Metal fusion bonding – With control means responsive to sensed condition – Work-responsive
Utility Patent
1999-02-12
2001-01-02
Ryan, Patrick (Department: 1725)
Metal fusion bonding
With control means responsive to sensed condition
Work-responsive
C228S020100, C228S037000
Utility Patent
active
06168065
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a method and apparatus for removing bridges and excess solder formed between or on soldered joints during an automated soldering process of electronic components so as to remove the bridges and excess solder without harming solder joints on the circuit board. The method and apparatus involve applying heated compressed air or an inert gas to selected portions of the printed circuit boards and flexible circuits after soldering.
BACKGROUND OF THE INVENTION
An electronic component is generally soldered onto printed circuit boards in one of two ways. Using a first approach, the component is mounted to a surface on one or both sides of the printed circuit board (known as “surface mount” technology). Alternatively, the component is mounted to the printed circuit board by mounting the body of the component on one side of the printed circuit board, inserting leads through holes in the board plated with a conductor (such as copper), and applying solder to the leads on the opposite side of the board (known as “through hole” technology). Through hole technology is well illustrated in
FIG. 1
which shows a cutaway view of a single lead
1
inserted through a through hole
2
covered with copper laminated pad
3
in printed circuit board
10
(made of fiberglass/phenolic) and solder
4
holding lead
1
. This patent application relates to both types of mounting in which the “bottom” side of the board will be “wave soldered”.
Automated wave soldering systems for wave soldering of through hole components and surface mount components include a series of apparatus or stations within the system, each of which performs one step in the soldering process. The soldering process begins with the application of flux and/or other fluid used in preparation for soldering onto a printed circuit board, followed by the preheating of the board and the application of solder to the leads (in “through hole” technology) or component (in “surface mount” technology) protruding from the bottom of the board. The board is transported in the wave soldering system along a conveyor.
A significant problem with this prior art automated process has been that in some instances a bridge of solder has formed between adjacent conductors causing a short circuit. In through hole technology (FIG.
1
), upon application of the solder to the bottom surface of the printed circuit board, strong adhesive forces bond the negatively-charged solder
4
which adheres by capillary forces between the wettable lead and positively-charged copper laminated pad
3
around through holes
2
and to the positively-charged copper leads
1
on the components passing through holes
2
. In surface mount technology, the same type of forces bond the solder to surface mount component terminations. The solder molecules bond to each other with less strong cohesive forces. Therefore, excess solder between joints typically falls away and does not create a bridge. However, where joints are very close together, where leads are improperly clinched, or where the components mounted to the board have long leads which extend from the surface of the printed circuit board more than approximately 0.030 inches, or 0.8 mm, the strength of the adhesive forces may cause a bridge to form. Several processing-related factors related to wave dynamics, conveyor speed, conveyor angle, and/or using insufficient or no flux prior to soldering or using nitrogen during soldering either of which may increase surface tension, may also result in the formation of bridges.
FIG. 2A
illustrates the bottom of a sample printed circuit board
10
before wave soldering. On this “mixed technology” board, to which this invention is by no means limited, some components are surface mounted and some components are mounted with through hole technology to form a printed wire assembly. Those components which are mounted with through holes to board
10
include D-Sub connectors
20
,
21
which are mounted through through holes in board
10
and pin headers
23
,
24
. Components which are surface mounted to the bottom of illustrated board
10
include SOIC's (small outline integrated circuits)
30
,
31
, SOT's (small outline transistors)
33
and chip resistors
34
-
37
. Also illustrated are large and small vias
25
,
26
, respectively, through which current is supplied to components on top of board
10
and mounting holes
22
. The top of board
10
is illustrated in FIG.
2
B.
FIG. 2C
illustrates board
10
of
FIG. 2A
after wave soldering (and without any debridging). As shown, bridges
40
,
41
,
43
in board
10
have formed between terminations on SOIC
31
, between chip resistors
34
-
36
, and between chip resistor
37
and SOT
33
, respectively.
FIG. 2D
shows a cross-sectional view of board
10
of
FIG. 2C
along line D-D′. In addition to bridges
40
,
41
,
43
, bridges
44
,
45
have formed between leads
50
,
51
on connector
21
and between leads
52
,
53
on pin header
24
during wave soldering.
The cohesive forces in the solder strengthen as the solder solidifies. Thus, a bridge can be removed without damaging the joint only if the solder has not solidified. The time of solidification is dependent on several variables including the solder alloy used, the solder temperature, board mass, solder mass and environment temperature. One time consuming method of removing a bridge is to inspect each board for bridges after it exits from the mass soldering system, manually removing the bridge of existing solder with a soldering iron and/or soldering wick and manually resoldering the joint.
Electrovert's Hot Air Knife
An alternative to manual debridging, currently in use in some commercial wave soldering systems, is to incorporate a hot air knife following the soldering station. This type of hot air knife is described in U.S. Pat. Nos. 4,401,253, 4,402,448, and Re. 32,982. The hot air knife shown in these patents (which is also implemented to some degree commercially by Electrovert Ltd., Grand Prairie, Tex.) comprises a manifold extending the entire width of the conveyor. The Electrovert hot air knife blows the entire width of the conveyor regardless of the width to which the conveyor is adjusted. Compressed air or inert gas is output from the orifice of the hot air knife at a high flow rate and is aimed at the bottom of the board following the application of the solder while the solder is still molten. While the adhesive and capillary forces retain the solder on the joints, the hot air knife breaks the cohesive forces and “relocates excess solder on, and/or blasts excess solder from the underside of the board, interconnections, leads, and bodies, and in doing so also minimizes the possibility of solder bridging or icicling or short formation upon solidification.” Re. 32,982, col. 5, lines 48-52.
Electrovert's AccuKnife has both a vertical adjustment in the height of the nozzle and an adjustment of the nozzle towards or away from the wave, both requiring movement of the entire manifold.
Electrovert currently manufactures commercial products known as AccuKnife, a hot air debridging tool, and CoN
2
tour
PLUS
, a similar device which performs the debridging function using inert hot gas. Re. 32,982 describes the stream as heated to a temperature in the range of 93° C.-350° C. It is applicants' understanding that in Electrovert's commercial products the stream of air or inert gas must be heated to a temperature above the temperature of molten solder (known as the eutectic point of solder, which for the current standard 63/37 tin/lead solder alloy is 183° C.).
Electrovert's present commercial hot air knives (including AccuKnife and CoN
2
tour
PLUS
), and the hot air knife shown in U.S. Pat. Nos. 4,401,253, 4,402,448, and Re. 32,982 present significant disadvantages because they extend over the entire width of the conveyor system carrying the printed circuit board and operate continuously at a high flow rate when the soldering system is on. These hot air knives, therefore, unnecessarily treat the en
Ryan Patrick
Soltec B.V.
Stoner Kiley
LandOfFree
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