WO1994017568A1

WO1994017568A1 - Self aligning surface mount electrical component

Info

Publication number: WO1994017568A1
Application number: PCT/US1994/000769
Authority: WO
Inventors: Allen David Hertz; David A. Tribbey
Original assignee: Motorola, Inc.
Priority date: 1993-01-22
Filing date: 1994-01-19
Publication date: 1994-08-04
Also published as: AU6032394A

Abstract

A surface mount electrical component (500) comprises a body portion (505) for housing an electrical circuit (510), wherein the body portion (505) is partially bounded by at least one side (525, 530, 535, 540), and conductive leads (515, 520) extend from the at least one (525, 530, 535, 540) for providing electrical signals to the electrical circuit (510). A first plurality (515) of the conductive leads (515, 520) have contact surfaces of a first size, and a second plurality (520) of the conductive leads (515, 520) are alignment leads (520) having contact surfaces (550) of a second size greater than the first size.

Description

SELF ALIGNING SURFACE MOUNT ELECTRICAL COMPONENT

Field of the Invention

This invention relates in general to electrical components, and more specifically to self aligning surface mount electrical components.

Background of the Invention

Modern selective call receivers, e.g., pagers, generally include circuit carrying substrates, such as printed circuit boards, having electrical components mounted thereon and interconnected thereby. Because pagers are intended for portable use and are usually carried by a user, smaller, less cumbersome pagers are considered very desirable. Additionally, smaller pagers are desirable because they are typically less noticeable, and thus more attractive, than large, bulky pagers. As market trends dictate the design and manufacture of smaller pagers, electrical components within the pager must become smaller to meet tighter space constraints. However, while pagers are becoming smaller, the features offered in many pagers are increasing. For example, more recently designed pagers often include real time clocks, alarms, alphanumeric displays, etc., all of which are considered desirable by users. These additional features require increasingly complex electrical components, many of which necessitate a large number of interconnects with other electrical components.

The need for smaller electrical components for use in pagers is in direct conflict with the need for more numerous interconnects between electrical components because smaller component dimensions increasingly limit the available space for leads with which an electrical component may be interconnected to other electrical components. As a result of these conflicting needs, many electrical components include an extremely large number of leads within a small amount of space. Additionally, the leads are generally smaller and spaced more closely together than in larger electrical components. The smaller electrical components, however, are often very difficult to process and are prone to manufacturing errors. One of the largest sources of error is the placement of the electrical component on the printed circuit board. More specifically, as the pitch between adjacent leads of an electrical component approaches the rrtirtirnurn pitch which can be accurately processed utilizing current manufacturing technology, alignment errors can easily occur in which leads are incorrectly placed on the printed circuit board. As a result, leads may be soldered incorrectly, if at all, thereby causing product defects and reliability problems which are reflected in an increased product cost.

Thus, what is needed is a method for alleviating the alignment errors that often occur when smaller electrical components having numerous leads are processed.

Summary of the Invention

According to an aspect of the present invention, a surface mount electrical component comprises a body portion for housing an electrical circuit, wherein the body portion is partially bounded by at least one, and conductive leads extend from the at least one side for providing electrical signals to the electrical circuit. A first plurality of the conductive leads have contact surfaces of a first size, and a second plurality of the conductive leads are alignment leads having contact surfaces of a second size.

Brief Description of the Drawings

FIG. 1 is a top plan view of a conventional electrical component. FIG. 2 is a side view of the conventional electrical component of FIG.

1.

FIG. 3 is a perspective view of the electrical component of FIG. 1 mounted to a conventional printed circuit board.

FIG. 4 is an illustration of a conventional manufacturing process which may be utilized to mount the electrical component of FIG. 1 to the printed circuit board of FIG. 3.

FIG. 5 is a top plan view of an electrical component having alignment leads in accordance with a preferred embodiment of the present invention. FIG. 6 is a perspective view of the electrical component of FIG. 5 and a printed circuit board on which it is placed in accordance with the preferred embodiment of the present invention. FIG. 7 is a side view of the electrical component of FIG. 5 and the printed circuit board of FIG. 6 in accordance with the preferred embodiment of the present invention.

FIG. 8 is an illustration of the misalignment of the electrical component of FIG. 5 with the printed circuit board of FIG. 6 in accordance with the preferred embodiment of the present invention.

