank sheet 7 is made with a final bond 4/3 . This bond attachment 4/3 can be used as a fixed point for tearing off the melting wire 3 , because it is designed solely for strength reasons.

The typical method steps for the method according to the invention can be explained particularly clearly on the basis of FIG. 2. First, a fuse element 3 is bonded to the sheet metal strips lying opposite one another. It can be done in the manner described in connection with FIG. 1, or the method according to the invention is used in its simplest form, that is to say using only two bond attachments.

Subsequently, a two-part housing is placed around the sheet metal strip 1 and finally attached, for example by snap locks, glue or by extrusion coating. In Fig. 2, a housing base 8 and a housing cover 9 can be seen, which are snapped together in a manner not shown Darge. The then flat sheet metal strips 1 are then freed from a metal body residue, so that they protrude laterally from the housing as a flat strip, which is indicated in FIG. 2 by dashed lines and the reference numeral 1 '. These free ends of the metal strips are then bent under the housing base 8 , so that an SMD fuse is created overall. The length of a fuse actually implemented is, for example, 6 mm or less.

In Figs. 3, 4 and 5 are by punching resulting pattern of metal bodies 10, 10 'and 10' 'shown that in the production according to the Ver invention drive a precursor for providing the sheet-metal strip 1 with the contact pads 2 form. The metal body 10 according to FIG. 3 is a copper sheet which is coated with a nickel layer. A wafer-thin gold layer of 0.5 or 2 μm thick is applied in the area of the contact surfaces 2 , the nickel layer serving as a diffusion barrier to the copper. After the fuse wire 3 has been bonded on, the housing is placed around the metal strips, each containing the contact surface 2 , which has been explained in more detail with reference to FIG. 2. Following this, cuts are placed along the dash-dotted lines, so that the shape of the metal strip 1 shown in dashed lines in FIG. 2 is created.

On the basis of Fig. 4 is illustrated, that this arrangement can be multiplied approximately as desired, for a mass production to achieve high volume. Fig. 5 shows an embodiment which is characterized by less stability, but by less waste on the side of the metal body 10 ''. In the case of the metal body 10 'and 10 ''according to FIGS. 4 and 5, an aluminum roll-clad copper sheet is used, whereby it is then determined that the fuse element 3 is also made of aluminum or an aluminum alloy. Although it is not possible to achieve optimal electrical properties with aluminum, since aluminum is much cheaper than gold, this material is completely sufficient for certain applications.

Claims (10)

1. A method for producing an electrical connection between two contact surfaces with the aid of a wire by bonding to form a fuse, in which a bonding head first places the wire guided along the head under an end-side squeezing surface onto the one contact surface and with simultaneous actuation of an ultrasound source deformed and thus fastened and then, while simultaneously allowing the wire to run out of the guide, moves over the second contact surface and fastens the wire in the same way, characterized by the process features and steps:

• a) providing a one-piece metal body which holds the two level contact surfaces ent,
• b) weak deformation of the wire during crimping,
• c) strong ultrasound dose, short duration,
• d) flat travel curve of the bond head relative to the substrate from one contact surface to the other without any significant bending of the wire after the first attachment,
• e) connecting the two contact surfaces by a housing and
• f) cutting off the metal body residues except for one soldering surface on each contact surface.

2. The method according to claim 1, characterized records that the metal body before or after the provision in whole or in part in the area the contact surface is coated. 3. The method according to claim 1 or 2, characterized ge indicates that as a metal body coated starting material is selected. 4. The method according to any one of the preceding claims, characterized in that as metal body is a copper sheet coated with nickel is selected, which is partially gold-coated, and which is chosen as wire gold or doped gold. 5. The method according to any one of claims 1 to 3, characterized characterized that as a metal body a copper sheet coated with aluminum was chosen and that as a wire aluminum or an aluminum alloy is selected. 6. The method according to any one of the preceding claims, characterized in that the metal body is produced by stamping, and that the stamping facing the bonding head at the contact surfaces becomes. 7. The method according to any one of the preceding claims, characterized in that the first Bond attachment another, zero bond attachment upstream and the second another, third bond attachment is subordinate, and that the further Bond attachments with the others by the same Wire are connected directly.   8. The method according to claim 7, characterized records that the parameters of the zeros and third bond attachment only according to Festig viewpoints are chosen, i.e. a relative severe deformation with a low ultrasound dose is combined with a long exposure time. 9. Fusible link, in particular manufactured according to claim 7 or 8, with two contact surfaces and a fusible wire applied by bonding to the contact surfaces, characterized in that the fusible wire ( 3 ) does not have a sheath, and that on both sides of the bond fastenings ( 4/1 , 4/2 ) each have a further bond attachment ( 4/0 , 4/3 ) of the same, elongated fuse wire ( 3 ) is provided. 10. A fuse according to claim 9, characterized in that the melting section ( 5 ) is flat and the other two sections ( 6 , 7 ) are curved as desired.