WO2007060661A2

WO2007060661A2 - Method and devices for surface mounting

Info

Publication number: WO2007060661A2
Application number: PCT/IL2006/001347
Authority: WO
Inventors: Joel Lang
Original assignee: Cellergy Ltd.
Priority date: 2005-11-23
Filing date: 2006-11-23
Publication date: 2007-05-31
Also published as: IL172146A0; WO2007060661A3

Abstract

A method and thermal shield for connecting heat sensitive electric/electronic component onto a printed circuit board, which method comprises providing a printed circuit board having solder material applied thereon, placing said component in a casing provided with a liquid whose boiling point is lower than the melting point of said solder material, positioning said casing on said printed circuit board such that connector leads, which extend from one or more of the lateral sides of said component and exit said casing are brought into contact with said solder material, heating the printed circuit board whereby said solder material is melted and subsequently gradually cooling the same to solidify the solder, thereby securing said component to said printed circuit board.

Description

METHOD AND DEVICES FOR SURFACE MOUNTING

The electronic industry commonly uses Surface Mounting Technology (SMT) to place and connect electronic and/or electric components, such as capacitors, transistors or inductors, to electronic circuit boards. This technology substantially improves placement accuracy and speeds up the manufacture process of such circuits in comparison to the manufacture of the same by mounting and connecting the individual components manually. Additional advantages of SMT are the reduction in Printed Circuit Board (PCB) size and weight, increased reliability and the fact that it is fully automated.

SMT processes typically comprise an initial step of preprinting tin paste onto the circuit board plate via a suitable screen printing technology. Thereafter, an assembly machine places the electronic and/or electric components on the board at exact locations. Upon finishing the placement process the circuit board is passed through an oven e.g., Infra Red (IR) or convection, which goes through a temperature ramp (e.g. 180-290⁰C) that exceeds the melting temperature of the tin paste. Thereafter, the circuit board and the components mounted thereon are subjected to a controlled cooling process, following which the electronic and/or electric components become connected mechanically and electronically to the board.

Many electronic components are heat sensitive due to the materials they are made of and/or fragility of their structure. For instance, a component which contains water (such as some batteries or super capacitors) , if heated to temperatures of about 250⁰C, will most probably undergo component failure or rupture, since the water contained therein will boil at these temperatures.

A heat sensitive electric/electronic component to be mounted on a printed circuit board according to the present invention may be described as a spatial body having two opposite faces, which in general are parallel to one another, and lateral sides. Thus, more specifically, the heat sensitive electric/electronic component is a polyhedron having two opposite faces lying in parallel planes and lateral sides. For the purpose of simplicity and ease of illustration, without limiting the generality of the present invention, a heat sensitive electric/electronic component which is essentially in the form of a rectangular parallelepiped is illustrated in Figure 1 by means of numeral 10; the dimensions of the component are designated by the letters w, h and d. Numeral 11 indicates connector leads (e.g., wire leads) that extend from one of the lateral sides of the heat sensitive electric/electronic component. Hereinafter, the term connector leads refers in general to electrical connecting means (e.g., metal wires) which are used for electrically (and optionally also mechanically) connecting electric/electronic components to printed circuit boards.

It has been found that it is possible to provide an effective thermal protection to a heat sensitive electric/electronic component during SMT processes, by placing said component in a casing, wherein the interior space of said casing is further provided with a liquid whose boiling point is lower than the melting point of solder materials commonly used in SMT processes (such as tin paste, whose reflow temperature is around 18O⁰C) . The casing to be used according to the present invention may be either manufactured from a material having thermal conductivity generally in the range of 10 to 450 W.irf ^■"^■.K^"1, such as for example, a metallic material (e.g. Copper 390 W. m^"1. K^"1) , or may be made of material characterized by low thermal conductivity, generally in the range of 0.003 to 10 W.m^.K^"1, preferably in the range of 0.003 to 1 W.m^.K^"1, most preferably about 0.3 W.m^.PC¹ (e.g. Low density polymers 0.3 W-itf^.K^"1). Regarding the latter type of casing, it should be noted that in the absence of a liquid within the interior space of such casing, it would have been necessary to use a casing having very thick walls in order to provide a sufficient thermal shielding to the heat sensitive electric/electronic component enclosed therein. For example, in the absence of an interior liquid, the wall thickness necessary for obtaining a suitable temperature gradient between the outer and inner surfaces of a casing would have been in the range of 2-3 mm. Consequently, the total thickness of the structure formed by enclosing a 3 mm thick component within the afore-mentioned thick casing would be in the range of 7-9 mm. It may be appreciated that the resulting bulky structure may ^■ impair the benefits associated with surface mounting technologies. Surprisingly, it has been found that the introduction of a suitable volume of water, or an aqueous mixture, into the interior space of a casing of poor thermal conductivity permits a considerable reduction in the thickness of the walls of said casing, as will be illustrated in the examples below.

