Patent Application
20160021747
This disclosure generally relates to a circuit board assembly, and more particularly relates to features that allow for electrically connecting a thru-hole component to a circuit board assembly by laser welding.
Thru-hole technology is used when an electrical component is too large for surface mounting. Examples of large electrical components include a connector (
It has been observed for automotive applications that soldering of thru-hole connections may provide adequate performance for vibration and mechanical shock resistance. However, soldering may not provide adequate performance for thermal shock resistance. Furthermore, it has been observed that in some instances the removal of lead (Pb) from solder further reduces the thermal shock resistance of a solder joint. What is needed is a cost effective means to electrically connect a thru-hole type component to a printed circuit board.
In accordance with one embodiment, a circuit board assembly is provided. The assembly includes a printed circuit board and a component. The printed circuit board includes a plated thru-hole through the printed circuit board. The component includes a lead to interconnect the component to the printed circuit board. The lead is formed to define an opening through the lead. The lead is placed into the plated thru-hole such that at least part of the opening is within the plated thru-hole. The lead is laser welded to the plated thru-hole.
Further features and advantages will appear more clearly on a reading of the following detailed description of the preferred embodiment, which is given by way of non-limiting example only and with reference to the accompanying drawings.
The present invention will now be described, by way of example with reference to the accompanying drawings, in which:
It is proposed herein to employ laser energy to weld contact surfaces between pins or wires (i.e. the leads) of relatively large electrical components to a plated thru-hole of a printed circuit board (PCB). After a lead of the large electrical component is inserted into the thru-hole of the PCB with small interference, a laser beam is directed to the tip of the lead. That is, the laser energy will not directed to the plated thru-hole of the PCB as the heat of the laser beam energy may affect the near-by copper traces and electrical components upon the circuit board assembly. The melted copper from the lead of the large electrical component is welded to the copper barrel of the plated thru-hole of the PCB by means of heat conduction from the lead to the barrel portion of the printed thru-hole. The size of the lead is about the same size as the plated thru-hole. This is to ensure contact between the lead and the plated thru-hole. An opening feature within the lead is provided to minimize the heat conduction away from where the laser is directed. That is, the opening inhibits heat conduction toward to the other end of the lead during laser welding. In general, the mechanical strength of the interconnection by laser welding is governed by the material strength of the copper alloy.
Preferably, the contact between the lead and the plated thru-hole will be an interference fitted. By way of example and not limitation, it is preferable that any gap between the lead and the plated thru-hole is less than 0.02 millimeters (mm) as it has been observed that such a sized gap can be filled by the melted copper from the lead. The thickness of the barrel portion of the printed thru-hole should be adequately large to cater for the melt thickness of 0.02 mm. The advantages of laser welding as described herein are: the mechanical strength of the interconnect has been observed to be stronger; the reliability of the interconnect has been observed to be improved for vibration, mechanical shock, and thermal shock resistance; and the material and processing costs are be reduced as compared to soldering.
The assembly 10 also includes a component 16 such as, but not limited to, a connector as illustrated in
Accordingly, the component 16 includes a lead 18 to interconnect the component 16 to the PCB 12. As used herein, the term ‘lead’ covers lead-frames or pins which are terms commonly used to refer to the electrically conductive pieces in a connector, and includes wires or terminals which are terms commonly used to refer to the electrically conductive pieces of electronic components, and includes other terms commonly used to describe some feature or object used as part of a thru-hole type interconnection to a circuit board. The lead 18 is advantageously formed of copper or a copper alloy. The material selected for the lead 18 and the plated thru-hole 14 need to be materials that are readily welded together, as will become apparent in the description below.
Preferably, the laser weld is formed by laser energy 22 projected onto or directed toward a tip portion 40 of the lead 18. That is, the laser energy 22 is focused such that the laser energy 22 is not projected onto or directed toward an annular ring 24 of the plated thru-hole. As such, the laser weld is formed by direct heating of the lead 18, and indirect heating of the plated thru-hole 14, where the indirect heating comes from heat dissipated by the lead 18 to the plated thru-hole 14. The laser energy 22 may be focused by a lens 26 as will be recognized by those in the art. While the illustration suggests that the laser energy 22 is broadly distributed over the surface of the tip portion 40, it is contemplated that the laser energy 22 could be scanned across the surface of the tip portion 40 such that only a portion of the tip portion 40 is ‘illuminated’ by the laser energy 22 at any moment. Also, the laser energy may be non-uniformly distributed in order to achieve a uniform temperature along the interface between the lead 18 and the plated thru-hole 14. The non-uniform distribution of energy is preferred as the presence of the opening 20 may cause non-uniform temperature distribution.
The opening 20 is provided to reduce the cross section of the heat path proximate to the opening 20 and thereby reduce the amount of heat that travels down the lead 18 in the direction shown by arrow 28. If the lead 18 does not have the opening 20, then the amount of heat conducted away from the interface between the lead 18 and the plated thru-hole 14 is increased and so more energy is needed to form a weld joint between the lead 18 and the plated thru-hole 14. In other words, the opening 20 restricts the amount of heat conducted away from the tip portion 40 so that the weld joint between the lead 18 and the plated thru-hole 14 can be more quickly formed.
Preferably, the body portion 30 has a body diameter 46 selected to provide an interference fit with the plated thru-hole 14. The tighter the fit (i.e. the more interference there is), the better heat is transferred from the lead 18 to the barrel portion 38 of the plated thru-hole 14. However, it is recognized that too much interference can damage the plated thru-hole 14, possibly breaking the electrical connection with the annular ring 24 and/or other conductor layers within the PRB 12.
In this example above, the body diameter 46 is greater than a wire diameter 34 of the lead 18. This configuration is advantageous as it allows the opening 20 to be more readily formed and the smaller size of the unformed portion 32 conducts less heat away from the body portion 30 than would be the case if the body portion 30 and the unformed portion 32 had the same diameter.
The body portion 30 includes a tapered tip, i.e. the tip portion 40 may be tapered. Having a tapered tip is advantageous as the lead 18 is more readily aligned with the plated thru-hole 14 when being inserted, and the laser energy 22 is more readily focused on the surface of the tip portion 40 when compared to a configuration where the tip portion 40 was not tapered so the taper angle 42 was zero.
In this example, the body width 56 is greater than a body thickness which is the same as the lead thickness 52. The body width 56 and the body thickness are cooperatively selected to provide an interference fit with the plated thru-hole 14. In this example the body width 56 is greater than the lead width 54 of the lead 18. Preferably the lead thickness 52 is smaller than the body width 56 so the gap between the body portion 30 across the lead thickness 52 is small enough so the welding process can fill the gap.
Like the circular lead 18A previously described, the body portion 30 of the rectangular lead 18B also advantageously includes a tip portion 40 characterized as a tapered tip which provides the same advantages as described above with regard to the tip portion 40 of the circular lead 18A.
Accordingly, a circuit board assembly (the assembly 10) is provided that is well adapted for laser welding of the lead 18 into the plated thru-hole 14. This ability helps to reduce the manufacturing costs of circuit board assemblies that are mostly built with surface mount type components, but need to also connect thru-hole type components to a printed circuit board after the surface mount type components have been attached using, for example, reflow soldering. Furthermore, the welded joining of the lead 18 to the plated thru-hole 14 is more reliable during thermal shock testing.
While this invention has been described in terms of the preferred embodiments thereof, it is not intended to be so limited, but rather only to the extent set forth in the claims that follow.
