Custom Modules and Encapsulation
There are four reasons to encapsulate an
assembly. All four reasons are valid and
more than one may apply to a project.
1.
High
Voltage Insulation
2.
Environmental
Protection
Epoxy can protect a circuit from the environmental elements
such as water, chemicals, etc. With the
epoxy completely surrounding the circuit, the effect of the elements is
eliminated.
3. Mechanical
Protection
Epoxy can also protect a circuit from mechanical shock and
vibration. When the circuit is
surrounded by epoxy the mechanical shock is applied to the entire assembly and
the components are not pulled apart or pushed together. The epoxy can also protect from damage of an
item accidentally hitting the circuit and breaking some of the components.
4.
Circuit
Security from Competitor
Epoxy encapsulation may not prevent competitors from getting
a look at the circuit, but what they find will be of little use. If the epoxy is chemically devolved, the
components will also dissolve. Mechanically
destroying the epoxy will also destroy the components. Your competitor may find out what is in your
circuit, but he will have to work very hard for the information.
When designing a PC Board to be encapsulated in a
mold (no case), there are a number of items which should be kept in mind. The purpose of this document is to list the
design considerations which should be kept in mind during the PC Board layout
phase of design. If any exceptions to
these rules are needed in order for the PC Board to function properly, please
contact
General: Most molds used to encapsulate or pot PC
Boards generally have 5 sides (left, right, front, back, and bottom) and an
open top through which the epoxy is poured and the cured part is removed. Although the five solid sides can have a
limited variety of features, the open top creates a pour surface which is
basically (but not perfectly) flat. In
addition, the four vertical sides are usually designed 1° or more off of
vertical so that the pour surface is larger than any parallel cross section
below it. This draft allows the
finished part to be removed without rubbing against the side of the mold, reducing
both labor costs and mold wear. Also,
the molds usually need some type of feature to keep the PC Board properly
positioned during the encapsulation procedure.
Finally, the corners of the mold itself are usually rounded, rather than
square, to preserve mold life and prevent the trapping of air bubbles.
Custom Modules and Encapsulation
PC Board Perpendicular to Pour Surface
Rule #1: Have at least one .15
to .20 diameter through-hole in the PC Board for every 2 to 3 square inches of
board area.
Epoxy bonds to itself
better than to the PC Board. It also
bonds better than the layers of the PC board bond to each other. By allowing the epoxy on one side of the
board to bond to the epoxy on the other side through the holes, the overall
strength of the final product will be greatly increased.
Rule #2: Keep the mounted components at least .10,
preferably .15, from the edge of the PC Board except for the pour and bottom
surfaces. High voltage (over 2000 Volts)
parts may require additional spacing.
Most molds are
designed with slots to grab about .05 of the vertical edges of the PC Board in
order to keep the board vertical during the pouring operation. Any board components closer than .10 to
these edges may prevent the mold design from having these slots.
Rule #3: Keep traces at least .10 from the edges of
the PC Board except on the pour and bottom edges. A possible exception to this might be a wide
ground trace (> .15 wide). High voltage (over 2000 Volts) parts may require
additional spacing.
Most molds are designed with slots to grab about
.05 of the vertical edges of the PC Board in order to keep the board vertical
during the pouring operation. Any traces
closer to these edges than this distance may be exposed. Exposure of these traces may result in cut
traces (due to external damage in shipping or handling), shorting from the
module to a grounded conductor near the exposed trace, or a voltage hazard during
operation.
Rule #4: All wires, connectors,
or other electrical, mechanical or visual circuit interfaces should exit
through the pour surface, if possible.
Although it is
possible to have holes in the sides of molds for wire exits and the like, the
costs in additional labor to seal them prior to potting, additional labor to
insert and remove parts, and reduced mold life will make the part much more
expensive.
Rule #5: Any tall components such as capacitors, transistors,
or end-mounted axial parts should be located as close to the pour surface as possible.
In order to remove the
cured assembly from the mold, the perimeter of the pour surface must be larger
than anything below it. For example: if
a funnel were filled with epoxy with the pour surface at the widest part, the
epoxy would come out of the funnel easily.
If, on the other hand, it were poured with the small end at the top, the
funnel would have to be destroyed to get the epoxy out. Any tall components need to have bulges in
the mold extend from the pour surface to a point slightly below the components
location. If these components are near
the pour surface, the additional epoxy needed to fill the rest of the bulge
will be minimized. This will result in
less epoxy and a less expensive part.
Custom Modules and Encapsulation
PC Board Parallel to Pour Surface
Rule #1: Placed as many .15 to .20 through-holes
on the board as possible, preferably within .75 of each other.
When epoxy is poured
into a mold, air tends to get trapped below horizontal surfaces, such as the PC
Board. Since bubbles are normally
removed by use of a vacuum chamber, the horizontal distance an air bubble must
travel until it can start rising to the surface should be kept to a
minimum. This is particularly true with
molds are more than 1 deep, where the head pressure of the epoxy tends to
inhibit the pressure reducing effect of the vacuum from reaching the bubbles. The presence of holes in the board reduces
this horizontal distance so the bubbles do not have to travel to its edge.
Rule #2: Keep traces located on the opposite side of
the board from the pour surface .10 to .15 away from the corners of the PC
Board and any additional support points.
A possible exception would be wide (>.15) ground traces. High voltage assemblies (>2000 V) may
require additional distance
In order to keep the
lead protrusions and/or components on the bottom side PC Board (side away from
the pour surface) from being exposed, small pedestals are formed in the mold to
lift the assembly off the mold bottom.
Typically, these pedestals are placed in the mold corners. They are also placed along the edges of large
boards which need additional support.
Any bottom-side traces at these support locations would be exposed because
the pedestals prevent epoxy from filling in these areas.
Rule #3: Lead protrusions through the PC Board
should be kept less than .10.
PC Boards in these
types of molds are usually kept off the bottom of the mold with .15 pedestals
(see Rule #2 explanation). Any leads
protruding more than .10 may stick through the epoxy and be exposed.
Rule #4: All visual, mechanical, or electrical
circuit interfaces, such as pins, wires, should exit pour surface, preferably
in non-precise locations.
Although it is
possible to have holes in the sides of molds for wire exits and the like, the
costs in additional labor to seal them prior to potting, additional labor to
insert and remove parts, and reduced mold life will make the part much more
expensive. In addition, precision
placement requires the use of locating fixtures which, again, add to the cost.