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Description  |
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TECHNICAL FIELD
This invention relates to a system and method for mounting integrated
circuits onto printed circuit boards. This invention also relates to a
method for testing integrated circuits and printed circuit boards.
BACKGROUND OF THE INVENTION
Electronic circuit boards for computers and other devices typically consist
of many integrated circuits mounted on a printed circuit board (PCB).
Individual integrated circuit (IC) packages have a semiconductor chip or
die enclosed within a protective plastic package. A lead frame consisting
of multiple conductive leads provide the electrical interconnect between
the enclosed semiconductor chip and components exterior to the plastic
package.
The PCB has many conductive traces arranged on its surface according to a
selected pattern for efficiently transferring electronic signals across
the board to and from individual IC packages. The traces on the PCB have
terminal locations or bonding sites which define the locations that
specific IC packages are to be located. As the IC package is being mounted
to the PCB, the conductive leads of the IC package are aligned with
corresponding bonding sites on the PCB. The conductive leads are then
soldered to the corresponding bonding sites to permanently mount the IC
package to the PCB.
One significant problem in the mounting process is how to initially
position the IC packages on the PCB and hold them there temporarily until
the soldering step is performed. According to conventional techniques, a
screen with multiple holes is positionally and carefully aligned with the
PCB so that the screen holes are superimposed on the bonding sites on the
PCB. Solder paste is then squirted through the holes of the screen onto
the bonding sites of the PCB. Solder paste is a conductive material which
forms a temporary glue.
After the solder paste has been applied to the bonding sites, the IC
packages are positioned atop the PCB. The solder paste temporarily adheres
the conductive leads of the IC package to the corresponding bonding sites.
Solder is then applied to the bonding sites, and the PCB is subjected to
ultraviolet light or other techniques for causing the solder to melt and
completely surround and secure the conductive leads of the IC package to
the bonding sites.
The use of solder paste for temporarily mounting IC packages onto PCBs
creates numerous problems. First, the process of positioning and aligning
the screen over the PCB and squirting the solder paste through the screen
holes is a difficult, time-consuming task. Second, the solder paste is an
extremely messy substance and can cause problems if not precisely applied.
Third, after the permanent solder is applied, the PCBs must be cleaned
with special cleaning materials, such as freon-type cleaners, to remove
all of the solder paste. Accordingly, the present mounting technique
introduces several additional manufacturing steps that reduce overall
manufacturing efficiency while increasing costs.
The present invention eliminates the above-mentioned drawbacks by providing
a system and method for mounting integrated circuits onto printed circuit
boards without the use of solder paste.
Another drawback in conventional manufacturing techniques is that, when an
IC package fails during the testing, it is often too difficult to break
the solder bonds and remove the defective IC package without damaging the
PCB or other components. Thus, the entire manufactured board is discarded,
even though most of the board and IC packages are operative. This causes
significant waste because otherwise operable components are needlessly
thrown away.
The present invention provides a method for testing integrated circuits and
printed circuit boards prior to the soldering step. This testing method
thereby permits removal of defective components without discarding the
operable components.
BRIEF DESCRIPTION OF THE DRAWINGS
Preferred embodiments of the invention are described below with reference
to the following accompanying drawings.
FIG. 1 is a top plan view of a portion of a PCB having an IC package
mounted thereon. FIG. 1 also shows a partial cut away view illustrating a
semiconductor chip within the IC package.
FIG. 2 is an enlarged top plan view of the semiconductor chip shown in FIG.
1.
FIG. 3 shows a cross-sectional view taken along line 3--3 in FIG. 1.
FIG. 4 is a diagrammatic illustration of a system and method for mounting
integrated circuits onto printed circuit boards according to this
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
This disclosure of the invention is submitted in furtherance of the
constitutional purposes of the U.S. Patent Laws "to promote the progress
of science and useful arts" (Article 1, Section 8).
According to one aspect, this invention encompasses a method for mounting
an integrated circuit (IC) device onto a printed circuit board (PCB), the
PCB having a plurality of bonding sites provided thereon, the IC device
having a plurality of conductive bonding portions for electrically
contacting corresponding bonding sites when the IC device is mounted to
the PCB, the method comprising the following steps:
providing a PCB having an upper surface;
providing magnetic material within the IC device;
inducing a magnetic field of a selected strength at the surface of the PCB
to hold the IC device onto the PCB; and
soldering the conductive bonding portions to the corresponding bonding
sites of the PCB while the IC device is held onto the PCB by the magnetic
field.
