Fastenerless sealed electronic module

A die cast housing having an open side receives a circuit which seats on a housing ledge near the open side. An accelerometer and other components carried by the board are enclosed by the housing and board. The outer side of the circuit board is covered with an adhesive sealant compound which secures and seals the circuit board to the housing without fasteners for efficient structural transmissibility, and also structurally dampens the circuit board. A sheet metal cover over the open side inhibits electromagnetic interference. Lateral ports on the housing are aligned with connectors on the circuit board and permit coupling to the circuit board by mating connectors which seal the ports. The connectors have an interference fit with webs on the housing to permit thrust testing before the adhesive sealant is cured.

FIELD OF THE INVENTION 
This invention relates to electronic assemblies and particularly to sealed 
modules requiring no fasteners for assembly. 
BACKGROUND OF THE INVENTION 
Airbag or supplemental inflatable restraint systems for automotive vehicles 
employ a sensing diagnostic module comprising an accelerometer and 
electronic circuits that are used to detect a vehicle crash and then 
determine whether the airbag in the vehicle should be deployed. Generally 
the accelerometer and other circuits are mounted on a single circuit board 
which is packaged in a protective die cast case. Typically such devices 
are assembled using selftapping screws. The units are sealed using a "form 
in place" gasket similar to an "o" ring type gasket. Grounding between the 
case and the circuit board is achieved via one or more of the screws which 
connects a ground plane on the circuit board with the case. Typically 3 or 
4 screws are require to secure the circuit board to the case and 4 or 5 
screws are used to compress the seal between a die cast cover and the 
case. The disadvantages of this arrangement are: 1) the circuit board is 
pre-stressed by the screws bearing down on the circuit board, 2) the 
circuit board is fastened to the case only where the screws are located, 
3) the case may be shipped or otherwise handled after the gasket is 
formed, thereby subjecting the gasket to damage, 4) the screws may provide 
less than optimum structural dampening of the circuit board, possibly 
requiring auxiliary dampening mechanisms and/or additional processing of 
the accelerometer output signal, and 5) due to installing the screws, the 
assembly of the module is labor intensive. 
It is preferred that the stress due to screws be eliminated, that the 
circuit board be held in place along its entire perimeter, the formed in 
place gasket be eliminated, the circuit board be structurally dampened, 
and the assembly labor be minimized. In addition, it is important that 
tamper deterrent and tamper evident features be employed. Further it is 
required that the assembly must be watertight, that there will be a 
structural transmissibility of 1 to 1 between the circuit board and the 
case, and that the circuit board be grounded, all without the use of 
fasteners of any type. 
SUMMARY OF THE INVENTION 
It is therefore an object of the invention to achieve a watertight 
electronic assembly having a structural transmissibility of 1 to 1 between 
a circuit board and a case. Another object is to include tamper deterrent 
and tamper evident mechanisms in such an assembly. Still another object is 
to achieve these advantages in an assembly without fasteners. 
A sensing diagnostic module for an airbag controller has a die cast case or 
housing with an open side and having a ledge on a wall near the open side 
for supporting a circuit board. A thick layer of adhesive sealant covers 
the open side of the circuit board and seals against the housing to form a 
watertight seal at the interface of the circuit board and the housing and 
rigidly secures the board in the housing. The sealant layer also affords 
damping of the circuit board. The other side of the circuit board carries 
a connector and the housing has a opening aligned with the connector for 
coupling with an external harness. The opening is surrounded by a collar 
which makes a sealed coupling with the harness connector. A ground clip on 
the circuit board connector contacts the housing. An interference fit 
between the connector and the housing transmits housing motion to the 
circuit board for thrust testing before the adhesive sealant is completely 
cured. A sheet metal cover over the open side of the housing protects 
against electromagnetic interference.

