Method of sealing electric parts mounted on electric wiring board with resin composition

A method of sealing electric parts mounted in a bare state on an electric wiring board uses a liquid sealing resin as a printing ink and a screen serving as screen printing means and having apertures in the same pattern as the electric parts mounted on the board. The lower end of a peripheral wall of the screen defining each of the apertures is substantially brought into line-to-line contact with the surface of the board for screen printing. The screen has a recess formed on the rear side of the line contact portion thereof and surrounding the contact portion. These features prevent the printing ink from adhering to the rear side of the screen and the collapse of the sealing resin layer that would occur if the ink adheres to the screen rear sides.

TECHNICAL FIELD 
The present invention relates to a method of sealing electric parts mounted 
on the surface of electric wiring boards with resin composition, and more 
particularly to a method of sealing electric parts mounted in a bare state 
on the boards with resin composition by screen printing means. 
BACKGROUND ART 
With an increase in the complexity of arrangement of parts mounted on the 
surface of electric wiring boards, for example, of printed circuit boards, 
electric parts such as integrated semiconductor devices are mounted in a 
bare state directly on printed boards and electrically connected thereto. 
In this case, the electric parts mounted in a bare state on the printed 
board must be sealed with an electrically insulating resin mainly so as to 
retain their quality. 
As such a method of sealing electric parts with resin, the so-called 
potting method has already been proposed wherein a pneumatically operable 
dispenser a is used for applying a liquid sealing resin d dropwise onto an 
electric part c mounted on a printed board b to seal the part as shown in 
FIG. 10. The potting method has the problems to be described below with 
reference to FIGS. 11 and 12. 
(1) As seen in FIG. 12, the resin portion d' sealing the electric part c 
appears convex in shape, has a thickness which is largest at the center 
and gradually decreases toward the outer periphery, and therefore fails to 
produce a uniform sealing effect. 
(2) The part c is usually square to rectangular, whereas the sealing resin 
portion d' is circular as shown in FIG. 11, so that the part c must be 
sealed over an excessive area. This is not desirable in view of the high 
complexity of the arrangement on the board. 
(3) Although the amount of resin d to be applied dropwise can be suitably 
determined according to the size of the part c, the resin d applied is 
difficult to control in thickness. 
(4) For application to LSIs or like large-scale electric parts, the resin d 
must be applied dividedly several times. It is then difficult to give a 
uniform appearance and a uniform thickness to the sealing resin layer to 
be formed. The part therefore can not be sealed effectively with good 
stability. 
(5) The method is low in productivity since parts c must be sealed 
individually one by one. 
(6) When the resin is to be applied to the next part after application to 
one part, cobwebbing of the resin is liable to occur. The thread of resin 
is then likely to soil the printed board b. 
We have already proposed a method of sealing electric parts mounted on the 
surface of electric wiring boards with resin using screen printing means 
(see, for example, Unexamined Japanese Patent Publication No.82639/1989). 
All the foregoing problems (1) to (6) involved in the potting method can 
be overcome by this method which employs the screen printing means. 
The proposed sealing method with use of the screen printing means employs a 
screen having apertures in the same pattern as the electric parts mounted 
on the board and is practiced by forcing a liquid sealing resin serving as 
the printing ink into a space formed between each of the apertures and the 
part registered therewith by the operation of a squeegee to fill the space 
and seal the part. However, when the printing cycle is repeated many 
times, the liquid sealing resin serving as the printing ink adheres to the 
rear side of the screen, and the resin portion adhering to the rear side 
is printed on the surface of the board in the form of a thin overflow 
portion around the sealing resin layer, consequently collapsing the resin 
layer slightly as will be described below in detail. 
Further when the liquid sealing resin is forcibly filled into the screen 
aperture by operating the squeegee, the amount of resin filled in is 
larger toward the end side of stroke of the squeegee than toward the 
starting side of the stroke. The uneven amount of resin filled in gives an 
uneven thickness to the sealing resin layer and results in reduced 
productivity as will be described in detail later. 
DISCLOSURE OF THE INVENTION 
An object of the present invention is to seal electric parts mounted in a 
bare state on an electric wiring board with resin using screen printing 
means without permitting printing of an overflow resin portion that would 
lead to a collapse of the sealing resin layer formed. 
Another object of the invention is to force a liquid sealing resin into 
each aperture of a screen by the operation of a squeegee to fill the resin 
into the aperture in an amount which is substantially uniform from the 
squeegee stroke starting side to the stroke end side thereof so as to give 
a uniform thickness to the resulting sealing resin layer and achieve 
improved productivity. 
