Assembly of multi-layer circuit boards secured by plastic rivets

An assembly of aligned, stacked, circuit boards for hot-pressing in an evacuated bag utilizes a sliding body formed of a thermoplastic material which is inserted into elongated peripheral board holes. The sliding body is dimensioned to allow the boards to be slideable in a longitudinal direction at the direction of the sliding body to allow thermal expansion to be compensated for. Disclosed is a tool for reforming the slilding body after insertion into the board holes has a cylindrical heating element with a central bore in which a centering pin is located. The heating element is surrounded by a tubular device which is biased in a sleeve by a compression spring. Application of the tool against the inserted sliding body causes heat forming of a portion of the sliding body to maintain the circuit boards in registry during the hot-pressing period.

BACKGROUND OF THE INVENTION 
The present invention relates to a method for positioning and fixing 
electronic circuits disposed in layers upon a plurality of insulating 
circuit boards in a hot-pressing process, whereby several circuit boards 
are stacked so as to be congruent in their circuit layout and wherein 
elongated holes are provided in the area at the edge of the boards. The 
present invention also relates to a means for carrying out the method and 
to a device employing the means. 
Multiple layer circuit boards are typically formed by pressing a plurality 
of circuit layers and bonding sheets held together by bolts in a vacuum 
whereby the boards are bonded together into so-called "multi-layers". The 
firm Fela Planungs AG offers a device for sale under the trademark 
"Multiklav" in which a plurality of assemblies of circuit layers are 
subjected to pressure and heat by a hydraulic fluid while in an evacuated 
bag made of a suitable elastomer, as disclosed in EP-A1-0 297 027. The 
individual circuit layers are stacked together with bonding sheets 
(prepregs) which are held in position temporarily during the press cycle 
by bolts extending through the elongated holes. 
In practice, it has been shown that the elastomeric bags used in the 
"Multiklav" unit are exposed to extensive mechanical loads at the points 
in the stack where the circuit boards are held together by the bolts and 
for this reason wear out prematurely. If failure occurs during the 
pressing process, the enclosed boards are contaminated by the hydraulic 
oil. 
Accordingly, an object of the invention is to provide a method and a means 
of carrying out the method so as to considerably lengthen the life of the 
bag and to exclude the danger of soiled boards due to hydraulic oil 
contamination. 
BRIEF DESCRIPTION OF THE INVENTION 
In accordance with the present invention, the method is characterized in 
that a "sliding body" formed of a thermoplastic material and having a 
flange at one end is inserted into the aligned, elongated holes of the 
stacked boards so as to be tightly fitting across the width of the slots. 
A portion of the sliding body which protrudes out of the stacked boards is 
formed and reshaped in such a way that the circuit boards are retained by 
the sliding body, but are slidable in a longitudinal direction over the 
sliding body to balance out and/or compensate for thermal expansion. The 
stacked circuit boards are thus also allowed to expand at high 
temperatures without mutual distortion and/or displacement. 
Due to the use of a thermoplastic material the sliding body may be secured 
mechanically by thermal flanging to the stack of boards in an efficient 
and economical manner, and which in addition insures that the boards are 
able to slide in the longitudinal direction. 
A preferable embodiment of the sliding body has a central bore and two 
parallel sliding surfaces. In general, any thermoplastic synthetic 
material may be used for the sliding body. The high temperatures and 
pressures employed, however, necessitate dimensional stability through at 
least 170 degrees C., measured according to Vicat. Synthetic materials 
such as polyamides, polyimides and polysulphones have proved suitable. 
The sliding bodies may be flanged with a device having an integral heating 
element associated with a spring-loaded mandrel or sleeve, slideable in 
the longitudinal direction. A centering pin made of graphite allows clean 
and mechanically faultless flanging, and results in flanging which does 
not burden the elastomeric bag mechanically.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
As seen in FIGS. 1a and 1b, sliding body 1 is generally cylindrical in 
length and is provided with a continuous central bore 2 in its center. The 
cylindrical body has two parallel, opposed flattened sliding surfaces 3. 
At the lower end of the cylindrical central portion of the sliding body is 
a thin, circular flange 4. As seen in FIG. 1c, several circuit boards 6 to 
10 may be secured together by the sliding body 1 being inserted through 
aligned bores in the boards, the upper edge of the cylindrical portion 
being formed into a collar 5 to retain the boards. 
In FIG. 3, a tool is shown which may be used to form the sliding body 1 in 
an especially effective and simple manner. A copper insert 12 is provided 
at one end of a commercially available heating cartridge 11. The heating 
capacity of the heating cartridge 11 is preferably about 50 watts, and may 
be adjustable as known in the art. Slid over the copper insert 12 is a 
cylindrical heating element 13 made of a vanadium steel alloy such as V2A 
or V4A and having a stepped bore 14 at its center. The heating element 13 
is securely screwed to the copper insert 12 by a hexagonal socket-head 
bolt 15 threaded into the heating element 13, the bolt 15 being recessed 
so as to not project beyond the outer surface of the heating element. 
