Preparation of fragile devices

A method of fabricating a generally planar device such as a lead frame, having fragile portions contiguous respectively with relatively robust portions, which includes in sequence, the steps of providing a robust, generally sheet-like blank, removing material from selective portions of a surface layer of the blank at one of its main surfaces to define first portions of the surface layer to be the relatively robust portions in the finished device and second portions to be fragile portions of the finished device, firmly embedding the second portions in a fixing material layer, and removing material from the other main surface of the blank in the region of the embedded portions to a sufficient depth to expose an underside of the surface layer, thereby to isolate the embedded portions from each other and to render them thinner and thus more fragile than the first portions. With such a method, there is fabricated a generally planar lead frame including several leads, each having a relatively robust portion and a contiguous, relatively fragile portion thinner and more narrow than the robust portion, the fragile portion having a flat face. The lead frame further includes a flat support in the plane of the lead frame, all of the fragile portions being embedded in the flat support such that only the flat face of each fragile portion is exposed in the plane for electrical contact, and each fragile portion is as wide at the flat face as it is beneath that flat face.

BACKGROUND OF THE INVENTION 
This invention concerns the preparation of fragile devices by the removal 
of part of a suitable body of material (by an etching or similar removal 
process), and relates particularly, though not exclusively, to the 
formation of lead-frames useful inconnection with the packaging of 
micro-electronic devices such as integrated circuits. 
The main concept of the invention is conveniently described with reference 
to lead-frames. 
As is now well known it is possible to manufacture very complex electronic 
circuits in the form of "chips", in which the circuit--containing both 
passive components, such as conductors, resistors and capacitors, and 
active components, such as diodes, transistors and other semi-conductor 
devices--is formed on and in the surface of a fingernail-sized chip of 
silicon. In a Large Scale Integrated device the number of connections in 
and out of the chip may well be at around the hundred mark, but frequently 
chip designers are pressing for lead counts of a much greater 
value--possible as high as 400. To fit these in within the available 
space, the actual conductors forming these connections have to be only 
about 1 thou (about 25 microns) wide; this is finer--and much more 
delicate--than the finest human hair. 
Naturally, the chip, and its connectors, has to be protected, and it is 
common to encase, or package, the chip, usually in a plastics or ceramic 
material, to make a larger, stronger article which may relatively safely 
be handled during the construction of the device of which the chip forms a 
part. One possible end result is the familiar Dual Inline Package (DIP), 
and oblong object the size of a LEGO brick with conductor "legs" extending 
downwardly therefrom along either side by which the DIP may be connected 
onto a Printed Circuit Board (PCB) via holes therein. Another--frequently 
used where the number of connectors to the chip is large--is the Flat Pack 
(FP), a square object that has its legs on two or all four sides and 
extending below and then outwardly so that it can sit on the PCB with its 
legs flush with, and soldered to, the tracks on the board's surface. Such 
a result (the DIP or FP) is preferably achieved by packaging the chip 
together with what is called a "lead-frame" (a framework of "radially" 
extending conductive leads that are very fine and fragile on the inside, 
where the 25 micron connectons are made from them to special pads on the 
chip itself, but are much coarser, and more robust, on the outside, where 
the connections are made to the PCB). The invention is concerned with the 
preparation of fragile bodies like this lead-frame. 
It is common practice to construct a lead-frame by starting with a foil or 
sheet blank of relatively thick, robust material (robust enough to make 
the legs of a DIP or FP, say), and to remove this material, often by 
etching, where it is not required at all (the empty spaces between legs). 