Other features of the present invention will become apparent from the 
following description. 
The present invention provides a method of sealing electric parts mounted 
in a bare state on an electric wiring board with a liquid sealing resin 
with use of a screen having apertures in the same pattern as the electric 
parts mounted on the board by forcing the resin in a liquid state into a 
space formed between each of the apertures and the part registered 
therewith through the aperture by the operation of a squeegee to fill the 
space, the method being characterized in that the lower end of the 
peripheral wall of the screen defining the aperture is substantially 
brought into line-to-line contact with the surface of the board, and that 
the line contact portion of the screen is formed at the back side thereof 
with a recess surrounding the contact portion. 
According to the present invention, the sealing method described above is 
further characterized by moving a doctor knife from the squeegee stroke 
end side of the screen aperture to an intermediate portion of the aperture 
in contact with the screen subsequent to the operation of the squeegee for 
filling the liquid sealing resin into the space to thereby render the 
space filled with the resin substantially uniformly from the stroke 
starting side of the aperture to the stroke end side thereof, and 
thereafter retracting the screen from the board to complete transfer of 
the sealing resin from the screen to the board. 
Examples of liquid sealing resins serviceable as printing inks for use in 
the present invention are resin compositions predominantly containing a 
resin such as silicone resin, epoxy resin, phenol resin, acrylic resin, 
acrylic epoxy resin, acrylic polyester resin, urethane resin or 
polybutadiene resin, and various other known resin compositions which are 
used in the art. A preferred liquid sealing resin is, for example, a 
single-pack epoxy resin composition consisting essentially of a liquid 
epoxy resin, a curing agent which is the reaction product of an aromatic 
diamine and a quaternary phosphonium salt, a flame retardant and an 
inorganic filler. The viscosity of the sealing resin is preferably in the 
range of 200 to 20000 poises at room temperature. When having a higher 
viscosity, the resin is difficult to forcibly fill into the screen 
aperture, whereas if the resin has a lower viscosity, the sealing resin 
layer formed will encounter difficulty in retaining its shape until it 
cures. Thus, the viscosities outside the above range are not desirable. 
Useful screens are mainly those made of an aluminum alloy, copper, nickel 
or stainless steel and further include those made of plastics. The screen 
has apertures formed in the same pattern as the electric parts mounted on 
an electric wiring board and to be sealed. 
The thickness of the screen must be larger than that of the electric parts 
to be sealed and is determined suitable usually from the range of from 0.1 
to 5.0 mm in accordance with the thickness of the electric parts to be 
sealed. 
The apertures are formed usually by etching. Each aperture is similar to 
the corresponding electric part in configuration when seen from above and 
has a larger size than the part. When the aperture is in register with the 
electric part, a space required for sealing is formed around the part in 
the aperture. 
The peripheral wall of the screen defining the aperture has its lower end 
substantially brought into line-to-line contact with the surface of the 
board. The screen is formed at the back side of the line contact portion 
with an annular recess surrounding the contact portion. The line contact 
portion and the annular recess surrounding this portion can be provided by 
forming in the rear side of the screen the annular recess which caves in 
upward from the rear side around the aperture. The width of the recess, if 
excessively large, is likely to warp or undulate the screen and must 
therefore be a minimum. The width is suitably determined usually from the 
range of about 1 to about 5 mm as will be described in detail later. 
The line contact of the lower end of the screen aperture defining a 
peripheral wall with the surface of the board and the annular recess 
surrounding the line contact portion prevent the adhesion of the printing 
ink to the rear side of the screen that would otherwise occur. 
Further according to the present invention, a doctor knife is moved from 
the squeegee stroke end side of the screen aperture to an intermediate 
portion of the aperture in contact with the screen surface subsequent to 
the operation of the squeegee for filling the liquid sealing resin into 
the aperture. Consequently, a raised excessive portion of the liquid 
sealing resin at the stroke end side is forcibly returned to the 
intermediate portion of the aperture, whereby the amount of resin filling 
the aperture is made substantially uniform from the stroke starting side 
to the stroke end side. This diminishes the amount of cobwebbing of the 
resin that will occur below the aperture. 
When the screen is retracted to complete the transfer of the liquid sealing 
resin from the aperture to the board, thereby shortening the time taken 
for the thread of resin to break to achieve improved productivity. 
