Metal fusion bonding – Process – Plural joints
Reexamination Certificate
2000-09-18
2002-05-14
Dunn, Tom (Department: 1725)
Metal fusion bonding
Process
Plural joints
C228S180210
Reexamination Certificate
active
06386429
ABSTRACT:
FIELD OF INVENTION
The invention generally relates to the art of printed circuit board (PCB) manufacture and more particularly to the manufacture of a PCB having a stacked header component.
BACKGROUND OF INVENTION
Many techniques are employed in the manufacture of PCBs in order to increase component density and reduce the area and/or size of the PCB. The exploded view of
FIGS. 1A and 1B
show the results of one such technique in which an integrated circuit (IC) semiconductor chip
20
is directly mounted onto a PCB
22
using a “direct chip attach” process. In this process, a solder mask is placed on an etched or coined PCB and solder paste is screened onto the copper lands of the PCB. The PCB
22
is then stuffed with components, including IC chip
20
. The IC chip
20
is not housed in a chip carrier or any other kind of package and thus is able to occupy a minimal footprint on one side the PCB
22
as compared to a fully packaged IC, e.g., one encased in a common dual in-line package (DIP). After stuffing, the PCB is heated by a heat radiation means to reflow the solder paste and electrically and mechanically connect the components to the PCB. The PCB is then washed to remove excess solder flux. Thereafter components such as the IC chip
20
are wire-bonded directly to the PCB. After wire-bonding and a potential testing phase, an encapsulant
24
is applied to the IC chip
20
and possibly other components in order to hermetically seal these components from the external environment.
In the illustrated embodiment, the PCB
22
includes a copper backplane
25
which provides a low profile means for dissipating heat. In circumstances where the IC chip
20
produces significant operating heat it is directly attached to the backplane
25
for efficient heat transfer. The encapsulant
24
protects the chip and wirebonds from the surrounding environment.
The component density of the PCB
22
is also increased by stacking a header
26
onto the PCB
22
after the encapsulant
24
is applied. The header
26
may carry on its underside
28
relatively large electronic components such as inductors
30
thereby eliminating the need to reserve a relatively large footprint on the PCB
22
for these bulky components. The header
26
includes a number of friction-fitted pins
32
. A portion
32
B of the pins extend from the underside
28
of header
26
for mounting it onto respective header pin lands
34
located on the PCB
22
. Some of the pins
32
and correspondingly some of the lands
34
serve to electrically interconnect the inductors
30
to the circuitry of the PCB. A portion
32
A of the pins
32
extend from a top-side of the header and may be used to mount the PCB/header assembly
22
and
26
to a host card or mothercard (not shown) in a larger system. In this case, some of the pins
32
and correspondingly some of the lands
34
may be electrically active and function as input/output interconnections between the PCB
22
and the host card. This feature also eliminates the need to dedicate a significant footprint of the PCB
22
for card edge connectors.
The header
26
is relatively large and may be sized as large as the PCB
22
itself as shown in
FIGS. 1A and 1B
, or may be somewhat smaller. Smaller headers may also be employed. As such, the header
26
must typically be mounted to the PCB
22
after the encapsulant
24
is applied. This creates certain thermal constraints in soldering the header pins
32
to the corresponding lands
34
. The principal constraint is that solder located under the directly attached IC chip
20
should not be allowed to reflow once the encapsulant
24
is applied. This is because the chemical composition and temperature profile of solder paste changes after the first reflow. The solder underneath the chip
20
may contain a number of small voids which, when subsequently reflowed, may coalesce to produce a large void. A direct mount chip with a large solder void underneath it is unable to efficiently dissipate heat to the copper backplane
25
and thus will have a very short field life.
In the past, the pins
32
were hand-soldered to the PCB
22
. This was a labour intensive and economically undesirable method of manufacture. The problem was exacerbated due to the thermally conductive copper backplane
25
which acted as an effective heat sink making it difficult to manually solder each pin.
Alternatively, a heat radiation and flux dispenser apparatus was employed to reflow solder (previously applied) on lands
34
in order to create a joint with the header pins
32
. This apparatus was often unable to create successful joints. In cases where the lands
34
were very close to the site of the IC chip
20
, e.g., less than 0.25 inches, the solder on lands
34
did not receive enough heat to reflow due to the aforementioned thermal constraint. If the heat radiation time was increased to reflow the solder on lands
34
, solder would also reflow under the IC chip
20
, creating unwanted voids and defective PCBs
22
. The problem is exacerbated due to the rapid heat conduction properties of the thermal backplane
25
to which the IC chip
20
is directly attached.
Furthermore, in an effort to keep within the limits of the aforementioned thermal constraint, the apparatus was used to reflow only one side of the PCB
22
at a time in order to keep the temperature of the solder underneath the direct mount IC chip
20
below the solder reflow point. This uneven heating of the sides of the PCB caused header
26
to tilt and reduced the number of successfully soldered pins on the opposite side of the PCB in the following manner: One side of the PCB was heated first. Assuming that the voiding described above did not occur, the solder was reflowed on the first side and the header pins travelled downward due to gravity to touch the underlying copper-plated surface or land of the PCB on that side. However, the solder on lands on the second side of the PCB
22
, being ball-like in shape, were still solid and high, causing the header
26
to tilt somewhat, with the first side down relative to the second side. The apparatus then advanced to reflow the solder on the second side of the PCB. However, the header pins
32
were high and would not travel down to meet the copper land of the PCB, since the header
26
is constructed from a solid plastic mould and the pins
32
are friction inserted into the plastic. This caused a great failure rate in the joints on the second side of the PCB.
SUMMARY OF INVENTION
Broadly speaking, the invention overcomes various problems of the prior art by employing a heat conduction, as opposed to heat radiation, approach to creating the header-PCB solder joint.
One aspect of the invention relates to a method for mounting a component having one or more pins onto a printed circuit board (PCB) having one or more respective lands for receiving the component pins. The method includes: (a) applying solder and flux, preferably in paste form, onto the lands; (b) bringing the pins in contact with the lands; (c) preheating the PCB to at least a flux-activation temperature; and (d) applying additional heat only to the pins in order for the pins to conduct sufficient heat to reflow the solder on the PCB lands.
The method may be advantageously applied to PCBs having pre-existing solder joints, such as an un-packaged IC chip directly mounted onto a copper backplane. In this case the PCB is heated in step (c) to a temperature approaching but not reaching the reflow temperature of the solder in the pre-existing joints, and in step (d) heat is applied so that the pins conduct only enough heat to locally reflow the solder on the lands without reflowing the solder in the pre-existing solder joints.
In the preferred embodiment the component is a header and its pins are exposed on top and bottom sides of the header. The top portions of the pins provide contact points for a heating element and the bottom portions of the pins provide a part for assembly onto the PCB.
The apparatus according to the preferred embodiment includes a nest for locating the he
Lekx David
Sani-Bakhtiari Paymon
Blake Cassels & Graydon LLP
Calestica International Inc.
Pittman Zidia
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