Unfortunately, the sheer quantity of material to be removed, coupled with 
the very significant problems of controlling the removal process so as to 
remove only what is required to be removed, means that it is very 
difficult to start with a thick and robust foil and end with a lead-frame 
the inner parts of which are sufficiently narrow and delicate. In one 
conventional method. quite suitable for making a Flat Pack lead-frame with 
only a few--say, four or five--connectors per side (making 16 or 20 in all 
four sides), the lead-frame blank is masked on either major face so as to 
define those areas wanted (the connectors, together with a few temporary 
sections holding the whole together) and unwanted (the spaces between 
connectors together with the central area where the chip will be), and 
placed in a bath of chemical etchant material until all the unwanted 
material has been etched away. The result is a "skeleton" of conductors 
uniformly as thick as the original sheet (typically 0.1 mm--100 
microns--or so) but wider on the outside (perhaps 1 mm--1000 microns) than 
on the inside (possibly 0.3 mm--300 microns). With a low lead count it is 
acceptable to form these inner leads with their inner ends free, for their 
thickness and width make them reasonably rigid. However, with a higher 
lead count--say 20 to 25 connections per side (making 80 to 100 in all) 
this is unacceptable, and it is necessary to leave unetched a central, 
lead-retaining, portion to which each fine lead end is attached by a very 
narrow neck, then to "fix" the leads by a packaging operation, and finally 
to chop away the lead-retaining portion (through the narrow necks) so as 
to leave the tips of the leads free and ready to be connected to the 
relevant chip pads. Unfortunately, with a lead count that is higher 
still--say, 50 connections per side (making 200 in all)--it becomes 
necessary to reduce the width of the individual leads at their "fine" end 
to as little as 0.05 mm (50 microns), and the sheer mechanics of fixing 
such fine leads, and then chopping away the lead-retaining portion, 
becomes almost impossible to manage. The invention of the copending 
British Application for Letters Patent No. 8519709 which has been 
assigned to the assignee of the invention and whose disclosure is 
incorporated herein by reference. seeks to mitigate the problems by 
providing the "fixing" support for these very fine leads not after they 
have been formed but before. More specifically, this other invention 
suggests a technique whereby most of the thickness of the material over 
the whole of the lead-frame foil blank's inner area (where the inner 
connections to the chip will eventually be) is removed from one face to 
make a generally robust foil with a centrally-located recess defining a 
much thinner inner section, the recess is filled with some "fixing" 
material that will both provide support for and hold in position the thin 
inner section, and further material is then removed from the other face of 
the blank so as to form the fine inner parts of the lead-frame, these 
being firmly supported, and held in place, by the "fixing" material 
filling what was the recess in the foil. Such a technique is, of course, 
applicable to the preparation of many devices in which a fragile area is 
formed by the removal of a part of a more robust area. 
The invention the subject of this other Application is defined as a method 
of fabricating a generally planar, sheet-like, device having a first, 
robust, area and, contiguous therewith, a second, fragile, area to be 
constructed by removal of some of the material of which the device 
consists, in which method: 
(a) starting with a corresponding generally planar, sheet-like blank, in 
which both first and second areas are robust, some of the material to be 
removed from the blank to make the second, fragile, area is so removed 
from one major face of the blank, to form a recess in the blank; 
(b) the thus-formed recess is filled with a fixing material, so as to 
support and fix in place the thin area of blank material constituting the 
base of the recess; and thereafter 
(c) the remainder of the material to be removed from the blank to make the 
second, fragile, area is so removed from the other major face of the 
blank, so forming the required second, fragile, area supported and fixed 
by the fixing material and contiguous with the first, robust, area. 
Unfortunately, though this defined method does enable there to be prepared 
devices of the type specified, and specifically to allow there to be 
formed by a through-mask ethcing process lead frames having extremely fine 
inner leads occupying the second (fragile) area, nevertheless it does 
suffer from one minor disadvantage for this particular use, which 
disadvantage arises because of the occurrence of mask undercutting during 
the stage of etching from the other major face. The problem can be 
explained as follows: 
The preferred technique (using this other invention) involves removing 
material from the blank by an etching process, specifically a chemical 
etching process. In this process the blank is first coated with a masking 
layer, that covers the areas not to be etched away, and is then contacted 
with the etch solution. Naturally, the etching occurs only where the blank 
material is exposed--and at first this is indeed the surface. However, as 
the etching progresses, cutting a deeper and deeper "groove" (into the 
blank material) bounded by "walls" of blank material underlying, and thus 
protected by, the mask, so the etchant attacks not merely the "floor" of 
the groove but also these very walls. Thus, the etch process eats 
"sideways" into the blank material into the areas beneath the mask, so 
undercutting the mask. Of course, the degree of undercut depends on a 
number of factors, including the detailed crystolographic structure of the 
blank material and the physical way in which the enchant is allowed to 
contact the surface; it is common, however, for the unwanted undercut to 
be about half the depth of the desired etching. The net result, though, is 
that when the mask is removed the fine leads produced by the etching 
process have quite substantial, wide, bases (where little sideways etching 
occurred), but have extremely narrow tops (where the maximum undercutting 
took place). And most unfortunately it is to these tips that there must be 
bonded the wires connecting the leads to the chip to be mounted in the 
lead frame; as can be imagined, bonding these wires to leads each of which 
have half--or less--than the thickness they should have is quite 
difficult. 