According to another aspect, this invention defines a system for mounting
an integrated circuit (IC) device onto a printed circuit board (PCB), the
system comprising:
a PCB having a plurality of bonding sites and an upper surface;
an IC device having a plurality of conductive portions for electrically
contacting corresponding bonding sites when the IC device is mounted to
the PCB, a portion of the IC device being formed of a magnetic material;
and
field creating means for controllably inducing a magnetic field at the
upper surface of the PCB, the magnetic field having a selected strength
sufficient to hold the IC device onto the PCB.
FIG. 1 illustrates a portion of a printed circuit board (PCB) 10 having an
integrated circuit (IC) device 20 mounted thereon. PCB 10 has multiple
conductive traces 12 formed in a selected pattern to bus electronic
signals across the board to and from IC device 20. Conductive traces 12
terminate on the surface of PCB 10 at bonding sites 13.
PCB 10 consists of multiple layers 14a-14c of insulative material such as
paper bonded with phenolic resin or glass fibers bonded with epoxy resin
(FIG. 3). The circuitry pattern of conductive traces can be provided on
the upper surface of the PCB 10 (as shown in FIG. 1) or on intermediate
layers as illustrated by traces 16 shown in FIG. 3. To access the
circuitry provided on intermediate layers, "through-holes" are formed in
the layers, as represented by through-hole 18. Conductive material fills
through-hole 18 to join conductive traces 12 on the upper surface of PCB
10 to conductive traces 16 provided on intermediate layer 14b of PCB 10.
Traces 12 and 16 are formed of a conductive material, such as copper.
According to one aspect of this invention, PCB 10 includes a layer 19 of
magnetic material interspersed between two layers of the laminated PCB.
Preferably, the magnetic material is a metal that is capable of being
magnetized. For instance, layer 19 is preferably formed of iron, nickel,
cobalt, mixtures thereof, or other materials composed at least partially
of these metals. Alternatively, as a less preferred approach, the magnetic
material has permanent magnet characteristics. This approach is less
preferred because employing a permanent magnetic material may adversely
affect normal operation of the PCB and IC devices thereon. Layer 19 is
discussed below in more detail.
IC device 20 is illustrated as an IC package consisting of a semiconductor
integrated circuit chip 22, a plastic casing 24 which encapsulates and
protects IC chip 22, and a lead frame 28 which provides an electrical
interconnection between IC chip 22 and traces 12 of PCB 10. Lead frame 28
has multiple conductive leads 30 which protrude from plastic casing 24 to
electrically contact bonding sites 13 on PCB 10. As shown in FIG. 3, the
conductive leads are "J"-shaped which is common for surface mounted IC
packages.
IC device 20 is shown as an IC package for purposes of explanation.
However, IC device 20 encompasses various other embodiments having IC
chips and interconnecting bonding portions that contact the PCB bonding
sites. For example, IC device 20 may consist of an unpackaged
semiconductor integrated circuit chip having multiple conductive bonding
pads (instead of leads) which mount directly to bonding sites on PCB 10.
Additionally, the conductive leads need not be "J"-shaped, but can be
straight (such as a dual in-line IC package) or have other geometries.
According to an aspect of this invention, the semiconductor IC chip 22 has
a layer of magnetic material. As above, the magnetic material is
preferably a metal that is capable of being magnetized (such as iron,
nickel, or cobalt), or less preferably, a metal having permanent magnetic
characteristics. As shown in the enlarged view of FIG. 2, a layer of
magnetic material 32 is provided on top of IC chip 22. The layer of
magnetic material is electrically isolated within the IC chip 22 by
insulating materials that are within the purview of one of ordinary skill
in the art. Here, layer 32 of magnetic material is spatially separated
from pads 34 which are used to interconnect with lead frame 28. Although
layer 32 is preferably provided on top of IC chip 22, layer 32 may be
interwoven within the semiconductor structure.
According to another aspect of this invention, lead frame 28 is formed of a
magnetic material.
