TECHNICAL FIELD

The present invention relates to an assembly for carrying electronic components on a printed circuit board.

BACKGROUND

In electronic design most electronic components emit heat when used. Certain components (in particular components in power applications) need additional cooling in order to be operational and to prevent damages to the component.

Cooling can be done in different ways. A common solution is to mount a heat sink to the component. The component is often mounted on a printed circuit board (PCB) by soldering the pins of the component to a conductive layer in the board. When the PCB is subject to vibration or chock (which is not uncommon in industrial applications) this can cause pin stress, especially if the component is relatively large

(such as a power module) and/or when a heat sink is attached to the component .

A known solution for reducing pin stress for components is therefore to add an extra support structure often fixed to the PCB that supports the component (and the heat sink) . A problem with an assembly comprising the support structure, the component and the PCB is that it can be too rigid. Thermal variations between the parts in the assembly can cause additional pin stress.

One known solution to overcome this problem is to use a thermal pad that is designed to handle mechanical and thermal deviations between the component and heat sink. A disadvantage when using a thermal pad is that the pad itself adds thermal resistance. This means that the efficiency of cooling the component is reduced compared to mounting the heat sink directly to the component. The thermal pad is also relatively expensive and adds significant cost to the design.

Another solution to reduce pin stress to a component is disclosed in US patent 6,310,773. In this patent a compressible support structure is disclosed comprising a deformable tube that is supported by a base structure. A disadvantage with this solution is that the tube adds extra cost to the design and that it requires an extra step in the process of mounting the support structure to the PCB.

Yet another solution is found in the paper 'Mechanic Interaction of Module, PCB, and Heat Sink in Power Applications' by RaIf Ehler and Michael Frisch, Tyco Electronics/Power systems Oct 04. This paper discloses an assembly where the pin stress is reduced by using specific pins provided with stress relief sections that allow for plastic deformation. A disadvantage with using this type of pins is that there is a risk for fatigue cracking of the stress relief sections due to deformation during vibration and/or when heat is transported through the pin. Another disadvantage is that the stress relief sections reduce the space between the pins. As the pins often are tin plated this can increase the risk for whisker growth which in turn can cause shortcuts. Yet another disadvantage is that this solution requires specific pins that are more expensive.

SUMMARY

The present invention relates to the problem of how to avoid the disadvantages mentioned above.

The problem is in the current invention solved by an assembly for carrying the component on the printed circuit board, comprising a support structure and specific electrical contact elements at the printed circuit board adapted to be electrically connected to the pins on the electrical component. The contact element comprises the conductive layer within a portion of the printed circuit board where said portion is resiliently movable relatively the remaining portion of the printed circuit board.

In one embodiment of the invention the resilient portions of the printed circuit board are produced by milling tracks in the board.

Optionally, a heat sink can be coupled to the support structure and brought in thermal contact with the electronic component so that the cooling of the component is increased.

The invention also comprises a method of mounting the electronic component to the printed circuit board.

An advantage with the invention is that the pin stress is significantly reduced as the resilient portions of the printed circuit board compensates for mechanical and thermal variations within the assembly. No thermal pad is needed if a heat sink is coupled to the electronic component and no specific pins with stress relief sections are needed.

Another advantage is that the tolerances between the electronic component and the printed circuit board can be greater when mounting the electronic component to the printed circuit board. The possible pin stress already created when mounting the component is significantly reduced.

Yet another advantage is that the resilient portions can be given different layouts in order to adapt to different pin configurations, to different characteristics of the PCB and to different types of mechanical and thermal variations.

The objective with the current invention is therefore to provide an assembly that reduces pin stress for electronic components on a PCB. The invention will now be described in more detail and with preferred embodiments and referring to accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Figures Ia and Ib are block diagrams illustrating assemblies comprising a heat sink and an electronic component mounted on a PCB.

Figure 2 is a block diagram illustrating an embodiment of an assembly according to the current invention.

Figure 3 is a block diagram illustrating a PCB with an embodiment of electrical contact elements according to the current invention.

Figures 4a, 4b and 5 are block diagrams illustrating further embodiments of an assembly according to the current invention.

Figure 6 is a block diagram illustrating a PCB with further embodiments of electrical contact elements according to the current invention.

Figure 7 is a block diagram illustrating an electronic component that is a power module comprising a set of other electronic components.

Figure 8 is a flow chart illustrating the method of mounting components to a PCB accoroing to the current invention.

DETAILED DESCRIPTION

A known assembly 150 for reducing pin stress for an electronic component 100 is illustrated in Figure Ia. This assembly 150 comprises a support structure 110 mounted on a printed circuit board PCB 120. The support structure 110 comprises a heat sink 145 and fastening elements 115. The pins 101,102 of the component 100 are soldered to an electrical conductive layer 121 in the PCB 120. Between the heat sink 145 and the component 100 a thermal conductive pad 140 and a base plate 130 is mounted. The thermal pad 140 has typically a thickness of approximately 2 mm and is designed handle deviations of approximately ±1 mm. The purpose of the thermal pad 140 is to reduce stress on the pins 101,102 when the PCB 120 and the support structure 110 is subject to mechanical and thermal variations. The base plate 130 is an optional element and is used to provide a well-defined and flat surface for heat dissipation when the heat sink 145 is mounted on the component 100. If the component 100 is a power module comprising a number of other small components, this base plate 130 can have different profiles in order to be in thermal contact with each individual component on the module.