The upper end of the heating element 13 forms a ring-shaped heating surface 
16. A headed centering pin 17 projects upwardly from the heating surface 
16 and is dimensioned to fit within and be retained by stepped bore 14. 
The centering pin 17, which is dimensioned to be insertable within the 
bore 2 of sliding body 1, consists of a material of lesser heat 
conductivity than the heating surface 16 and may preferably be of 
graphite. An insulator, such as aluminum oxide, may also be used. 
Slid over the heating element 13 is a tubular device 18 which has a flange 
19 at its lower end. The tubular device 18 is preferably made of 
polytetrafluorethylene (PTFE) reinforced with carbon, heat-resistant to 
temperatures above 300 degrees C. Mounted lower down on the heating 
cartridge 11 is cylindrical ring-piece 20, which acts as a support for a 
compression spring 21, which is biased to push the tubular device 18 
upwards by contacting its lower surface. Ring-piece 20 may be formed of an 
aluminum-magnesium-alloy ("Anticorodal"). 
Slid over the tubular device 18 and the ring-piece 20 is brass sleeve 22, 
having a first inner bore 23 joined to a second, smaller inner bore 24 to 
create an inner peripheral stop or shoulder 25. The first inner bore 23 
corresponds to the diameter of the ring-piece 20 and the flange portion 19 
of tubular device 18. The second inner bore 24 corresponds to the diameter 
of the upper, narrower part of the tubular device 18. Provided in the 
ring-piece 20 are two opposed threaded bores 26, which are aligned with 
two threaded bores 30 in the sleeve 22. Alternatively, the bores 30 may be 
unthreaded. The sleeve 22 and the ring-piece 20 are fastened to the 
heating cartridge 11 by means of two hexagonal socket-head bolts 27 which 
are screwed into the boreholes 30 and 26, and which contact the sides of 
the cartridge. As may be appreciated, the distance between the upper edge 
of the tubular device 18 and the heating surface 16 may be adjusted by 
shifting the ring-piece 20 and sleeve 22 vertically on the heating 
cartridge 11. 
FIG. 2 shows several boards 6 to 10 each which may comprise a substrate 
upon which film circuit elements are deposited on the boards as known in 
the art. The circuit layers are stacked with the intermediate bonding 
sheets, and form a multi-layer, held together by sliding bodies 1. The 
sliding bodies 1 are each inserted within the elongated holes 28 which are 
in the peripheral area 29 at the edge of the circuit boards, the parallel 
surfaces 3 of the bodies being aligned with the elongated parallel sides 
of the holes. Each of the sliding bodies 1 is formed of a thermoplastic 
material which has a melting point-like fluid region of up to at least 180 
degrees C. and dimensional stability through at least 170 degrees C., 
according to Vicat. Polyamides, polyimides, and polysulphones are 
especially suitable. The sliding body is then formed using the tool of 
FIG. 3. 
The tubular device 18 is positioned over a sliding body 1 with centering 
pin 17 aligned with the central bore 2. The tool is then pressed into 
engagement with the board through which the sliding body projects, spring 
21 compressing as tubular device 18 retracts into sleeve 22 until top 
surface 16 of heating element 13 engages and shapes the upper edge of the 
sliding body 1 into the flattened and widened collar 5, as seen in FIG. 
1c. The tool is then withdrawn slightly so that the surface 16 is no 
longer in contact with the sliding body while tubular device 18 remains in 
contact with the topmost circuit layer, maintaining the stack of boards 
together. The sliding body 1 is thus allowed to cool and harden to retain 
the boards together. 
Since the sliding bodies 1 have a high body stability during a hot-pressing 
process, and particularly in an isostatic pressing process in an autoclave 
wherein the circuit boards 6 to 10 are held together and put into a 
continuously evacuated bag, the epoxy resin from the circuit boards 6 to 
10 flows around the sliding bodies 1 without being bonded to the latter, 
which is often the case when metal bolts are used. Since each of the 
elongated holes 28 is situated along a line defining the midpoint 0 of the 
boards, thermal expansion is balanced out and/or compensated for. Thus, 
displacement of the circuit layouts relative to each other may be 
prevented. This is very important because the circuit layers 6 to 10 are 
almost always slightly undulated, which normally leads to such 
displacement and distortion. 
After the hot-press cycle whereby the circuit layers 6 to 10 are 
permanently bonded, the sliding bodies 1 are removed from the elongated 
holes 28 with a punch the shape of which duplicates the shape of the 
sliding body. The elongated bores can then be utilized as part of a 
reference system for hole drilling and for etching the outside conductor 
patterns of the multi-layer circuit which has been produced. 
It is of course recognized that other adhesives, which are specifically 
adapted for circuit boards or similar components, may be used instead of 
the currently used epoxy-composite films and the teaching of the invention 
may be applied thereto. 
In addition, the device presented herein in the form of a hand tool may be 
similarly employed as part of a mechanized system, for example, in 
combination with positioning and punching means. It is preferred to use 
brass instead of graphite for the centering pin, since brass is easier to 
machine and mechanically more stable.