According to this invention, IC devices are temporarily mounted onto PCBs
using a magnetic coupling approach. With reference to FIG. 3, a magnetic
field F of a selected strength is controllably induced or created by a
field creating means at the upper surface of PCB 10. This magnetic field F
attracts the magnetic material provided in IC device 20 (such as in lead
frame 28 or in layer 32 of IC chip 22). The magnetic field has sufficient
strength to hold IC device 20 onto PCB 10 at least temporarily until
conductive leads 30 can be soldered to corresponding bonding sites 13 of
the PCB. As shown in FIG. 1, IC device 20 is preferably held by the
magnetic field in such a manner that conductive leads 30 are aligned with
corresponding bonding sites 13. The field strength used to hold the ID
devices onto the PCBs varies depending upon the distance between the field
creating means and the ID devices and any material therebetween. More
specifically, the magnetic field strength is inversely proportional to the
distance between the ID devices and the PCBs.
According to one preferred embodiment of the field creating means, layer 19
of magnetic material is magnetized to create magnetic field F. The
magnetic field flows into the metal provided in IC device 20 and turns the
metal into a temporary magnet, whereby the magnetic layer 19 attracts the
magnetic metal in IC device 20. According to an alternative embodiment of
the field creating means, magnetic field F can be created by a magnetic
source external to both PCB 10 and IC device 20. For example, an
electromagnet could be provided beneath PCB 10 to induce a magnetic field
of sufficient strength to hold IC device 20 onto PCB 10. This alternative
embodiment is particularly useful if PCB 10 does not have a layer 19 of
magnetic material which can be magnetized to attract IC device 20.
FIG. 4 diagrammatically illustrates a system for mounting an IC device onto
a PCB. The system includes a mounting station 40, a testing station 50,
and a soldering station 60. PCBs 41, 51, and 61 are shown at respective
stations. The PCBs are moved in sequence through mounting station 40,
testing station 50, and soldering station 60 by a conveyor mechanism,
manually, or by some other apparatus. PCBs 41, 51, and 61 have the same
structure as discussed above with respect to PCB 10 in that all three
contain a layer of magnetic material. The layers of magnetic material
within PCBs 41, 51, and 61 are selectively magnetized and demagnetized by
magnet control system 70. Magnet control system 70 is electrically or
magnetically coupled to the individual PCB layers of magnetic material, as
is illustrated by intercoupling lines 72-74. According to this
arrangement, magnetic control system 70 can independently magnetize or
demagnetize any one of the PCBs without similarly magnetizing or
demagnetizing the remaining PCBs.
At mounting station 40, individual IC devices 20 are transferred or moved
from a supply location 42 to PCB 41 using a transfer mechanism 44.
Transfer mechanism 44 preferably comprises a robotic arm 45 and a
controller 46. Robotic arm 45 has a handling instrument 48 provided at the
distal end thereof for contacting and handling individual IC devices 20.
According to this invention, the handling instrument has an electromagnet
provided therein with a relatively flat surface for contacting and mating
with the relatively flat upper surfaces of IC devices 20. An electromagnet
consists of a coil of wire wound around a magnetizable core, such as an
iron core. When current flows through the coil, it creates a magnetic
field having a strength dependent upon the amount of current fed through
the coil. When current is stopped, the magnetic field ceases. Controller
46 is coupled to selectively activate and deactivate the electromagnet
provided in handling instrument 48.
In operation, robotic arm 45 positions handling instrument 48 above an IC
device 20 at supply location 42. Control system 46 activates the
electromagnet in handling instrument 48 to induce a magnetic field which
attracts and holds IC device 20 against the surface of handling instrument
48. Robotic arm 45 then moves IC device 20 to PCB 41 and aligns the
conductive leads of IC device 20 with the corresponding bonding sites of
PCB 41. Robotic arm 45 then places the conductive leads in contact with
the corresponding bonding sites. Next, controller 46 deactivates the
electromagnet in handling instrument 48 to eliminate the magnetic field
and thereby release IC device 20 from handling instrument 48.
Magnet control system 70 magnetizes the layer of magnetic material within
PCB 41 to induce a magnetic field at the upper surface of PCB 41. The
magnetic field has a selected strength effective to hold IC devices 20
onto PCB 41. PCB 42 is then transferred to testing station 50, as
represented by PCB 51, while the magnetic field is maintained.