The present invention seeks to solve this problem by the extremely simple, 
but nevertheless very effective, expedient of, as it were, performing the 
method of this other invention in reverse--that is to say, not of first 
removing material from one face of the blank to form a recess, and then 
removing the remainder of the material from the other major face, but 
instead of first removing material to define the components of the 
proposed fragile area of the blank, supporting these with fixing material, 
and then removing the remaining material (to make a recess) from the other 
face. In this way, when using the method to make by etching the fine inner 
leads of a lead frame, it is the wide bases of the leads that are revealed 
and to which the wire connections may be bonded, rather than the 
"undercut" upper surfaces. 
In one aspect, therefore, the present invention provides a method of 
fabricating a generally planar, sheet-like, device having a first, robust, 
area (portion) and, contiguous therewith, a second, fragile, area 
(portion) to be constructed by removal of some of the material of which 
the device consists, in which method: 
(a) starting with a corresponding generally planar, sheet-like blank which 
can be thought of as consisting of two layers--a surface layer and a 
base--so that of the two major surfaces of the blank, one is formed on the 
base, the other on the surface layer, and in which both first and second 
areas are robust, some of the material to be removed from the surface 
layer of the blank to make the second, fragile, area is so removed from 
one major face of the blank over a proportion of its thickness, to form 
fragile components in the second areas upstanding from, and supported by, 
a remaining base of the blank material, 
(b) the thus-formed components (those portions of the) are intimately 
surrounded by a fixing material so as to be firmly embedded therein, so 
that the fixing material supports and fixes the component in place, and 
thereafter; 
(c) the remaining base from which the fragile components are upstanding is 
removed from the other major face of the blank to leave the components 
separated from each other, to form the required, second, fragile area 
having fragile portions supported and fixed by the fixing material and 
contiguous with portions in the first, robust, area such that each 
component which is formed by the removal of the base has a wide exposed 
surface. 
The method of the invention can, as stated hereinbefore, be applied in the 
fabrication of numerous types of device, and examples of these are contact 
sets for electrical switches, and positional sensors. Nevertheless, it is 
especially suited to the manufacture of lead-frames, as described above, 
and is for the most part discussed further hereinafter with reference to 
such a use. 
The inventive method is used in the formation of a generally planar (that 
is to say, flat), sheet-like, device, and starts with a corresponding 
generally planar, sheet-like, blank. This blank is a sheet, or foil, of 
material--thus, an article having an area (and length and breadth 
dimensions) large relative to its thickness, and so has two major faces 
(the two "sides" of the sheet). 
The material of which the blank is constituted naturally depends upon the 
nature of the device to be made. For a lead-frame the material must be an 
electrical conductor, especially one that is sufficiently malleable to 
allow itself to be shaped by a bending operation (subsequent to the method 
of the invention the robust area of the blank will be made into the "legs" 
of the package, and will need to be bent--and perhaps re-bent--out of the 
sheet plane and into the correct shape). Typical lead-frame materials are 
made of a suitable metal; nickel-iron alloys of the KOVAR or INVAR type 
are very suitable, as are copper and copper-based alloys such as 
beryllium-copper, phosphor bronze (an alloy of tin and copper with up to 
0.5 wt % phosphorus) and copper-iron-phosphorus alloys like OLIN 194. 
The device to be fabricated is one having a first area that is robust 
and--contiguous therewith (and probably wholely surrounded thereby)--a 
second area that is fragile. The two terms "robust" and "fragile" are used 
in a fairly relative sense--the "fragile" area is one that is 
significantly less robust than the robust area, and vice versa--though 
where a lead-frame is concerned the "robust" area ends up as the "legs" of 
the final package, tough enough to withstand handling, while the "fragile" 
area ends up as extremely fine conductors rather more delicate than human 
hair. 
