Assembly of electronic components onto substrates

In a method of assembling an electronic component onto a substrate, a fiducial is defined by a solder mask positioned upon the substrate. A fiducial-defining window in the mask has a base of the window completely provided with a material of different light-reflecting quality than the mask surface. This provides for the peripheral edge of the window, i.e. the mask, to be the edge of the fiducial. Windows in the mask which expose terminal pads upon the substrate and fixed in position relative to edges of the fiducial and both these windows and the fiducial are determined by the mask. Surface mount components are then located upon the substrate relative to the window positions and not relative to the terminal pad positions. This process reduces the number of incorrect connections of terminals of components to terminal pads.

This invention relates to assemblies of electronic components onto 
substrates. 
In the manufacture of printed circuit board structures which include a 
substrate (i.e. a printed circuit board) and electronic components mounted 
upon the substrate for automatic assembly of the components onto the 
substrate, components need to be positioned substantially exactly in their 
required location to ensure the terminals on the components are aligned 
correctly with terminal pads on the substrate, and to which they are to be 
soldered. For this purpose, registration datum positions are provided upon 
a substrate. These datum positions are located by a sensor of a robot 
machine which is programmed to move the electronic components to specific 
coordinate and positions based on the datum positions, for positioning the 
components at their required locations, which coincide with the coordinate 
positions. The datum positions, or origins or zeros of reference, are 
referred to in the printed circuit board manufacturing industry as 
"fiducials". The terminal pads, upon which the terminals of the components 
are to be placed, are exposed through windows of a solder mask covering 
the substrate surface. The fiducials are represented by metal pads which 
are also exposed through windows of the solder mask with clearance being 
provided between the pads and edges of the windows. Thus, the peripheral 
edges of the metal pads are identified by the control equipment to set the 
fiducial position. Fiducial position is in a desired and known location 
with reference to the terminal pads on the substrate, and with which the 
terminals of the electronic components need to be aligned. However, slight 
misalignments are known to occur between the position of the solder mask 
on the one hand, and the fiducials and terminal pads on the printed 
circuit board on the other hand. This may not cause a problem with windows 
of large size, but some windows are only slightly larger than the terminal 
pads which are exposed within them. This occurs when there is small 
terminal spacing as for terminal connections of chip or ball grid arrays 
to substrate terminal pads, and the margin for misalignment error becomes 
much smaller. In such cases, it is not unknown for edges of windows to 
interfere and prevent correct terminal placement upon terminal pads, 
thereby providing defective printed circuit board structures. 
The present invention provides a method of assembly of electronic 
components onto a printed circuit board, and which will reduce the 
resultant defective structures. 
According to the present invention, there is provided a method of 
assembling an electronic component onto a substrate, comprising providing 
the substrate having on one surface a plurality of electrical terminals 
for interconnection with the electronic component; 
providing a solder mask upon the substrate, the mask having terminal 
windows which expose the terminal pads through the terminal windows, the 
mask also having fiducial means comprising at least one fiducial defining 
window having the whole base surface area on one side of the peripheral 
edge of the fiducial defining window, and within the fiducial defining 
window having a different light reflecting quality from the surface area 
of the mask on the other side of the peripheral edge of the fiducial 
defining window; 
determining the datum position of the at least one fiducial defining window 
by reference to the peripheral edge position of the fiducial defining 
window; 
using the determined datum position of the fiducial defining window for 
guiding the electronic component to a desired location and a desired angle 
of orientation in accordance with coordinate positions dependent at least 
in part upon the determined datum position of the fiducial-forming window 
so as to align the terminals of the component with respective terminal 
windows; and 
then soldering the terminals to the terminal pads which are exposed by the 
terminal windows. 
It follows from the method of the invention that the fiducial defining 
window determines the fiducial datum position. As a result, if the solder 
mask is slightly misaligned with the substrate, and with the terminal pads 
still exposed through their respective terminal windows, then the 
electronic component is guided into position relative to the terminal 
windows of the terminal pads, and not relative to the terminal pad 
positions themselves. Hence, the terminals of the electronic component may 
be slightly misaligned with the terminal pads on the substrate, but are 
aligned with the terminal windows themselves. It follows that while the 
terminals of the electronic component and the respective terminal pads may 
be slightly misaligned, the soldering operation is still successful 
without interference in the soldering operation by edges of the terminal 
windows of the terminal pads. Hence, the misalignment of the solder mask 
with the substrate terminal pad positions does not interfere with 
placement of the terminals upon the terminal pads of the substrate and 
soldered joints are not suspect. 
In one arrangement, only one fiducial defining window is provided. In this 
arrangement, the peripheral edge of this window is non-circular and the 
method comprises determining the datum position and angle of orientation 
of the fiducial-defining window by reference to the peripheral edge 
orientation and position of the fiducial-defining window. 
However, it is preferred to provide at least two spaced-apart 
fiducial-defining windows and the datum position of each fiducial-defining 
window is determined by reference to the peripheral edge position of each 
of these windows. 