Furthermore, cobwebbing occurs at the intermediate portion of the 
aperture, so that the threadlike portion of the resin, upon breaking, is 
absorbed and collected by the intermediate portion of the sealing resin 
layer transferred to the board and thereafter flows toward opposite sides 
to form a smooth surface. As a result, a uniform thickness can be given to 
the sealing resin layer despite the absorption of the broken threadlike 
portion of the resin. 
The aperture defining wall surface can be treated with fluorine or the like 
to permit the resin to flow smoothly therealong and thereby shorten the 
time taken for the resin thread to break. 
Examples of electric parts mounted in a bare state on electric wiring 
boards and to be sealed by the method of the invention are integrated 
semiconductor devices, resistors, photocouplers, capacitors, sensors, 
transistors, thyristors, diodes, transformers, varistors, operational 
amplifiers, filters, windings, switches, relays, etc. Integrated 
semiconductor devices include, in addition to SSI, MIS, LSI, VLSI, 
photosemiconductors, compound semiconductors, Josephson devices and like 
single components, complex devides such as hybrid ICs and IC cards. 
Such electric parts can be mounted on boards, for example, by the wire 
bonding method, TAB method, HLIP chip method or soldering method.

BEST MODE OF CARRYING OUT THE INVENTION 
The mode of practicing the present invention will be described below with 
reference to the accompanying drawings. 
The screen printing means for use in the method of the invention will be 
described with reference to FIGS. 1 and 2A. In FIGS. 1 and 2A, a printed 
board 1 has a multiplicity of bare electric parts 2 mounted thereon. On 
the other hand, a screen 3 has a pattern of apertures identical with the 
pattern of parts 2 mounted. 
The upper diagram of FIG. 1 shows a first step, in which the screen 3 is 
fixed in place on a screen printing press (not shown), and the board 1 is 
fixed to a slide table 4 of the printing press. Further a liquid sealing 
resin 5 is supplied in a predetermined amount to a specified position on 
the screen 3. Subsequently, the printing press is operated to register the 
apertures 3a (see FIG. 2A) of the screen 3 with the electric parts 2 on 
the board 1 and press the screen 3 against the board 1. The screen 3 as 
pressed against the board 1 is shown on an enlarged scale in FIG. 2A. A 
space 6 necessary for sealing each part 2 is formed in the aperture 3a 
around the part 2 accommodated therein. 
In the second step shown in the lower diagram of FIG. 1, the liquid sealing 
resin 5 is forced into the space 6 through the aperture 3a to fill the 
space by operating a squeegee 71. After the space 6 or the aperture 3a has 
been thus filled, the printing press is operated to retract the screen 3 
to the initial position, whereby the electric part 2 is sealed with a 
resin layer 5a. 
In the case where the sealing resin layer is formed by printing using the 
screen printing means, cobwebbing of the liquid resin occurs at the 
apertured portion when the screen is retracted, and after the cobweb or 
thread of the resin breaks, the lower portion of the thread is absorbed by 
the sealing resin layer on contraction, and the upper portion thereof by 
the liquid resin portion adhering to and remaining on the apertured 
portion. If the screen is flat over the entire area of its rear side in 
this case like the conventional one, the liquid resin portion to be 
absorbed by the apertured portion on contraction partly adheres to the 
rear side of the screen at the lower end of the aperture and is 
consequently printed around the sealing resin layer as an overflow. The 
principle of this phenomenon will be described below with reference to 
FIG. 5B. 
The first diagram of FIG. 5B shows a thread of resin immediate after a 
break. When the broken threadlike portion 5'b1 of resin is absorbed by the 
resin portion 5'b2 adhering to and remaining on the peripheral wall of the 
apertured portion 3'a upon contracting from the illustrated state, the 
threadlike resin portion partly adheres to the rear side of the screen 3' 
as shown in the second diagram. During the next printing cycle, the resin 
portion 5'b3 adhering to the rear side comes into contact with the surface 
of the board (not shown) and is collapsed to spread as shown in the third 
diagram. In the following printing cycle, a thread is formed again as 
shown in the fourth diagram. Upon the thread breaking, the resulting resin 
portion partly adheres to the rear side of the screen 3' in the same 
manner as above to increase the amount of the resin portion 5'b3 on the 
rear side. These phenomena are thereafter repeated to increase the amount 
of the rear-side resin portion. 
