Metal fusion bonding – Process – Plural joints
Utility Patent
1998-10-14
2001-01-02
Ryan, Patrick (Department: 1725)
Metal fusion bonding
Process
Plural joints
C219S121630, C219S121640
Utility Patent
active
06168070
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to soldering electronic components to circuit boards, and more particularly to a method for soldering DPAK-type electronic components to circuit boards.
2. Disclosure Information
DPAKs are relatively high-power electronic switching devices (e.g., transistors) that are commonly used on circuit boards. As illustrated in FIGS.
1
A-C, a typical DPAK
10
includes an integrated circuit die
12
attached atop a heatspreader
14
, with one or more selected sites on the top surface of the die wirebonded
16
to a like number of leads
18
, and with a plastic or ceramic molded housing
20
encasing the die
12
, the wirebond wires
16
, a portion of the lead(s)
18
, and a portion of the heatspreader
14
.
A typical DPAK has two leads
18
, but other DPAK-type components
10
may have one lead (e.g., a diode), three leads, and so forth. Typically the heatspreader
14
itself serves as an electrical interconnect, such as in transistor DPAKs where the heatspreader
14
is electrically connected to the base of the transistor die
12
. (Usually the bottom surface of a transistor die serves as the base interconnect point, with the emitter and collector interconnect points being situated on the top surface of the die.)
As illustrated in
FIG. 1B
, DPAK-type components
10
are electrically and mechanically connected to circuit boards
30
by applying solder paste
36
to the heatspreader/lead mounting pads
32
/
34
, positioning the component
10
with its heatspreader
14
and leads
18
resting atop their respective solder-pasted pads
32
/
34
, and sending the component/board combination through a conventional reflow oven. For most types of electronic components, reflow soldering is an efficient and effective way of forming robust solder joints connecting a component's leads/terminations/etc. to their corresponding mounting pads.
However, conventional reflow soldering is not as effective at forming robust solder joints on DPAK-type components
10
. Because the heatspreader
14
of a DPAK-type component is so large, it is often difficult to provide enough thermal energy to such components during reflow to melt all of the solder paste
36
sandwiched between the heatspreader
14
and its mounting pad
32
. One approach for alleviating this problem is to simply increase the reflow oven peak temperature and/or increase the circuit board's exposure time to the peak temperature, but this approach has the drawback of risking damage to other components and/or to the substrate. Another approach is to use a laser rather than a reflow oven to solder the components. In laser soldering, a laser beam is directed at the component leads, terminations, exposed heatspreader portion, and/or the heatspreader mounting pad
32
, so as to melt the solder paste
36
on the pad
32
. However, as with reflow soldering, the heatspreader
14
of a DPAK-type component
10
is often too large to permit robust solder joint formation, even if a high laser power setting and long laser time are used.
In both of the foregoing approaches, it is often the case that the solder paste
36
immediately adjacent the exposed portion
14
e
of the heatspreader is the only portion thereof that gets melted. This results in a solder joint that forms only proximate the exposed portion
14
e
of the heatspreader, with no joint being formed underneath the component.
It would be desirable, therefore, to provide a way of soldering DPAK-type components to circuit boards without the aforementioned drawbacks.
SUMMARY OF THE INVENTION
The present invention overcomes the disadvantages of the prior art approaches by providing a method for soldering an electronic component having a heatspreader on a bottom surface thereof and at least one lead (e.g., a DPAK-type component) to a circuit board having a heatspreader mounting pad and at least one lead mounting pad, comprising the steps of: (a) depositing solder paste on the heatspreader mounting pad and on each of the at least one lead mounting pad; (b) placing the electronic component on the circuit board such that the heatspreader rests atop the heatspreader mounting pad and each lead rests atop a respective one of the at least one lead mounting pad; (c) directing a laser energy beam from a diode laser at the heatspreader and/or heatspreader pad for a first predetermined length of time, thereby heating the solder paste on the heatspreader mounting pad; and (d) continuing to direct the laser energy beam at the heatspreader and/or heatspreader pad for a second predetermined length of time while simultaneously feeding a predetermined amount of flux-core wire solder into the laser energy beam proximate the heatspreader, such that the wire solder melts and flows onto at least one of the heatspreader and the heatspreader mounting pad.
It is an object and advantage that the method of the present invention is effective at forming robust solder joints on DPAK-type electronic components.
Another advantage is that the method of the present invention may be used with low-melting-point substrate materials that cannot be exposed to the temperature profiles of conventional reflow soldering.
Yet another advantage is that the present invention may be easily utilized in conjunction with conventional laser soldering.
These and other advantages, features and objects of the invention will become apparent from the drawings, detailed description and claims which follow.
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Hodges Leslie C.
Pittman Zidia T.
Ryan Patrick
Visteon Global Technologies Inc.
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