It is of the essence of the method of the invention that the light 
reflecting quality on one side of the peripheral edge of the 
fiducial-defining window is different than on the other side. This may be 
achieved by providing a surface of the substrate, and which provides the 
base surface of the window, with a reflectivity which is different from 
that of the mask surface. It is preferable, however, to provide upon the 
substrate surface, two fiducial-forming elements, e.g. of copper, which 
extend completely across and occupy the whole of individual base surface 
areas of the fiducial-defining windows. This is achievable by causing the 
mask to overlap edges of the fiducial-forming elements. 
According to a further aspect of the present invention, there is provided 
an assembly of a substrate with an electronic component mounted upon one 
surface of the substrate, comprising: 
a substrate having electrical terminal pads; 
a solder mask extending across the one surface of the substrate, the mask 
formed with terminal windows with terminal pads exposed through the 
terminal windows, and also formed with fiducial means comprising at least 
one fiducial-defining window having the whole base surface area on one 
side of the peripheral edge of the fiducial-defining window, and within 
the fiducial-defining window, having a different light reflecting quality 
from the surface area of the mask on the other side of the peripheral edge 
of the fiducial-defining window, whereby the peripheral edge of the 
fiducial-defining window coincides with the peripheral edge of the 
fiducial; and 
an electronic component having terminals electrically connected to 
respective terminal pads of the substrate. 
In a practical construction, to ensure that the fiducial-forming elements 
extend completely across the base area of their respective windows, then 
the fiducial-defining windows are smaller in area than the 
fiducial-forming elements and extend across edge regions of the fiducial 
forming elements so as to obscure these edge regions.

As shown by FIG. 1, a prior printed circuit board and solder mask 
construction 10 for use in a conventional method of assembling electronic 
components onto the board, comprises a printed circuit board 12 having a 
solder mask 14 upon it. The solder mask is provided with fiducial 
clearance windows 16 which expose copper fiducials 18 within the base 
areas of the windows 16. These fiducials 18 are of smaller area than the 
windows 16 so that the windows have a clearance around the fiducials as 
shown particularly by FIG. 1. In addition to this, and as shown in detail 
by FIG. 3, the mask 14 is provided with terminal pad windows 20 which 
expose terminal pads 22 on the board surface. These terminal pads are 
provided for solder connection to terminals of electronic components to be 
mounted thereon. 
As may be seen from FIG. 2, it is not uncommon for the solder mask on a 
conventional circuit board to be slightly misaligned with the board so 
that the fiducials 18 are misaligned with the windows 16. FIG. 2 may show 
an exaggeration of this misalignment, but it is understood that in 
practical terms the misalignment may be significant as will now be 
discussed, even though in dimension the misalignment may be small, e.g. 2 
mil or greater. 
To dispose an electronic component upon the board where its terminals are 
to be soldered to the terminal pads 22, a vision system (not shown) is 
used to determine the position of the fiducials 18. This determination is 
made by sensing the light reflected from the fiducials, which is of higher 
intensity than the background provided by the board and mask. Each 
fiducial is conventional in that it is made from copper with an oxide 
protective surface which provides high intensity of light reflection. The 
protective surface may be a tin/lead finish, an electroplated nickel with 
an immersion gold plating or an organic antioxidant. The fiducial 
positions then provide datum locations used to control a robot equipment 
for moving the various electronic components to correct coordinate 
positions and angular positions on the board (e.g. x, y, theta) to 
register the terminals of the components exactly in line with the terminal 
pads 22 to enable these to be soldered together. However, in this 
registration process, no consideration is given to any misalignment of the 
solder mask which cannot be read by the vision system. As a result, 
windows for exposing the terminal pads for the respective electronic 
components must also be misaligned with the terminal pads. In some 
instances, this is of no due concern as the windows may be made 
sufficiently large to render the misalignment insignificant. However, with 
certain constructions, the windows need to be made extremely small, i.e. 
only slightly larger in area than the terminal pads which they are 
provided to expose. Such a situation occurs for location of "flip" chips, 
"solder bumped" chips, or ball grid arrays upon a printed board in which 
the terminals of the electronic component are exceedingly close together, 
e.g. of the order of 12 mil. FIG. 1 shows, as an example, three location 
areas 23 for chips, the chaindotted lines representing the lines of 
closely positioned terminal pads provided upon the board for chip terminal 
connection. As a result, and as shown by FIG. 3, the mask at such terminal 
positions as windows 20 which are misaligned with the terminal pads 22 
upon the board 12, and the misalignment may even be so great that the mask 
overlaps one side of each of the terminal pads. Hence, when the associated 
terminal 24 of the respective electronic component 26 is disposed in 
position, it is registered relative to the respective terminal pad 22 and 
during its location, it is forced to overlap at least partly the solder 
mask which extends over the terminal pad. As may be seen from this, the 
solder which is already aligned within the window 20 has much less surface 
contact with both the terminal pad 22 and the terminal 24 than it would 
have if the solder mask were correctly aligned with the terminal pad 22. 