FIG.5C shows a sealing layer 5'a as formed by the conventional method. As 
seen in FIG. 5C, an overflow portion 5'al is printed around the lower end 
of the resin layer 5'a owing to the adhesion of liquid resin to the rear 
side of the screen 3'. Since the resin layer 5'a has the same composition 
as the overflow portion 5'al and is compatible therewith, the shoulder of 
the resin layer 5'a tends to flow outward toward the lower overflow 
portion 5'al owing to a difference in level during curing following 
printing, collapsing the resin layer to result in impaired dimensional 
accuracy as indicated in a phantom line in the drawing. 
To overcome the above problem, the screen 3 of the present invention is 
adapted to come into line-to-line contact with the surface of the board at 
the lower end of the peripheral wall defining each aperture 3a of the 
screen 3, and a recessed portion 8 is formed around the line contact 
portion 7 at the back side thereof. The adhesion of the liquid resin to 
the rear side of the screen 3 can be prevented by the line contact portion 
7 and the recessed portion 8. This will be described below with reference 
to FIG. 5A. 
The upper diagram of FIG. 5A corresponds to the first diagram of FIG. 5B 
and shows a threadlike resin portion 5b1 upon a thread breaking. This 
portion contracts from the illustrated state and is absorbed by a resin 
portion 5b2 adhering to and remaining on the peripheral wall defining the 
aperture 3a. According to the present invention, the lower end of the 
peripheral wall defining the aperture 3a is so shaped as to come into 
line-to-line contact with the surface of the board. For example, it is 
shaped with an acute angle as illustrated. A recessed portion 8 is formed 
around the line contact portion at the back side thereof. Accordingly, the 
threadlike resin portion 5b1 is nearly entirely absorbed on contract by 
the resin portion 5b2 adhering to and remaining on the apertured portion 
3a almost without adhering to the rear side of the screen 3. This results 
in reduction or removal of resin on the rear side of the screen 3. 
Further at the back side of the line contact portion 7, the rear side of 
the screen 3 is at a distance 9 from the surface of the board 1 during 
screen printing as shown in the lower diagram of FIG. 5A. This obviates 
the likelihood that a portion of the resin, if adhering to the rear side, 
will be printed on the board surface. 
With use of the method of the present invention, therefore, no overflow 
resin portion will be printed with the sealing resin layer 5a as joined 
therewith even if the printing cycle is repeated many times as shown 
schematically in FIGS. 3 and 4, with the result that the sealing resin 
layer 5a having no overflow portion can be printed on the surface of 
boards 1 as positioned as specified repeatedly and reliably with good 
stability using the screen printing means. When having an overflow 
portion, the sealing resin layer 5a formed by screen printing partly flows 
toward the overflow portion gradually and collapses due to the interfacial 
tension until the layer cures, whereas when having no overflow portion, 
the resin layer remains in shape reliably with good stability until it 
cures. Thus, the sealing resin layer 5a can be printed with the specified 
dimensions. 
FIGS. 6 to 8 shows various modified screens 3 for use in the method the 
invention. 
The screen 3 shown in FIG. 6 is in the form of a single sheet of a metal 
such as stainless steel and has an aperture 3a and a recessed portion 8 
which are formed by etching. The etching treatment is conducted once. The 
aperture 3a is formed by etching the sheet from both the front and rear 
sides thereof, while the recessed portion 8 is formed by etching the rear 
side to a depth of one-half of the thickness of the screen 3. In the case 
where the recessed portion 8 is formed in the rear side of the screen 3 by 
etching, the recessed portion, if occupying an excessively large area, is 
liable to warp or undulate the screen 3, so that the area to be occupied 
by the recessed portion 8 is preferably as small as possible. The recessed 
portion 8 shown in FIG. 6 is formed in the rear side of the screen 3 to 
surround the aperture 3a and has a width w which is suitably about 1 to 
about 5 mm. If the width is less than 1 mm, the resin will not be 
effectively prevented from adhering to the rear side, while if it is more 
than 5 mm, the screen will not retain satisfactory dimensional accuracy, 
so that widths outside the above range are not suitable. 
With reference to FIG. 6, the lower portion of the screen 3 is formed with 
a peripheral wall 10 having a small thickness and separating the recessed 
portion from the aperture 3a. The lower end of the peripheral wall 10 
comes into substantial line-to-line contact with the surface of the board 
for screen printing. It is suitable that the peripheral wall 10 be about 
0.1 to about 1.0 mm. If less than 0.1 mm, the wall 10 is difficult to form 
by etching. When having a width larger than 1.0 mm, the wall 10 tends to 
come into surface-to surface contact with the board and is not desirable. 
