Aircraft PCBs

Electronic equipment operating in severe vibration environments plays a critical role in military and aerospace hardware. A crucial element of most modularized electronic equipment are the printed circuit boards (PCBs). Structural failures of such boards are almost entirely the result of imposed mechanical vibration. It is desirable to incorporate structural guidelines and analysis techniques at an early stage in board development when deficiencies may easily be corrected.

However, there are a number of problems connected with such an approach. A computer program has been developed to address the considered situation. The program is to reduce PCB structural analysis to a level readily understood by the average designer. Another objective of the program is to encourage use of structural analysis at an early stage of PCB design and development. Attention is given to board geometry, program organization, board response computation, and a sample problem.

Generally, it is known that structures of this type currently in use come in the form of cases in which printed circuit boards are fitted by sliding. The entire rear side of these cases supports mechanical and electric connecting elements suited to those of the printed circuit boards.

Experience has proved that this solution is unsuited to the electronic equipment taken on board aircraft where shock and extreme vibratory conditions prevail.

In fact, when a printed circuit board inside such a rack has been subjected to intense vibrations for a certain period of time, the board's connectors and the weld seams ensuring the mechanical and electric contacts between the board and its electronic components tend to deteriorate.

Under these circumstances, it is thus necessary to fix these printed circuit boards without play and in a way that they cannot be deformed when submitted to vibrations. Furthermore, it is necessary that these cards be easily locked in place and unlocked in order to facilitate their maintenance.

Moreover, the available space on board aircraft is usually very limited. The printed circuit boards of onboard electronic equipment are therefore grouped together in a very small space. It ensues that the heat emitted by the electronic components borne by these boards is not easy to evacuate.

The known structures are not very satisfactory from this point of view, as they are ill-suited to the heat exchanges required to evacuate the heat thus emitted.

It is therefore necessary to provide an efficient means of evacuating this heat in the racks destined to equip aircraft.

WHAT IS NEEDED....

a rigid and good heat conducting frame supporting the printed circuit board, comprising two opposite posts susceptible of slidably fitting into two respective U-shaped slide rails, in good heat conducting material, and integral with the rack;

a means of putting the posts into thermal contact with the heat-generating electronic components borne by the board;

a means of locking the frame into the slide rails, which ensures application of a lateral side of the posts against a corresponding wing of the slide rails so as to obtain maximum heat exchange between the posts and the slide rails.

By way of these arrangements, the aerospace printed circuit boards are rigidified by the frames which are firmly held in the slide rails. This does away with the mechanical stresses brought to bear on the connectors connecting the boards to the rack and their components, when the latter are subjected to intense vibrations.

Furthermore, to efficiently evacuate the heat emitted by the electronic components borne by the board, the slide rails need only be brought into contact with heat dissipators e.g. integrated into the underframe of the rack.

According to a feature of the invention, the locking means comprises:

two runners formed respectively in the posts of the frame, susceptible of fitting slidably into the slide rails, each having two ramps that are symmetrical in relation to a plane perpendicular to the posts;

two mobile wedging blocks per post, each having a bevelled side coming to rest against the two respective ramps of the post's runner, with a longitudinal motion of the wedging blocks in relation to the runner generating a relative transversal motion of the wedging blocks in a direction perpendicular to the plane of the frame; and

a tightening means susceptible of displacing the wedging blocks towards one another longitudinally in relation to the runner.

Thus, each of the two posts of the frame can be firmly fastened to one of the two wings of a U-shaped slide rail, the blocking force exerted on the frame being a function of the locking strength of the two wedging blocks against each runner. This arrangement ensures good thermal conduction between the frame and the slide rails of the rack.

Advantageously, the locking means comprises, for each post, a cap screw of which the threaded rod passes freely and successively through a first of the two wedging blocks and through the runner, to screw into the second wedging block.

In order to avoid the wedging blocks falling to the bottom of the rack when dismounting the frame from the rack and the ensuing risk of causing deterioration, the frame forms, in the axis of each fastening screw, a first stop intended to retain the second wedging block, and a second stop retaining the screw and in which a through bore is made to enable the passage of a tightening tool susceptible of cooperating with the head of the screw.

In this way, when wishing to loosen the fastening screws in order to remove a frame, the second stop prevents the screw from being removed from the frame. As for the first stop, it enables avoidance of complete withdrawal of the screw from the second wedging block.

The risk of losing parts when dismounting a frame is thus suppressed, and storage of the frame facilitated.