FIG. 9 is a chart depicting the percent increase in component centering over a conventional electrical component, in accordance with the preferred embodiment of the present invention, for the electrical component of FIG. 6.

FIG. 10 is a top plan view of an electrical component having alignment leads in accordance with an alternate embodiment of the present invention.

FIG. 11 is an illustration of a data communication receiver utilizing the electrical component of FIG. 6 in accordance with the preferred embodiment of the present invention.

FIG. 12 is an electrical block diagram of the data communication receiver of FIG. 7 in accordance with the preferred embodiment of the present invention.

Description of a Preferred Embodiment

FIG. 1 is a top plan view of a conventional surface mount electrical component 100, such as an integrated circuit chip, which comprises a component housing 105 within which circuitry (not shown) is enclosed. Conductive leads 110 extending from the component housing 105 provide electrical signals to and from the circuitry. These leads 110 are generally rectangularly shaped and formed as shown in FIG. 2.

FIG. 2 is a side view of a conventionally shaped conductive lead 110 utilized to provide the electrical signals to and from the circuitry enclosed within the electrical component 100. The lead 110, which may be, for example, a "gullwing" (as shown) or a "J" lead, extends from the electrical component 100 and is formed such that a flat rectangular contact surface 115 of the lead 110 contacts a circuit carrying substrate, such as a printed circuit (pc) board, as may be better understood by referring to FIG. 3.

FIG. 3 is a perspective view of the conventional electrical component 100 mounted to a pc board 120. Typically, the electrical component 100 is placed on the pc board 120 such that contact surfaces 115 of the leads 110 are aligned with and electrically coupled, such as by solder bonding, to component pads (not shown) on the pc board 120, thereby holding the electrical component 100 in a fixed position above the pc board 120. In this manner, the electrical component housing 105 is not damaged by contact with the pc board 120. Printed circuit runners 130 formed on the pc board 120 generally interconnect the electrical component 100, via the leads 110 and the component pads, to other circuitry mounted to the pc board 120.

In many typical manufacturing environments, the electrical component 100 is mounted to the pc board by an automated process, such as the one illustrated in FIG. 4. Generally, a photo-lithographic processor 200 is employed to deposit patterns of a photo-imageable etch resist, such as Dupont Vacrel, onto the pc board substrate, which could be constructed, for example, of FR4 (a flame retardant classification) glass epoxy material. The substrate is thereafter plated by plating equipment 205 with a metal, such as one-half ounce copper covered with a hot-air-solder-leveled tin- lead alloy, for use in forming the runners 130 (FIG. 2) and component pads of the pc board 120, which are not covered by the deposited etch resist. Next, the substrate passes to a chemical etcher 210, which completes the formation of the pc board 120 by etching surplus metal from the substrate with an etchant, e.g., ferric chloride.

In a similar manner, the photo-lithographic processor 200 and the chemical etcher 210 are utilized to manufacture from a material such as stainless steel a solder stencil having apertures matching the locations, sizes, and shapes of the component pads formed on the pc board 120. Subsequently, the solder stencil is positioned within the frame of a solder printer 215, and the pc board 120 is aligned with the solder stencil. The solder printer 215 then uses a wiper blade to wipe solder paste across the solder stencil, thereby pushing the solder paste though the apertures of the solder stencil onto the component pads of the pc board 120. Typically, the pc board 120 is next processed by automated placement equipment 220, which robotically places the electrical component 100 (FIG. 2) on the pc board 120 such that the leads 110 are placed on top of the solder paste.

Next, the pc board 120 and the electrical component 100 placed thereon are passed through a reflow oven 225 for liquefying solder in the solder paste. The reflow oven 225 usually has an inert atmosphere to control oxidation of materials while in the reflow oven 225. Once liquefied, the solder "wets", i.e., uniformly covers and adheres to the leads 110 and the component pads. When the pc board 120 and the electrical component 100 emerge from the reflow oven 225, the solder re-solidifies, thus bonding the leads 110 of the electrical component 100 to the component pads of the pc board 120. Finally, during final assembly 230, the pc board 120 may be cleaned and assembled to other devices to form a final product, such as a data communication receiver.