At testing station 50, PCB 51 and IC devices 20 are tested to determine
whether the completed board has any defects. Accordingly, another aspect
of this invention relates to a method for testing an integrated circuit
(IC) device and a printed circuit board (PCB), the PCB having a plurality
of bonding sites provided thereon, the IC device having a plurality of
conductive bonding portions for electrically contacting corresponding
bonding sites when the IC device is mounted to the PCB, the method
comprising the following steps:
inducing a magnetic field at the bonding sites of the PCB;
magnetically holding the conductive bonding portions of the IC device in
electrical contact with the corresponding bonding sites of the PCB using
the magnetic field; and
testing the PCB and IC device while the IC device is being held onto the
PCB by the magnetic field.
Testing station 50 has a testing unit 52 which evaluates the PCB and IC
devices while the IC devices 20 are held onto PCB 51 by a magnetic field.
The magnetic field is maintained at the surface of PCB 51 by magnet
control system 70 from the time it was initiated at mounting station 40,
during transfer from mounting station 40 to testing station 50, and during
the evaluation period at testing station 50. Test unit 52 includes a probe
54 having multiple prongs 58 for contacting the edge connector of PCB 51
and imparting test signals to PCB 51 and IC devices 20, and a test
controller 56 for controlling the testing patterns sent to PCB 51 and
evaluating the data returned therefrom.
If the board tests positively, PCB 51 is transferred to soldering station
60, as represented by PCB 651, while the magnetic field is maintained. On
the other hand, if the board fails any testing protocol, the inoperative
components are identified and removed from the board.
This method of testing is advantageous over prior art techniques in that
the IC devices 20 are temporarily held onto PCB 51 during testing by a
magnetic coupling. The IC devices 20 have not yet been permanently
soldered to the PCB. Accordingly, if a defect is discovered in one of the
IC devices or in the PCB, the defective component can be removed and the
remaining components recaptured for later use on other boards. This
effectively eliminates the needless waste of discarding otherwise operable
components that commonly occurs under conventional manufacturing and
testing procedures.
According to another aspect, the invention also encompasses a method for
testing a die wherein the die has circuitry provided on a semiconductor
substrate. The method comprises the following steps: (1) magnetically
holding the die in a selected orientation; (2) applying a testing device
(such as a probe or test prongs) to the die; and (3) evaluating the die to
determine whether the die is defective. The evaluation step might consist
of transferring test data to and from the die through the testing device
to ascertain whether the die is properly processing or storing the data.
At soldering station 60, a soldering unit 62 solders the conductive leads
of IC devices 20 to corresponding bonding sites on PCB 61 while IC device
20 is held onto PCB 61 by a magnetic field. Soldering unit 62 comprises a
solder applicator 64 and a solder controller 66. Magnetic control system
70 maintains the magnetic field at the surface of PCB 61 during its
transfer from testing station 50 and at least the initial soldering
process until the IC device is adequately secured to PCB 61. Thereafter,
magnetic control system 70 ceases to create a magnetic field at the
surface of PCB 61.
The above-preferred system includes a field creating means that induces a
magnetic field at the surface of the PCBs by magnetizing layers of
magnetic material provided within the PCBs. Alternatively, for PCBs that
are constructed without these layers, the field creating means could
consist of electromagnets are provided beneath the PCBs to selectively
create a magnetic field, or cease to create a magnetic field, at the
surface of the PCBs. In this alternative embodiment, magnet controller 70
would interface with such electromagnets to provide selective control of
the magnetic fields. The electromagnets would be assigned to individual
PCBs and carried with them through the various stations to insure that a
magnetic field is maintained at mounting station 40, testing station 50,
and soldering station 60.
The above process is provided in its preferred sequence of operation.
However, certain steps may be conducted in a different order than
described. For example, at mounting station 40, the magnetic field created
by magnet control system 70 may be generated prior to deactivating the
magnetic field induced by the electromagnet in handling instrument 48.
This invention is advantageous over prior art mounting techniques in that
it eliminates the use of solder paste. Accordingly, the inefficient steps
of positioning and aligning a screen and then applying soldering paste to
the many bonding sites on the PCB are eliminated. Additionally, the mess
associated with the application and cleaning of solder paste is also
eliminated.
In compliance with the statute, the invention has been described in
language more or less specific as to structural and methodical features.
It is to be understood, however, that the invention is not limited to the
specific features described and shown, since the means herein disclosed
comprise preferred forms of putting the invention into effect. The
invention is, therefore, claimed in any of its forms or modifications
within the proper scope of the appended claims appropriately interpreted
in accordance with the doctrine of equivalents.
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