The line contact portion 7 and the recessed portion 8 therearound need not 
always be formed by etching but can alternatively be formed by machining 
or laser beam machining. In this case, the peripheral wall 10 can be made 
as thin as about 0.02 mm. Further as seen in FIG. 7, an annular member 11 
having a triangular cross section may be affixed to the rear side of the 
screen 3 in register with the aperture 3a. The annular member 11 may be 
made of the same material as the screen 3 or of a different material. For 
example, the screen 3 may be made of a metal, and the annular member 11 a 
plastics material. 
As shown in FIG. 8, the aperture 3a can be provided with a porous portion 
12 at its upper end. When the aperture 3a has a relatively large diameter, 
e.g., larger than 20 mm, the porous portion 12 is effective for giving a 
smooth surface to the filled resin layer. The porous portion 12 need not 
be provided when the aperture 3a is up to 20 mm in diameter. The opening 
area of the porous portion 12 is such that the flow of resin to be filled 
through the aperture 3a by the squeegee will not be impeded and is usually 
30 to 70% of the aperture area. 
According to the present invention, the liquid sealing resin is forcibly 
filled into the screen apertures for printing by the operation of the 
squeegee. 
The upper diagram of FIG. 9B shows the liquid sealing resin 5 immediately 
after filling. A raised portion is formed toward the end side of stroke of 
the squeegee 71. Consequently, cobwebbing of the resin 5 occurs at the 
apertured portion toward the end side when the printing cycle is completed 
as seen in the middle diagram, and the cobwebbing occurs in an increased 
amount since the resin is in contact with the screen 3 over a large area. 
The thread 5A breaks when allowed to stand for a given period of time, 
whereas the thread in the increased amount takes a longer period of time 
to break to result in lower productivity. Further since the broken 
threadlike resin portion is absorbed by the sealing resin layer 5a at one 
side thereof, the layer 5a becomes no longer uniform in thickness as shown 
in the lower diagram. 
To solve this problem, the countermeasure shown in FIG.9A is taken by the 
present invention. When the resin 5 is filled by the operation of the 
squeegee 7, the resin 5 inevitably forms a raised portion 5B toward the 
end side of stroke of the squeegee insofar as it is moved in the usual 
manner as shown in the first diagram of FIG. 9A, so that a doctor knife 14 
is moved for cutting from the end side toward the other side subsequent to 
the filling operation to thereby forcibly return the raised resin portion 
5B to the intermediate portion of stroke distance of the squeegee. When 
the resin portion is thus returned, the aperture becomes filled with the 
resin 5 substantially uniformly from the end side 12 toward the starting 
side 13. When the resin is completely transferred from the screen in this 
state, the resin layer can be separated off the screen smoothly at both 
the end side and the starting side substantially without cobwebbing at 
these portions. Although the lower end of the doctor knife 14 produces a 
thread 5A by contact with the returned resin, the amount of cobwebbing is 
small because the contact area is small, permitting the thread to break in 
a short period of time to diminish the waste of time due to cobwebbing, 
hence improved productivity. Further since the break of the thread occurs 
at the intermediate portion of the stroke distance, the broken threadlike 
resin portion is absorbed by the intermediate portion of upper surface of 
the resin layer 5a, whereupon the resin flows toward both sides to form a 
substantially flat surface. Thus, the problem of uneven thickness due to 
the break can also be obviated. 
Although the above procedure has been described with respect to one 
aperture 3a with reference to FIG. 9A for the sake of convenience, the 
screen 3 has a multiplicity of apertures 3a, for each of which the resin 
is cut with the doctor knife 14. To cut the resin by a single movement of 
doctor knives 14 for the apertures, the doctor knives 14 are held to a 
support member (not shown) therefor and arranged in the same pattern as 
the aperture pattern. 
The electric parts mounted on the surface of the wiring board include those 
which must be sealed with resin like those exemplified above, and 
relatively small electric parts which need not be sealed with resin, such 
as capacitors, resistors, windings or the like. 
The latter electric parts may be mounted in place on the board after the 
former pats have been sealed with resin, whereas it is advantageous from 
the viewpoint of productivity to mount all the former and latter parts 
first and thereafter seal the former parts only with resin. 
Such mode of sealing can be accomplished by forming the rear side of the 
screen 3 with cavities 8' in the same pattern as the arrangement of the 
electric parts 2' which need not be sealed with resin for accommodating 
these parts 2' as schematically shown in FIG. 2B. In this case, the 
recessed portion 8 formed around the aperture 3a in the screen 3 for 
preventing the printing ink from adhering to the rear side can be utilized 
as a portion of the accommodating cavity 8'.