The liquid, e.g., water, present in the interior space of the casing according to the present invention absorbs the thermal energy transferred to the printed circuit board during the SMT process, and is transformed into a gaseous state (e.g., steam) once a certain amount of thermal energy accumulates therein. Table 1 compares the heat capacity and the amount of thermal energy required to bring a) water and b) a solid Phase Change Material (PCM) to about, and above, 100⁰C. As seen in Table 1, the amount of energy required to heat water to its boiling temperature (100⁰C) is significantly large, thus make it particularly suitable for use in accordance with the present invention.

Table 1

For the purpose of illustration and in order to demonstrate the feasibility of the method provided by the present invention, an electrochemical double layer capacitor was tested as follows (The capacitor, CLG03PO20L17 , manufactured by Cellergy, Israel, may be schematically represented by the structure shown in Figure 1, with dimensions of 17x17x2.5 mm (hxwxd, respectively) ) :

i) The capacitor was wrapped with a 1.5 mm thick film of wet clay (e.g., such as used by children and artists). The edges of the clay film were joined over the capacitor, thereby sealing it thereinside such that only its conducting connector leads protrude outwardly therefrom.

ii) In a similar manner, the capacitor was wrapped in a 1.5 mm thick film of baking dough (e.g., pizza dough comprising approximately 50% water and 50% flour) . The baking dough film was wrapped around the capacitor and the edges of the dough film were joined over the capacitor, thereby sealing it thereinside such that only its connector leads protrude outwardly therefrom.

The resulting, wrapped component is shown if Figure 2 (numeral 20) , wherein numerals 10 and 11 indicate the body of the capacitor and the connector lead that protrude laterally therefrom, respectively, whereas numeral 18 indicate the wrapping material. The wrapped capacitor may be kept in a 100% humidity chamber until it is tested in an SMT oven. The protected capacitor was passed through an SMT oven, following which several electrical properties thereof were measured. It has been established that no considerable damage was caused to the capacitor during the SMT process, due to the protection afforded by the water coupled thereto.

Accordingly, in a first aspect, the present invention relates to a method for connecting heat sensitive electric/electronic component onto a printed circuit board, which method comprises providing a printed circuit board having solder material applied thereon, placing said component in a casing provided with a liquid whose boiling point is lower than the melting point of said solder material, positioning said casing on said printed circuit board such that connector leads, which extend from one or more of the lateral sides of said component and exit said casing are brought into contact with said solder material, heating the printed circuit board whereby said solder material is melted and subsequently gradually cooling the same to solidify the solder, thereby securing said component to said printed circuit board.

In general, simply enclosing the heat sensitive component in a casing with a suitable quantity of water (typically in the range between 0.2cc to 1.0 cc, in accordance with the size and properties of the component to be protected) is expected to give satisfactory results. The water may be poured into the interior of the casing/shell or may be trapped in a polymer contained therein. Alternatively, it is possible to introduce into the casing a small amount of powder (e.g., 1-50 mg of a super water absorbent material) which is capable of absorbing at least a portion, or all, of the liquid poured thereinto (e.g., Sodium Polyacrylate, hydro-gels, silica).

More preferably, however, the component is placed in a rigid casing whose inner surfaces are at least partially covered with absorbent layers that are impregnated with water. It should be noted that the order of operations in introducing the heat sensitive component and the liquid within the casing may vary as desired. For example, the heat sensitive component may be wrapped by the absorbent material before inserting it into the casing. In addition, the absorbent material may be wetted with the liquid before and/or after wrapping said component therewith. Additionally or alternatively, the liquid may be introduced into the hollow interior of the casing before and/or after insertion of the absorbent material and heat sensitive component therein.