DESCRIPTION OF THE INVENTION 
The ensuing description is directed to a sensing diagnostic module for a 
supplemental inflatable restraint for use in an automotive vehicle. It 
will be appreciated, however, that the same principles of construction 
embodied in the invention are applicable to other electronic modules. 
Referring to FIG. 1, a die cast aluminum housing 10 is a generally 
rectangular shallow box and is open at one side. The inner wall 12 defines 
a ledge or shelf 14 extending around the housing near the open side to 
support a circuit board 16. The housing has a rear flange 18 with mounting 
holes 20 and a locating pin 22, and a front mounting lug 24 with a 
mounting hole 26. Another locating pin 28 is mounted at one side of the 
housing. The front face 30 of the housing 10 has a pair of openings 32 to 
receive harness connectors to be attached to the module. An external 
collar 34 surrounds each opening 32. 
An accelerometer 33 and electronic components 35 are mounted on the inboard 
surface 37 of the circuit board. A circuit board connector 36 is fastened 
by snap-in pins 38 to the inboard side of the circuit board. The connector 
36 has two sections, each aligned with one of the openings 32. As seen in 
FIGS. 2a and 2b, the sides of the connector 36 have a pair of vertical 
slots 42 spaced a fixed distance from the front plane of the connector. 
Each slot is formed by a pair of spaced flanges 41, and a crushable rib 43 
extends inward from one flange toward the other. The housing has short 
support webs 44 extending inward from the housing wall 12. The webs engage 
the slots 42 of the connector, when assembled, and hold the connector in 
position. The ribs 43 interfere with the webs 44 so that during assembly 
the webs crush the ribs 43 resulting in an interference fit. Due to the 
interference fit between the webs and the connector, the connector 36 can 
be precisely located relative to the openings 32 during assembly. When a 
mating harness connector is coupled to the connector 36, the insertion 
force is applied to the housing through the webs 44, and not through the 
circuit board. One end of the connector 36 holds a ground clip 46 which 
has a spring arm 48 for conductively engaging the inner wall 12 of the 
housing. The connector 36 has terminal pins 50 which are soldered at one 
end to respective conductors in the circuit board, and in the same way the 
ground clip 46 is soldered to a ground plane of the circuit board. 
The circuit board 16 is assembled in the housing by a light press which 
seats the circuit board on the ledge and effects the slight interference 
fit between the webs 44 and the connector. Then, as shown in FIG. 3, a 
dispenser 54 applies potting compound 56 over the outboard surface 55 of 
the circuit board. The potting compound, when applied is in a liquid state 
and the ledge 14 prevents leakage of the compound into the housing beneath 
the circuit board. The compound does, however, fill any gap between the 
circuit board edge and the wall. When cured, the compound then transfers 
housing movement to the board with a unity transfer function so that the 
accelerometer senses the acceleration of the housing. A thrust test can be 
performed on the assembly before the compound is cured because the 
interference fit between the webs 44 and the connector 36 provides 
sufficient rigidity for testing an arming sensor on the circuit board. 
The compound is an adhesive sealant which seals the interface of the 
circuit board 16 and the portion of the housing wall 12 above the ledge. 
The potting compound is preferably a polyurethane two part fast cure 
adhesive sealant which is applied to a thickness of about 0.125 inch. To 
assure a fast cure the circuit board and the housing are preheated to 
85.degree. C. In about 3 to 5 minutes the compound will gel sufficiently 
to allow the assembly to be turned over to perform a thrust test on the 
assembly, and the compound will completely cure in approximately 24 hours. 
In its liquid form as applied to the assembly, the adhesive sealant has a 
viscosity of 3000 to 5000 centipoise. The cured compound has a coefficient 
of thermal expansion of 10.sup.-5 to 10.sup.-6, which is between that of 
FR-4 circuit board material and the aluminum housing. In the event 
components are carried on the outboard surface of the circuit board and 
are thus covered by the compound, a Shore A 40 to 50 hardness of the 
compound is preferred. 
The two part adhesive sealant 56 structurally bonds the board to the case 
around the entire perimeter of the circuit board without inducing 
localized stress points to the circuit board and seals the board-housing 
interface. The layer of adhesive sealant structurally dampens the circuit 
board to reduce accelerometer noise. The adhesive sealant also is a tamper 
deterrent and if tampering is attempted damage to the compound will be 
evident. 
A sheet metal cover 58, shown in FIG. 4, is applied over the open end of 
the die cast housing 10. The cover has edge flanges 60 with a row of 
dimples 62 which snap over a rim 64 on the housing outer wall to retain 
the cover. The cover does not touch the layer of potting compound 56. The 
purpose of the cover is to provide a shield against electromagnetic 
interference. 
Conventional automotive harness connectors (not shown) are inserted through 
the collars 24 to couple to the connector 36. Such harness connectors have 
pliable seals which mate to the inside surfaces of the collars. Thus the 
assembly is completely sealed when the harness connectors are installed. 
It will thus be seen that the sealed module is secured without screws or 
other fasteners and thus is easier and faster to assemble and eliminates 
localized stress on the circuit board. The module also has circuit board 
damping to reduce accelerometer noise. The slight interference fit holds 
the circuit board in place while the adhesive sealant is applied and allow 
thrust testing before the adhesive sealant is cured. The interference fit 
also provides accurate locating of the connector relative to the connector 
collar to enhance a good seal at the connector collar and takes connector 
insertion loading off the circuit board.