Conventionally, the automated placement equipment 220 is programmed to align the conductive leads 110 (FIG. 1) of the electrical component 100 with the component pads of the pc board 120. This process is usually accurate for electrical components having a small number of fairly large leads. However, as technology advances, conventional electrical components are beginning to include more complex circuitry which must be interconnected with a greater number of exterior components. As a result, the number of leads extending from electrical components has increased. Furthermore, as market trends have driven the miniaturization of electrical components, the leads have had to become smaller and closer together to take advantage of the decreased amount of space on the electrical component. Therefore, automated placement equipment is often utilized to place electrical components which are extremely small and have a large number of narrow, finely pitched leads extending therefrom.

By way of example, many very complex conventional electrical components, referred to as "quad flat packs" (QFPs), have either thirty-two leads uniformly distributed at an approximate 0.8 mm lead pitch or eighty leads uniformly distributed at an approximate 0.5 mm lead pitch. It may be appreciated that electronic components having these configurations are very difficult to correctly align with component pads using current manufacturing technology. Incorrect alignment of complex electrical components, therefore, is a common problem that often results in defective pc boards having incorrectly soldered electrical components or electrical components having leads that have not been soldered at all. Time and money is then wasted in analyzing and repairing or simply discarding defective pc boards, thereby increasing the manufacturing cost and the cost of the final product.

It may further be appreciated that, although these complex electrical components may be aligned and placed on a pc board manually, such a process is extremely slow and tedious, not to mention next to impossible with the naked eye. However, when the number and pitch of the leads extending from an electrical component makes it necessary, electrical components may be aligned by a human operator using a microscope. This process, similar to the automated process, increases the time and expense needed for the manufacturing process, and therefore also increases the cost of the final product. Referring next to FIG. 5, a top plan view of a surface mount, self aligning electrical component, such as a QFP 500, is depicted. The QFP 500 comprises a body portion 505 for housing an electrical circuit 510. The electrical circuit 510 may be, for example, receiving circuitry (as shown), decoding circuitry, filters, etc. Preferably, the QFP 500 includes a plurality of conventional conductive leads 515 and a plurality of conductive alignment leads 520 distributed along four sides 525, 530, 535, 540 of the component housing. In accordance with the preferred embodiment of the present invention, eight alignment leads 520 are approximately located at the edges of the four sides 525, 530, 535, 540, i. e., two at each corner of the QFP 500. The contact surfaces 550 of the alignment leads 520 are greater in size than the contact surfaces 522 of the conventional conductive leads 515 and are preferably elliptical in shape, although other shapes, such as circular or rectangular, may be employed.

It may be appreciated by one of ordinary skill in the art that, although the electrical component in accordance with the preferred embodiment is described as being a QFP 500, alternate embodiments of the present invention are envisioned in which other types of electrical^' components, such as those having conductive leads extending from only two sides, include alignment leads having contact surfaces which are greater in size than the contact surfaces of other conductive leads.

With reference to FIG. 6, a perspective view of the QFP 500 depicts the placement of the QFP 500 onto a circuit carrying substrate, such as a pc board 600, which has component pads 610 formed thereon through use of the process described in FIG. 4. Preferably, the component pads 610 include both conventional rectangular pads for placement of the conventional leads 515 and elliptically shaped pads for placement of the elliptically shaped alignment leads 520, as shown. By referring to FIG. 7, which is a side view of the QFP 500, it may be seen that, by using the conventional solder printer 215 (FIG. 4), solder paste 650 is applied to the component pads 610 with which the leads 515, 520, which are, for example, "gullwing" or "J" type leads, are aligned as accurately as possible by the automated placement equipment 220. As described above, electronic components, such as the QFP 500, having numerous finely pitched leads are very difficult to align correctly utilizing automated or manual processes. As a result, the QFP 500, like conventional complex electronic components, may be misaligned during the placement process or during transport to the reflow oven 225. An example of such a misalignment is depicted in FIG. 8. As shown, the QFP 500 is shifted to the right such that many of the leads 515, 520 (FIG. 5) are making only minimal contact with the component pads 610. However, in accordance with the preferred embodiment of the present invention, this misalignment problem may be conveniently corrected during the reflow process in which the pc board 600 and the QFP 500 are heated by the conventional reflow oven 225.