The method of the invention involves removing material from the second area 
of the blank (and so making that area fragile). Naturally, the 
distribution of this removal--where, and how much, material is 
removed--depends the type of device. For a lead-frame, however, there are 
two distinct classes of material to be removed. One is material removed 
simply to "thin" the sheet over the whole of the defined, second, area, so 
that it becomes more suitable for forming into the inner leads of the 
lead-frame. The other is material removed from between those portions that 
are to be the leads, so separating each lead from its neighbours. In the 
first stage the between-lea blank material is removed from one of the 
blank's major faces, so leaving the formed leads flush with this major 
face, and standing proud from the remaining "thickness" of the blank 
(which therefore constitutes a base carrying the formed leads), while in 
the second stage that base is removed from the other major face, so ending 
with the leads in this area having a depth equal to roughly half the 
blank's original thickness. 
Removal of material from the blank is accomplished by any technique suited 
to the intended device. Thus, the methods of removal include particle 
ablation and laser trimming, but--especially for lead-frames--chemical 
etching is the preferred technique. Of course, in this latter case the 
nature of the etchant will be chosen to fit the blank material; the 
methods are well known in the Art, and need no detailed comment 
here--though it may be useful simply to note that nickel iron alloys can 
best be etched by ferric chloride/hydrochloric acid mixtures, or nitric 
acid-based materials, while copper and its alloys are preferably dealt 
with using either ferric chloride or ammoniacal cupric chloride (REGENEX, 
for instance). It is also worth pointing out that etching (and, indeed, 
other techniques of material removal) has to be done selectively--some 
areas are to be removed, some not--and a convenient way to arrange this is 
to "mask" the areas not to be etched with a protective coating that is 
unaffected by the etchant to be employed. Within reason--as explained 
hereinbefore a degree of "undercutting" of the mask is, unfortunately, 
inevitable, and is the greater as the depth of the etching increases--this 
masking ensures that only the unmasked, exposed, areas are etched away. 
In the first stage of the method of the invention material is removed from 
the second area of the blank (that area destined to become the fragile 
area of the device), to leave the intended fragile components upstanding 
from, and supported by, a base constituted by second area blank material 
that has not been removed. Thus, where in the final device, there is to be 
no blank material, then in those areas the material is removed--but only 
to a depth that is a fraction of the thickness of the blank, so that those 
parts of the blank that will eventually form the fragile components are 
left upstanding/projecting from a "base" of the remaining thickness of the 
blank. The actual disposition of the material to be removed will, 
naturally, be appropriate to the device being fabricated; for a lead-frame 
it is that material lying between the portions defining the inner leads, 
the intended fragile components being these inner leads themselves. 
At the same time as material is removed from the second area of the blank 
it is, of course, possible to remove material from the first area (the 
area destined to become the robust area of the device). For example, when 
the device is a lead-frame it is naturally a requirement that the robust 
area end up as leads the outer part of which make the legs of the final 
package, so that somewhere along the way there must be removed from the 
blank's robust area all the material between those portions destined to be 
these leads. At least some of this surplus material can, then, be removed 
at the same time as the second area is thinned (when using a through-mask 
etching technique this is arranged quite simply by so designing the mask 
that the "between-leg" portions remain exposed). Over the whole of the 
first area the between-leg portions are consequently subjected to the same 
thinning as that effected to the second area, and this is in fact 
particularly advantageous because (as described further below) it greatly 
reduces the degree of undercutting of these leg portions as compared with 
a process wherein all of the depth of the between-leg portions is removed 
at one go. 
Having removed material from the second area of the blank the method of the 
invention results in the intended fragile components being upstanding from 
a base. The material forming this base will, in the second stage, be 
removed to leave the fragile components free, so forming the final, 
fragile area--which, in a lead-frame, is the assembly of inner leads. 