This may lead to a defective solder joint. In addition to this, and as may 
be seen from FIG. 3, the adjacent window 20 is also misaligned with its 
terminal pad 22, and this misalignment brings it towards the position at 
which the next terminal 24 is being registered. Thus, the solder in this 
window 20 approaches close to the next terminal 24 and, should the 
misalignment be sufficient, then an undesirable electrical connection 
could be made. 
The invention minimizes the possibility of such a problem as described 
above from occurring. This is apparent from the embodiment described 
below. 
In a first embodiment, a printed circuit board assembly 30 comprises a 
printed circuit board 32 having a solder mask 34. The board assembly is 
similar to the assembly 10, except for in one major respect. As shown by 
FIG. 4 and in great detail in FIG. 5, board 32 does not have fiducials in 
the form of the copper pads 18. Instead, the board 32 has fiducial-forming 
elements 36 which are also copper pads and provided in the same locations 
as described above for the prior art and having oxide protective surfaces. 
The solder mask 34 is provided with windows 38 (FIG. 6) for exposing the 
terminal pads 40 of the board, and it also has fiducial-defining windows 
42. Each window 42 is of smaller cross-sectional area than its 
fiducial-defining element 36 in that the fiducial-forming element extends 
completely across the base of the fiducial-forming window 42. In fact in 
the embodiment, each fiducial-forming element 36 extends beneath the mask 
34 as the mask overlaps the fiducial-forming element around its edge 
region thereby obscuring it. In essence therefore, the fiducial-forming 
elements 36 are not themselves fiducials and the windows 42 are not merely 
windows to expose such fiducials. In contrast, each fiducial in the 
embodiment is provided both by the fiducial-forming element 36 and its 
respective fiducial-defining window 42 in that the area of the 
fiducial-forming element 36 which is exposed in the base of the window 42 
is sensed by the vision system in determining the datum position of the 
fiducial, and the peripheral edge of the fiducial-defining window 42 
provides the peripheral edge of the fiducial. Hence, the fiducial-defining 
window does in fact determine the position of the fiducial and, thus, its 
datum location. 
As a result of the structure in the first embodiment as described with 
reference to FIGS. 4 and 5, even if the solder mask 34 is slightly 
misaligned with the printed circuit board 32, then with the mask 
overlapping each fiducial-forming element 36, then the peripheral edge of 
the window 42 still determines the fiducial position. It follows from this 
therefore, that each electronic component is controlled by the robot 
equipment to be positioned to its correct x, y, theta registration 
positions not in relation to the positions of the terminal pads on the 
board 32, but in relation to the positions of the windows 38 which expose 
those terminal pads. This is because the windows 38 of the terminal pads 
are in predetermined positions relative to the fiducial-defining windows 
42. As a result, should misalignment between the board and the mask occur 
then, as shown in FIG. 6, the windows 38 are misaligned with the terminal 
pads 40 in a manner similar to the misalignment of windows 20 and terminal 
pads 22 in the conventional structure as described above, and shown 
particularly in FIG. 3. However, the misalignment in the first embodiment 
cannot have such disastrous results, as may occur in the conventional 
method of assembly. As shown by FIG. 6, each of the electronic components 
46 is moved in the x, y, and theta directions to its registration position 
shown by a respective location area 23, and the terminals 44 of each 
component 46 are automatically aligned with the respective windows 38. As 
may be seen from FIG. 6, the method of the embodiment and of the invention 
ensures that the terminals 44 in registering with the windows 38 maintain 
a respectable and safe distance from an adjacent window 38, which may also 
contain solder paste or solid solder. It follows that the chances of a 
terminal being incorrectly soldered to an adjacent terminal pad are 
insignificant. 
As may be seen therefore, the present invention, and as explained in the 
first embodiment, provides a method of assembly of electronic components 
onto a printed circuit board which overcomes soldering inaccuracy problems 
when a solder mask is misaligned with the printed circuit board upon which 
it lies. Improved assembly yields of boards and electronic components thus 
result thereby reducing product wastage and resultant economic loss. 
In a second embodiment as shown by FIG. 7, the assembly 52, which is 
otherwise similar to the first embodiment, has only one fiducial 54. This 
fiducial must be non-circular, and is of such a shape as to enable the 
vision system to determine the position and angle of orientation of the 
peripheral edge 56, which is the edge of the fiducial-defining window 
providing the fiducial. In this case, as with other embodiments, a 
fiducial-forming member 58 extends completely across the base area of the 
window and the edges of the mask overlap around the edges of the member 
58, as shown in FIG. 7. 
In all instances, it is not necessary for a fiducial-forming element to be 
provided for each fiducial. This may be the case where there is sufficient 
difference in reflectivity between the surface of the mask and the surface 
of the substrate within the base area of each fiducial-defining window 
that the peripheral edge of the window can be determined by a vision 
system. Such is the case in a third embodiment as shown in FIG. 8 in 
which, in an assembly 60, the substrate 62 is surmounted by a mask 64. The 
substrate 62 is formed of a material which is of much lighter color than 
the mask 64, so that the light reflecting quality on one side of the 
peripheral edge 66 of the fiducial-defining window 68 is easily detectable 
by a vision system for determining the datum position of the window.