During the reflow process, the solder paste 650 on each of the component pads 610 liquefies, and the solder wets, as described above. When the solder wets, it spreads uniformly across the component pads 610 and exerts a surface tension on the leads 515, 520. The surface tension exerted on the alignment leads 520 is great enough to pull the QFP 500 , which floats freely during reflow, back into the proper alignment in which all of the leads 515, 520 contact their respective component pads 610 properly. Therefore, placement errors are advantageously corrected during the reflow process as long as the alignment leads 520 are contacting the correct component pads 620 within predetermined tolerances. As a result, defects due to misalignment of the QFP 500 are reduced significantly, thereby decreasing the manufacturing costs of the final product.

With reference to FIG. 9, a chart is shown in which alignment leads 520 having different sized contact surfaces 550 (FIG. 5) are compared for a QFP having thirty-two leads with a 0.813 mm lead pitch. The chart depicts the percent increase in component lead centering over conventional electronic components (vertical axis) versus the lead misalignment (horizontal axis) for alignment contact surfaces of different sizes. This increase has been charted for eight alignment contact surfaces having diameters of 0.457 mm (curve 700), 0.610 mm (curve 705), 0.762 mm (curve 710), and 0.914 mm (curve 715). It is clearly evident that a 32x32 QFP having alignment leads with contact surfaces of approximately 0.914 mm in diameter provides for an extremely advantageous increase (92 - 160%) in realignment of a misaligned QFP, thus decreasing the number of incorrectly reflowed QFPs during the manufacturing process. Referring next to FIG. 10, an alternate embodiment of the present invention is depicted in which a QFP 500' includes centrally located alignment leads 520'. In this alternate embodiment, a smaller number of alignment leads 520' are utilized to pull the misaligned QFP 500' back into alignment with component pads of a pc board. This alternate embodiment, however, might not exhibit the same realignment characteristics as those of the QFP 500 according to the preferred embodiment, as it has fewer alignment leads 520'.

FIG. 11 shows a miniature electronic device, such as a data communication receiver 800, in which the QFP 500 according to the present invention might be advantageously utilized. As can be seen, electronic components such as the QFP 500 are mounted to the pc board 610, which is enclosed by a device housing 805 when the data communication receiver 800 is assembled. Referring next to FIG. 12, an electrical block diagram of the data communication receiver 800 is depicted. The data communication receiver 800 comprises receiving circuitry 850 for receiving and demodulating a radio frequency signal and a decoder/controller 855 coupled to the receiving circuitry 850 for recovering a selective call message from the demodulated signal. The recovered selective call message is thereafter stored by the decoder /controller 855 in a memory 860. Additionally, a time value associated with the selective call message may be provided by a real time clock 865 and stored in the memory 860. In response to decoding the selective call message, the decoder/controller 855 activates an alert mechanism 870, such as a transducer, which then generates a sensible alert to announce reception of the selective call message to a user. The selective call message, and perhaps the time value associated therewith, may subsequently be retrieved from the memory 860 and provided to a display 875, such as an alphanumeric liquid crystal display, for presentation thereby.

Because a large number of features, e.g., clocks and alphanumeric displays, have become popular for use in data communication receivers, the components utilized by data communication receivers have become more complex. At the same time, the size and weight of data communication receivers is decreasing because consumers prefer to carry lighter and smaller receivers. As a result, complex electronic components having small, numerous leads are beginning to be used in data communication receivers. Therefore, the use of electronic components having alignment leads, in accordance with the preferred embodiment of the present invention, could be very useful in data communication receivers.

In summary, the surface mount electrical component as described above includes a plurality of conductive leads for contacting component pads formed on a circuit carrying substrate, such as a pc board. In accordance with the preferred embodiment of the present invention, a number of these leads are alignment leads having elliptically shaped contact surfaces of a size greater than the contact surfaces of the other conductive leads. When the electrical component is placed, either manually or automatically, onto the pc board, alignment errors may easily result in which the conductive leads are not properly aligned with the component pads. However, according to the present invention, this problem may be corrected during the reflow process. When the pc board and the electrical component are heated in a reflow oven, solder disposed between the alignment leads and the corresponding component pads liquefies, and the surface tension exerted thereby conveniently pulls the electrical component into the correct orientation in which the conductive leads contact the component pads of the pc board within predetermined tolerances. As a result, pc boards manufactured with electrical components according to the present invention may have fewer defects than pc boards manufactured with conventional electrical components. Therefore, because less time is wasted in troubleshooting and repair, the final cost of a product, such as a data communication receiver, utilizing electrical components having alignment leads will be less.