However, because this final form is so fragile it is desirable to support 
the whole area before the second stage removal is effected, and this is 
done, in the method of the invention, by filling the gaps between the 
upstanding fragile components with a fixing material that will, as 
required, support and fix them in place. This fixing material can, 
naturally, be any appropriate to the circumstances--and, where it is not 
to be removed after the second stage material removal has occurred, but is 
to stay as a part of the final device (as is preferred where the device is 
a lead-frame), this fixing material will also be appropriate to the type 
of device. A lead-frame device is to be packaged with an 
electrically-insulating packaging material; conventional packaging 
materials are plastics (such as silica-filled epoxies like POLYSET 410B 
and NITTO HC10 type 2) and ceramics (such as alumina and beryllia), and 
clearly the fixing material should be compatible with the packaging 
material. Fixing materials compatible with one or other plastics packaging 
material are generally the thermosetting resins, specifically the epoxies 
(such as those named above) while those compatible with one or other 
ceramics packaging material are generally glasses and glass-ceramics. 
The actual mode of emplacement of the fixing material may be whatever is 
appropriate. For example, a fixing material that is a polymerised plastics 
might be "injected" or "squeezed" into place in the form of a pasty but 
flowable pre-polymer mixture (the final polymerisation then taking place 
in situ); a thermoplastics fixing material might even be "melted" into 
place. Similarly, a glassy fixing material might likewise be applied, in 
an un-fired form, and then fired in situ. In one preferred case the fixing 
material is a glass-like lithium alumina silicate, and is deposited in a 
glassy state and fired in situ to convert it to a glass-ceramic state. 
As intimated above, the fixing material either may be removed at some later 
stage or may be left in place serving some useful function. 
Once the inter-fragile-component spaces have been filled with the fixing 
material, the material constituting the supporting base of the fragile 
components is removed from the other major face. As in the case of the 
removal of blank material in the first stage, the removal technique may be 
any appropriate, as described above. Moreover, as in the first stage, 
blank material can be removed not only from the second area but also from 
the first area. For example, to make a lead-frame half the thickness of 
first area material in the between-leg portions is removed (from one major 
face) in the first stage, and the rest--so separating the "legs" one from 
another--is removed (from the other major face) in the second stage. 
Any "masking" techniques required, particularly to ensure registration of 
wanted and unwanted portions on one face with the corresponding wanted and 
unwanted portions on the other face, are well known, and need no comment 
here. 
Having removed blank material in the second area both from one major face, 
and then, after supporting the formed fragile components, from the other 
major face, the device is complete. lf it is a lead-frame, for example, it 
can then be used--a chip can be mounted in the centre of the second, 
fragile, area surrounded by the free ends of the fine inner leads (this 
central portion will normally have been removed, either during the etching 
process or by some appropriate alternative method, perhaps as a 
preliminary measure), these leads can be connected (by wire-bonding, 
perhaps) to the relevant chip pads, the whole may then be encased in some 
suitable protector material (often by placing it in a small "box" sealed 
with a separate lid), but with the remains of the robust blank area--the 
"leg" portions--projecting therefrom and ready to be bent or otherwise 
mechanically shaped into the desired "leg" form. Alternatively, the 
lead-frame may be incorporated within an empty, chip-less open-top package 
(so that at some subsequent time the chip may be inserted into place and 
its pads connected up to the inner leads, and a lid attached and sealed 
shut). The process of making such a device is described hereinafter in 
more detail with reference to the accompanying Drawings and the Examples. 
It is here useful to note that the lead-frame as first constructed 
commonly has integrated therewith a whole series of strengthening and 
retaining members (some of which may be of a plastics, or other 
non-conducting substance, that is laid in place after the first face 
treatment but before the second face treatment), together with a series of 
test points conductively connected to the "leg" portions; some of these 
extra features of the lead-frame are removed once the completed 
chip-containing package has been made, while others are removed after the 
package and its contents have been tested. 
The invention extends, of course, to a device, particularly a lead-frame, 
whenever fabricated by the method of the invention.

It is convenient first to consider the sequence of operations illustrated 
by the FIG. 5A to G series. It is generally as follows: 
(A) The starting point is a blank (10)--a sheet of the appropriate material 
(in a leadframe this would be a conductive metal such as NILO K). 
(B) The blank is protected on one surface--the upper surface as 
viewed--with a mask (14) defining the form of the required fragile area. 
In a leadframe this area would be that containing the fine inner leads. 
(C) Blank material is removed, through the mask, to roughly half the 
blank's depth. Undercutting (as 19) takes place. 
(D) After removal of the mask (not shown separately) the thus-formed 
fragile area is encapsulated with fixing material (13). When making a lead 
frame this material is conveniently a non-conductive glass. 