Figure Ib illustrates another assembly 155. The support structure 110 with the heat sink 145 and the fastening elements 115 could be the same as in Figure Ia. The difference between the assembly 150 and 155 is that the thermal conductive pad 140 has been left out and that a component 105 is using pins 106, 107 having stress relief sections that allows for plastic deformation.

Figure 2 illustrates an embodiment of an assembly 200 for reducing pin stress according to the current invention. The assembly 200 comprises a support structure 210 with fastening elements 215 adapted to carry at least one electronic component 100 on a PCB 220. Optionally a base plate 130 is mounted between the component 100 and the support structure 210 but no thermal conductive pad is used. The electronic component 100 has a plurality of pins 101,102. The PCB 220 comprises at least one electrical conductive layer 221 to which the pins 101,102 are brought in electrical contact. The electrical conductive layer 221 in Figure 2 is placed embedded in the PCB 220 but could also be placed on the surface of the PCB 220. To bring a pin 101 in electrical contact with the electrical conductive layer 221, a hole 223 is drilled in the PCB 220. Instead of using a thermal conductive pad or pins with stress relief sections, the inventive concept is based on using a resilient contact element on the PCB 220 itself. This resilient contact element is manufactured by milling a track 250 around the hole 223 so that a resilient portion 222 of the PCB 220 including the conductive layer 221 is produced. Figure 3 illustrates the layout of this track 250 seen from above. A part 251 of the portion 222 is not milled so that the portion 222 is resiliently movable relatively the PCB 220. Figure 3 also illustrates a second track 310 and a second resilient portion 311 having a conductive layer (not shown) . The second resilient portion 311 also has a hole 312 that is adapted to receive the other pin 102. When all the resilient portions 222,310 are manufactured, the electronic component 100 can be mounted to the PCB 220 and the pins 101 and 102 can be brought in electrical contact with the conductive layer 221.

The pins 101,102 can be fastened to the PCB 220, by soldering, welding, using POP rivets or even bending the end of the pin 101, 102.

Depending on the type of electronic component 100, the characteristics of the PCB 220 and on different tolerances (mechanical or thermal) different shapes of the resilient portions 222,311 can be produced. The portions 222,311 in Figure 2 are formed by milling elongated U shaped tracks 250,310 surrounding the holes 223,312 in the PCB 220 resulting in tongue shaped portions 222,311 that are resiliently movable relatively the remaining portion of the PCB 220. Figure 6 illustrates resilient portions 623,624 with alternative layouts. The portion 623 is formed by milling an elongated U shaped track 620 around a hole 601 but with additional tracks perpendicular to and inside the main U shaped track 620. The other portion 624 is formed by milling a helix shaped track 621 around a hole 602. Other layouts are possible and again it is possible to use different layouts to meet different demands.

The Figures 4a and 4b illustrate further embodiments 400, 450 of an assembly according to the present invention. Assembly 400 corresponds to assembly 200 but where a heat sink 415 is included and brought in thermal contact with the component 100 (optionally via the base plate 130) . In the assembly 450 a multilayered PCB 451 is used. In addition to mill the track 455 so it completely penetrates the PCB 451, one or several layers from the multilayered PCB 451 are removed from the resilient portion 452 so that only one layer remains.

Yet another embodiment is to mill an enclosed track forming an aperture 456 in the PCB 451. In the aperture 456, a flex circuit such as a Flexboard® is mounted to an edge 453. The flex circuit acts in this embodiment as the resilient portion 452 to be mounted to the pins 101,102 of the component 100.

Figure 5 illustrates twc assemblies 500,510 according to another embodiment of the invention. Assemblies 500,510 comprise two PCBs 512,522 that are mounted to a common support structure 550 including a backplane 540 and two corresponding heat sinks 513,523. The two heat sinks 513,523 are mounted so that they can carry at least one electronic component 511,521 each. The PCBs 512,522 are provided with resilient portions 524,525 according to any of the embodiments described above to which the pins 531,532,533,534 of the electronic components 511,521 are mounted.

The inventive concept also allows for different types of electronic components 100 to be used in the different embodiments described above. Figure 7 illustrates a power module 700 that basically comprises a small PCB 710 with pins 701,702 and a plurality of other second electronic components 711,712,713,714.

Figure 8 is a flow chart illustrating a method to mount the component 100 on the PCB 220. To prepare the PCB 220 to receive the pins 101,102 one option is in step 800 to produce holes 223,312 in the PCB 220. The holes 223,312 are drilled and the inside of each hole 223,312 is plated in order to provide better electrical contact between the pins 101,102 and the conductive layer 221. When using the option of soldering the pins 101,102 directly to a conductive layer 221 on the surface of the PCB 220, step 800 is not necessary. In step 801 the tracks 250,310 are milled in order to produce the resilient portions 222,311 of the PCB 220. In step 802 the component 100 is mounted to the support structure 210 and the PCB 220 and the pins 101,102 are in step 803 brought in electrical contact with the electrical conductive layer 221 in the resilient portion 222,311.

The step 802 of mounting the component 100 can also include a step of fastening the pins 101,102 to the PCB 220 for example by soldering, welding, using POP rivets or even bending the end of the pin 101,102.