It may be appreciated by now that there has been provided an electrical component that alleviates defects caused by the alignment errors that often occur during the placement process.

Claims

1. A surface mount electrical component, comprising: a body portion for housing an electrical circuit, wherein the body portion is partially bounded by at least one side; and conductive leads extending from the at least one side for providing electrical signals to the electrical circuit, wherein a first plurality of the conductive leads have contact surfaces of a first size, and wherein a second plurality of the conductive leads are alignment leads having contact surfaces of a second size larger than the first size.

2. The surface mount electrical component according to claim 1, wherein the contact surfaces of the alignment leads are substantially elliptical in shape.

3. The surface mount electrical component according to claim 1, wherein the conductive leads are constructed from a solderable material.

4. The surface mount electrical component according to claim 1, wherein the at least one side comprises four sides and the alignment leads are located at predetermined distances from edges of the four sides.

5. The surface mount electrical component according to claim 4, wherein the alignment leads comprise eight alignment leads approximately located at the edges of the four sides.

6. The surface mount electrical component according to claim 4, wherein the alignment leads are centrally located along the four sides.

7. The surface mount electrical component according to claim 1, wherein the conductive leads are constructed from copper.

8. The surface mount electrical component according to claim 7, wherein the conductive leads are plated with tin.

9. An electrical assembly, comprising: a circuit carrying substrate having component pads disposed thereon; an electrical component mounted on the circuit carrying substrate, the electrical component comprising: a body portion for housing an electrical circuit, wherein the body portion is partially bounded by at least one side; and conductive leads extending from the at least one side for providing electrical signals to the electrical circuit from the component pads, wherein a first plurality of the conductive leads have contact surfaces of a first size, and wherein a second plurality of the conductive leads are alignment leads having contact surfaces of a second size larger than the first size; and solder disposed between the component pads and the conductive leads for electrically coupling the component pads and the conductive leads.

10. The electrical assembly according to claim 9, wherein a portion of the component pads are alignment pads, and wherein the alignment leads are oriented with the alignment pads such that, when the solder disposed therebetween is liquefied, the surface tension of the solder moves the conductive leads into a predetermined alignment with the conductive pads.

11. The electrical assembly according to claim 10, wherein the contact surfaces of the alignment leads are substantially elliptical in shape.

12. The electrical assembly according to claim 10, wherein the at least one side comprises four sides and the alignment leads are located at predetermined distances from edges of the four sides.

13. The electrical assembly according to claim 12, wherein the alignment leads are centrally located along the four sides.

14. The electrical assembly according to claim 10, wherein the conductive leads are constructed from a solderable material.

15. A data communication receiver, comprising: a circuit carrying substrate having component pads disposed thereon, wherein a plurality of the component pads are alignment pads; receiving circuitry mounted on the circuit carrying substrate for receiving data, wherein the receiving circuitry comprises at least one electrical component mounted on the circuit carrying substrate, the at least one electrical component comprising: a body portion for housing an electrical circuit, wherein the body portion is partially bounded by at least one side; and conductive leads extending from the at least one side for providing electrical signals between the component pads of the circuit carrying substrate and the electrical circuit, wherein a first plurality of the conductive leads have contact surfaces of a first size, and wherein a second plurality of conductive leads are alignment leads having contact surfaces of a second size greater than the first size; solder disposed between the component pads and the conductive leads for electrically coupling the component pads and the conductive leads, wherein the alignment leads are oriented with the alignment pads such that, when the solder disposed therebetween is liquefied, the surface tension of the solder moves the conductive leads into a predetermined alignment with the conductive pads; and a housing for enclosing the circuit carrying substrate and the receiving circuitry mounted thereon.

16. The data communication receiver according to claim 15, wherein the alignment leads are substantially elliptical in shape.

17. The data communication receiver according to claim 15, wherein the at least one side comprises four sides, and wherein the alignment leads are located at predetermined distances from the edges of the four sides.

18. The data communication receiver according to claim 17, wherein the alignment leads are centrally located along the four sides.

19. The data communication receiver according to claim 15, wherein the circuit carrying substrate is a printed circuit board.

20. The data communication receiver according to claim 15, wherein the conductive leads are constructed from a solderable material.