(E) The lower surface (as viewed) of the blank is now protected with a 
second mask (11) defining the area that is about to be transformed into 
the desired fragile area. 
(F) Blank material is removed, through this second mask, right "down" to 
the fixing-material-supported parts of the fragile area (the leadframe's 
fine inner leads), so forming the desired fragile area (generally 15) 
supported by the fixing material 13. 
(G) The mask is then removed. 
The sequence of operations illustrated by the FIGS. 1A, B, C and D series 
is generally as follows: 
(A) a planar, sheet-like blank 10 of leadframe material that is overall of 
a robust construction is protected on one surface (the upper surface, as 
viewed) with a mask (11) defining the form of the required fragile area. 
(B) Blank material is then removed, through the mask, to a depth of roughly 
half the blank's thickness, so forming a recess (12) in the upper surface, 
and this recess is then filled with sufficient fixing material 13 to 
support both the fragile area formed in the final stage, and the immediate 
surrounds thereof. 
(C) The lower surface (as viewed) of the leadframe blank is now protected 
with a second mask 14 defining the whole of the area which is to be 
transformed into the required fragile area. 
(D) Blank material is then removed, through this second mask, right through 
to the fixing-material-filled recess 12, so forming the desired fragile 
area 15 supported by the fixing material 13. 
FIGS. 2A, B and C are a series of perspective views from above (A and B) 
and from below (C) showing various stages in the formation of a leadframe. 
FIG. 2A depicts the stage after removal of blank material from one major 
surface so as to define the fragile inner leads (as 25). At the same time, 
there is removed half the thickness of the blank material elsewhere--the 
between-leg portion--so as to form in part the outer leads/legs (as 22) of 
the desired leadframe. In FIG. 2B there can be seen in place the fixing 
material (23), which takes the form of a square "ring" of material moulded 
snugly over the inner leads and leaving a chip-sized central area (24) 
unfilled. Finally, FIG. 2C shows from below (as viewed) the leadframe 
after the remaining unwanted blank material has been removed. From this 
Figure it will be clear how the fragile inner leads 25 are safely 
supported by the ring of fixing material 23 while the outer leads 22 now 
stand free, and support themselves. 
The views of FIGS. 3A, B and C are top plan views of three different lead 
frames. That of FIG. 3A corresponds to the leadframe of FIG. 2C, that of 
FIG. 3B relates to a yet more realistic leadframe, while that of FIG. 3C 
is a slightly enlarged view of a real leadframe. 
The leadframe of FIG. 3B is a four-sided device with 13 leads (as 31) a 
side (making 52 leads in all). Each lead has a fine, fragile, inner 
portion (as 31i) and a relatively robust outer portion (as 31o). Within 
the general area of the inner leads 31i there is a square ring-like lead 
support (of lead fixing material) whose inner edge is defined by the inner 
tips of the leads and whose outer edge is shown by the perimeter line 
(32). Further out from this perimeter is a line (33) indicating where the 
boundary of the package, forming the desired flat pack, will lie, and just 
beyond this is a first lead support ring (34) of a nonconductive plastics. 
Much beyond that, in the robust outer lead area, is a second 
non-conductive lead support ring (35), and beyond that still is a 
peripheral band of leadframe blank material (36) temporarily holding the 
outer ends of the leads together in registration. Finally, just within the 
second support ring 35 each lead widens into a test contact portion (as 
37). 
The preparation of the leadframe of FIG. 3B is as follows. First, from one 
major side of a blank, there is removed, to half the depth of the blank, 
all the material between both the inner leads and the outer leads/legs, 
save for the outer ring 36. Next, the fixing-material inner ring (like 23 
in FIG. 3A) is placed in position. Then, all the rest of the blank 
material between the outer leads/legs, and all the remaining material 
within the recess periphery (as defined by line 32) is removed from the 
other side. The finished lead-frame may then be attached to the material 
which serves as the base of the flat pack. Finally, after possibly bending 
the leads into the shape required for use, the two support rings 34, 35 
are placed in position. 
At this stage, a chip may be mounted and connected up to the leadframe, the 
outer blank material ring 36 may be cut off, and the correct working of 
the chip tested by signals and power sent in and out via the test pads 37; 
a protective covering may be added; and finally the leads may be cut just 
outside the first support ring 34 and bent (if not already done) into the 
required shape for use. 