As will be appreciated by those skilled in the art, the connector leads of the protected component should be electrically isolated in order to prevent electrical shorts therebetween. The electrical isolation of the connector leads may be achieved via any suitable means, such as epoxy resins and shrinking sleeves.

According to a particularly preferred embodiment, the casing is a rigid casing defined by two opposite bases, which in general are parallel to one another, and lateral sides, with an opening provided in one of said lateral sides, such that the interior space of said casing preferably corresponds in form and size to the heat sensitive electric/electronic component to be protected thereby, and wherein one or more layers are applied onto the inner surfaces of said casing, said layers being capable of trapping water therein. Preferably, the layers deposited onto the inner surfaces of the casing^' are made of absorbent material capable of holding more than 0.1, and preferably between 0.2 to 0.7 cc of water.

The aforementioned casing, which forms another aspect of the present invention, is illustrated in Figures 3A and 3B (the latter Figure shows the casing with the capacitor positioned therein) . The casing 12 is configured in a shape of a box having an upper base 12u and a bottom base 12b, connected by lateral sides 12s and rear side 12r, and provided with a front opening 32. Most conveniently, the casing is manufactured by drilling a suitable solid body made of a thermosetting plastic or reinforced plastic, ceramic matrial, or metal, preferably from Ultem or PPS (Polyphenylene Sulfide) . Autoclavable plastic has been found to be particularly suitable for use in construction of casing 12 due to its low thermal conductivity and its high temperature stability. The sizes of casing 12 may generally range between 14*14*4 to 50*32x6 mm, and the thickness of its walls may generally be in the range of 0.3 to 1 mm, preferably about 0.5 mm.

The inner surfaces of casing 12 are covered by one or more layers (15 in Fig. 3B) of an absorbent material (e.g., filter paper) for holding the volume of water (e.g., 0.5 cc) needed for the SMT process. The layers of the absorbent material may be made of a super-absorbent, hydro-gel, Silica or clay, and are attached to the inner surfaces of casing 12 by any suitable mean, for example Oasis type 2111 (Technical absorbents) . Additionally or alternatively, the liquid introduced into the casing may contain additives, such as, but not limited to, sodium polyacrylate, or it may be introduced in a form of a gel, such as, but not limited to, gelatin.

Another casing suitable for use according to the present invention is shown in Figure 5. The protected component is enclosed in a casing 14 having a box and lid portions, which may be altered between a "closed" and "open" states. In Fig. 5 casing 14 is shown in the "open" state.

Box portion 14b of casing 14 comprises a base having elevated lateral sides adapted for receiving the protected component 10 thereinside. Lid portion 14a of casing 14 preferably has the same spatial dimensions as box portion 14b, to allow fitting it over box portion 14b and closing it therewith. Additional fastening means (e.g., clasps - not shown) may be provided for enabling tight attachment between lid 14a box 14b portions. Box 14b and lid 14a portions are preferably connected by hinge 14h mounted between respective lateral sides of box 14b and lid 14a.

Lid 14a may be also configured in a shape of a box such that the upper portion of the protected component 10 may be received therein when it is fitted over box 14b. Casing 14 may include bores 14c provided in one or more lateral sides thereof and designed for passing connecting leads 11 of the protected component 10 outwardly therethrough. In the example illustrated in Fig. 5 bores 14c are formed in the "closed" state of casing 14 from respective grooves provided on the upper and bottom surfaces of corresponding lateral sides of box 14b and lid 14a.

The inner surfaces of box portion 14b (and/or lid portion 14a) may be covered by one or more layers (not shown) of absorbent material, for holding a suitable volume (e.g., 0.2 to 0.7 cc) of a liquid. For example, the absorbent layers may be made from an absorbent or super-absorbent material, hydro-gel, silica or clay. Advantageously, the one or more layers of absorbent material may be made from a filter paper (e.g. Whatman N⁰I) or from a super-absorbent cloth (e.g. Oasis type 2111) . Alternatively, said liquid, or liquid and additives (e.g., sodium polyacrylate) or a gel (e.g., gelatin), may be dispensed directly into box portion 14b. Additionally or alternatively, an absorbent material (e.g., absorbent cloth) may be wrapped around the protected component 10, and immersed in a liquid before, placing it in casing 14.

