Method for bonding LLCCC-components using a leadframe

A method for electrically bonding surface-mountable integrated circuits (LLCCC-components) onto a printed circuit board. A specially shaped flexible bonding element having a semicircular central portion is used to flexibly connect the LLCCC components to the circuit board so that stresses due to thermal expansion of the components are absorbed.

BACKGROUND AND SUMMARY OF THE INVENTION 
This invention relates to a method for electrical bonding of 
LLCCC-components on printed circuit boards. 
The use of LLCCC (Leadless Ceramic Chip Carrier) components in space 
technology has so far been possible only on ceramic substrates or special 
substrates with adapted coefficients of expansion, due to the very 
different physical characteristics of the component and the printed 
circuit board. These surface-mountable integrated circuits are situated in 
a hermetically sealed ceramic housing, and no longer have any flexible 
connections; thus, the multiple bonding--up to 68 connections--to the 
ceramic substrate must be established by way of soldering surfaces. FIG. 1 
of the drawing illustrates a standard bonding for LLCCC-components 
according to the state of the art. This bonding technique, however is not 
sufficiently reliable for the use of such components on printed circuit 
boards in space technology because the stress resulting from temperature 
changes encountered in such applications causes excessive expansion 
problems. 
It is an object of the invention to provide a bonding method which permits 
the use of LLCCC-components in electronic systems for space applications, 
and which reliably accommodates the high temperature stress which occurs 
on these components in a space environment. 
This object is achieved by the use of specially shaped flexible bonding 
elements to secure the LLCCC component to the printed circuit board. In a 
preferred embodiment, the bonding elements are bent to a semicircular 
curvature which provides the flexibility necessary to .absorb stresses due 
to differing coefficients of thermal expansion among the various 
components. 
Other objects, advantages and novel features of the present invention will 
become apparent from the following detailed description of the invention 
when considered in conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE DRAWINGS 
At present, epoxy-fiberglass sheets are used as the base material for 
printed circuit boards in space technology FIG. 1 is a cross sectional 
view of such a bonding arrangement according to the prior art. As is 
apparent, if the respective components have different coefficients of 
thermal expansion, any substantial change in the ambient operating 
temperature will give rise to stresses at the soldering joint which in the 
long run will result in an unacceptable failure rate. 
This problem is resolved simply and effectively by the mounting technique 
according to the invention, which is illustrated in partial cross section 
in FIG. 3. As can be seen, the LLCCC element is held in place by bonding 
elements 10 which are soldered to the LLCCC element at one end 10b and to 
a strip conductor 12 mounted on the printed circuit board 13 at the other 
end 10c. in order to allow for the absorption of stresses due to thermal 
expansion properties of the respective components, the bonding element 10 
are bent in a semi-circular configuration at 11. 
The bonding elements 10 are etched out of a sheet of nickel foil (Ni-foil) 
100 which is electrotinned on both sides to an extent of 10 to 15 .mu.m by 
means of Sn60Pb. As shown in FIG. 4, an edge strip 101 of the foil 100 is 
not electrotinned, in order to permit measurement of the effective tinning 
extent on it, and to permit determination of the rolling direction. 
Subsequently, the bonding elements are provided with a semicircular 
curvature or bending 11 in their center area, as shown in FIG. 2. As with 
all other dimensions in the drawing, the radius of this bending 11 is 
enlarged in the illustration. In reality, the radius in this case amounts 
to, for example, 0.5 mm. After this bending is established, the bonding 
elements are cut to length on one side; that is, one of the cross webs 
10a--as shown--is cut off at the point determined by the desired element 
length. 
The bonding elements 10, which on one side are still connected with one 
another by means of the other cross web 10a, are individually soldered 
together with the LLCCC-component. After this soldering-together, the 
other cross web 10a is then cut off at the required web length, and the 
free ends of the bonding elements 10, which are now fixed in their 
position on the LLCCC-component by means of the preceding soldering, are 
soldered to the strip conductor 12 of the printed circuit board 13, and 
the electrical bonding is therefore established between the component and 
the printed circuit board. 
By means of this method, the previous expansion problems caused by the 
stress of changing temperatures in travel are eliminated, and thus the use 
of LLCCC-components is permitted in space technology. However, the 
suggested method also offers other advantages, particularly a good visual 
control of the soldering joints. Furthermore, the soldered-on components 
can be exchanged easily and absolutely without any problems. In the case 
of vibrations and stress caused by temperature changes, the soldering 
joints must absorb virtually no more special stress; and finally, this 
method also significantly expands the application spectrum of highly 
integrated printed circuit boards. 
Although the invention has been described and illustrated in detail, it is 
to be clearly understood that the same is by way of illustration and 
example, and is not to be taken by way of limitation. The spirit and scope 
of the present invention are to be limited only by the terms of the 
appended claims.