FIG. 3C shows, slightly enlarged, the lead layout of a real leadframe. It 
is comparable to FIG. 3B without the latter's two non-conductive support 
rings, but has 50 leads a side (making 200 leads in all). 
Finally, the sequence of FIGS. 4A, B and C shows various views of a simple 
Dual Inline Package using a leadframe fabricated in accordance with the 
invention. FIG. 4B gives a general, perspective view from above, while 
FIGS. 4A and B are respectively a see-through end elevation and a 
see-through top plan. From these two see-through views it will be apparent 
that the leadframe has 12 leads (41, 6 on each of the two opposed long 
sides) that each have a fragile inner portion (41i) and a robust outer 
portion (41o). The inner portions 41i are supported by a "slab" (42) of 
fixing material, and mounted on that, and connected by wires (as 43) to 
the inner portions 41i, is the chip (44). The whole is encased in a 
protective package (45), with the ends of the lead outer portions 41o 
projecting therefrom to form legs. 
The following Example is given, though again by way of illustration only, 
to show details of one preferred method of the invention. 
EXAMPLE: PREATION OF A LEADFRAME 
Stage 1 Preliminary Preparation of the Leadframe Blank 
In the normal way a 0.1 mm sheet of NILO K (an alloy of 54 wt % iron, 29 wt 
% nickel and 17 wt % cobalt) was cut to size, pierced with the requisite 
tooling/locating apertures, and then vapour degreased to remove its oily 
protective coating. The surface was then de-oxidised and slightly 
roughened (abraded) by passing it through a conventional pumice jet 
scrubber. 
Stage 2 First Masking and Etching 
Using a standard dry film resist laminator, a 0.025 mm layer of a 
UV-sensitive photo-resist (Dupont's RISTON, a polyvinyl-glycidyl acrylate) 
was applied to each side of the blank. The coated blank was then located 
in an imager (a Dupont PC Printer Lightbox), and exposed for a suitable 
intensity/time to UV light through an appropriate artwork master mask, 
causing partial polymeration of the unshielded resist layer areas. After 
removal of the unexposed (unpolymerised) resist, by passing the blank 
through a resist developer containing CHLOROTHENE (a trichloroethane-based 
solvent), the remaining resist was baked at 80.degree. C. (to complete the 
polymerisation). The thus-masked blank was then spray etched, using a 
conventional ferric chloride etchant at 30.degree. C. for sufficient time 
(a few minutes) to allow the etchant to penetrate about 0.05 mm--half the 
blank thickness--into the blank material. After washing, the remaining 
resist was then removed by passing the blank through a bath of SYNSTRYP 
solvent (based on dichloromethane). At this stage the blank had the 
complete lead pattern--both the larger outer leads and the very much finer 
inner leads--etched on one side to a depth halfway through the material. 
Stage 3 Glassing the Fine Inner Leads 
In order to enhance the adhesion of the subsequent glass material to the 
leadframe, a "furnace oxide" coating was then formed on the blank surface 
by heating the blank in nitrogen with some oxygen in a furnace to around 
900.degree. C. over a period of half an hour. Then, using a graphite frame 
to locate and contain it, a finely powdered lithium aluminosilicate glass 
was applied to the half-etched fine inner lead area of the blank and fixed 
into place by fusing in a furnace to about 900.degree. C. In this way the 
glass flowed around, and "encapsulated", the fine leads. 
Stage 4 Second Masking and Etching 
After deoxidising the blank (by a 5 minute dip in 50% hydrochloric acid at 
65.degree. C.), the techniques described in Stage 2 above were used to 
form a photoresist coating on the non-glassed surface, convert this into 
an appropriate etch-resistant mask (the glassed surface was additionally 
given an etch-resistant coating of Robertsons Chemicals' Resist 1993, an 
air-drying liquid resist), etch away the unwanted blank material (so 
defining the "bottom" half of the large outer leads, and removing all the 
material in the area "underneath" the fine inner leads), and remove the 
residue of the photoresist coating. The result was the desired leadframe, 
having large, self-supporting outer leads having a depth equal to that of 
the original blank and being integral with the glass-supported fine inner 
leads having half that depth.