Casing 14 may be manufactured from a type of plastic, thermosetting plastic, reinforced plastic, ceramic material, or metal, preferably from Ultem or PPS (Polyphenylene Sulfide) , by means of metalworking, plastic extrusion or molding, for example. Typical sizes of electrical/electronic components range from 12x12x2 mm to about 48x30*4 mm, and in accordance the sizes of casing 14 may generally range between 14x14x4 to 50^χ32^χ6 mm. The thickness of the walls of casing 14 may generally be in the range of 0.3 to 1 mm, preferably about 0.5 mm.

Regarding the other parameters related to the surface mounting method according to the present invention, the following information is provide: the soldering material is typically a tin paste, the heating is conducted either by convection or infra-red radiation according to heating profiles well known in the art (such as described in Altera Corporation Reflow Soldering Guidelines for Surface-Mount Devices www. altera . com/'literature/'an/'anO81.pdf) .

The method of the present invention may be used to provide thermal shielding to a variety of electrical/electronic device, such as, but not limited to, capacitors, energy cells, integrated circuits, CPU's (central processing units) and memory chips .

Brief Description of the Drawings

The present invention is illustrated by way of example in the accompanying drawings, in which similar references consistently indicate similar elements and in which:

Fig. 1 schematically illustrates the structure of certain types of electrical/electronic components;

Fig. 2 demonstrates an implementation of the thermal shield of the invention in which the protected component is wrapped in wet clay;

Figs. 3A to 3C schematically illustrate the casing and sealing of an electrical/electronic component in a thermal shield comprising a liquid;

Figs. 4A and 4B schematically illustrate the wrapping and sealing of an electrical/electronic component by layers of a ribbon; and

Fig. 5 schematically illustrates a thermal shield casing comprising a box and lid portions.

In the following examples, several implementations of the thermal shield of the invention, wherein the thermal shield comprises a liquid, are described. In the various implementations described in the following examples the thermal shield of the invention was tested on EDLC (Electrochemical Double Layer Capacitor) capacitors manufactured by Cellergy, such as CLG03PO20L17, the dimensions of which are 17x17x2.5 mm (hxwxd)

Examples

Example 1

A 19x19x4.5 mm full mass cube made of an auto-clavable plastic (Ultem of General Electric) was drilled by manual milling machine to obtain one open side box. The dimensions of the hollow interior of the resulting shell were 18x18x3.5 mm. The external dimensions of the shell were 19x19x4. The thickness of the shell walls was t'=0.5 mm. The structure of the shell is shown in Figure 3A.

As shown in Figures 3B to 3C, the heat sensitive component 10 was cased in the aforementioned t^s=0.5 mm thick shell 12, such that component 10 can be inserted and removed therefrom via opening 32. A piece of super-absorbent cloth 15, such as Oasis type 2111 (Technical absorbents) , is placed inside shell 12, such that it surrounds the outer surface of component 10 disposed thereinside. Cloth 15 is then wetted with water and thereafter opening 32 is sealed by a hermetic pressure sensitive film 13 to entrap the water thereinside. The minimal quantity of water that absorbent 15 should be provided with, for component 10 with spatial dimensions of about 17*17*2.5 mm, should be at least 100 mg. Table II below summarizes the results collected after the protected heat sensitive component was passed through an SMT oven at maximum temperature peak of 290 °C:

Table 1

'^*> ESR: Equivalent Serial Resistance measured at 1 kHz, ^(*ML.C: Leakage Current.

Example 2

As shown in Figures 3B to 3C, a heat sensitive component 10 was cased in a t^x=l mm thick shell 12 made of a thermosetting plastic (PPS) , such that component 10 could be inserted into, and removed from, the hollow interior of said shell via an opening 32 provided in one of its lateral sides. The dimensions of shell 12 were 18*18x3 mm. A layer of filter paper 15 (e.g., Whatman no. 1) was placed in shell 12 between the inner wall of the shell and the external surface of component 10. After arranging component 10 inside shell 12 such that it became wrapped in filter paper 15, 0.5 cc of water were introduced into the shell 12 via opening 32. Opening 32 was then sealed with hermetic pressure sensitive film 13 (e.g., 3M Scotch tape) to entrap the water in the hollow interior of shell 12. It was found that the minimal quantity of water that may be absorbed in the filter paper 15 wrapped around a component having spatial dimensions of about 17^χl7^χ2.5 mm should be at least 100 mg. Example 3

In this example component 10 is shielded in a manner similar to the shielding used in examples 1 and 2, which were described with reference to Figs. 3A to 3C. However, in the present example, the water which was introduced into shell 12 was mixed with glycerol to reduce the drying rate of the liquid.

Example 4

Figs. 4A and 4B demonstrate another possible embodiment of the invention, wherein the heat sensitive component 10 is wrapped by several layers of a ribbon 16, for example a sticky ribbon, made from a thermally stable material capable of withstand temperatures in the range 200-300⁰C. Ribbon 16 may wrap all side of component 10 and thus seal it hermetically. Alternatively, the front and rear sides of component 10 may be sealed by hermetic pressure sensitive film 13. The layers of ribbon 16 may comprise absorbent material (e.g., a adhesive paper tape having 17 mm width which is wrapped around the protected component) , which is wetted with a suitable liquid, e.g. water.

All of the abovementioned parameters are given by way of example only, and may be changed in accordance with the differing requirements of the various embodiments of the present invention. Thus, the abovementioned parameters should not be construed as limiting the scope of the present invention in any way. In addition, it is to be appreciated that the different casing, components, and other members, described hereinabove may be constructed in different shapes (e.g. having oval, square etc. form in plan view) and sizes differing from those exemplified in the preceding description.

The above examples and description have of course been provided only for the purpose of illustration, and are not intended to limit the invention in any way. As will be appreciated by the skilled person, the invention can be carried out in a great variety of ways, employing more than one technique from those described above, all without exceeding the scope of the invention.

Claims

1. A method for connecting heat sensitive electric/electronic component onto a printed circuit board, which method comprises providing a printed circuit board having solder material applied thereon, placing said component in a casing provided with a liquid whose boiling point is lower than the melting point of said solder material, positioning said casing on said printed circuit board such that connector leads, which extend from one or more of the lateral sides of said component and exit said casing are brought into contact with said solder material, heating the printed circuit board whereby said solder material is melted and subsequently gradually cooling the same to solidify the solder, thereby securing said component to said printed circuit board.

2. A method according to claim 1, wherein the casing contains one or more layers applied onto its inner surfaces, said layers being capable of trapping water therein.

3. A method according to claim 2, wherein the inner surfaces of the casing are at least partially covered with absorbent layers that are impregnated with water.

4. A method according to claim 3, wherein the casing is a rigid casing made of a material having thermal conductivity in the range of 0.003 to 10 W.m^"1.^¹, wherein said material is selected from the group consisting of plastic, thermosetting plastic, reinforced plastic and ceramics.

5. A method according to claim 4, wherein the one or more layers applied onto the inner surfaces of said casing are made of a material selected from the group consisting of: super- absorbent cloth, hydro-gel, silica, clay and filter paper.

6. A rigid casing suitable for thermally protecting a heat sensitive electric/electronic component, said rigid casing is defined by two opposite bases, which are parallel to one another, and lateral sides, with an opening provided in one of said lateral sides, and wherein one or more absorbent layers are applied onto the inner surfaces of said casing, said layers being capable of trapping water therein.

7. The rigid casing according to claim 6, wherein said rigid casing is made from a material having thermal conductivity in the range of 0.003 to 10 W.m^.K^"1.

8. The rigid casing according to claim 7, wherein said rigid casing is made from one of the materials selected from the group consisting of: plastic, thermosetting plastic, reinforced plastic, ceramic material.

9. The rigid casing according to claims 6-8, wherein the one or more layers applied onto the inner surfaces of said casing are made from a material selected from the group consisting of: super-absorbent cloth, hydro-gel, Silica, clay and filter paper.

10. The rigid casing according to claims 7, wherein the sizes of casing range between 14*14x4 to 50^χ32^χ6 mm, and the thickness of its walls is in the range of 0.3 to 1 mm.

11. The rigid casing according to claim 9, wherein the layer is capable of holding between 0.2 to 0.7 cc water.