Patent Application
20060065431
This invention relates to a novel printed circuit board (PCB) that incorporates an embedded electric heater into its substrate. Said electric heater becomes the only source of heat that allows to attain a self-reflowing process for simultaneously soldering multiple electronic components to said PCB face.
The manufacture of most modern electronic products require a PCB that allows to electrically interconnect a variety of electronic components and also holds them together in a relatively rigid condition. Many types of components are placed over a PCB such as; resistors, capacitors, inductors, transformers, integrated circuit (IC) packages, connectors, headers, RF shields, LEDs, switches, board interface systems, battery sockets, etc. that are electrically connected and restrained by means of soldered joints either over one side or both sides of a PCB. These joints can be attained by three methods: hand soldering, the wave soldering process and by the oven reflow soldering process.
Manufacturing electronic products utilizing PCBs requires a few sequential steps performed by different machines. For example, such steps may comprise: (1) printing the PCB with soldering paste (an operation generally performed by stencil printing equipment), (2) placing surface-mount electronic components on that PCB face (an operation performed by an automated computer-controlled “pick-and-place” machine or by any other type of component placement equipment), (3) soldering the assembly (an operation, until now, performed inside a reflow oven or by a wave soldering machine), (4) cleaning the completed assembly (an operation that may involve washing and drying) and (5) testing the assembly for proper functionality (detects components damaged during step (3) and the presence of defective soldered joints.) Rework or rejection may be required after operation (5).
Mass production almost exclusively utilizes the oven reflow soldering process to accomplish above step (3). This process exhibit inherit disadvantages that, indeed, increase the cost of the final product, generate rejects, require rework and reduce the reliability of the final product. The electronic manufacturing industry accepts these inherit drawbacks and shortcomings, and works around them, for lack of a more suitable soldering process.
The oven reflow soldering process, that is carry out inside a reflow oven, simultaneously heats up the entire assembly (meaning the PCB and all of the components been soldered to the PCB) to a temperature ranging from about 20° C. (degree Celsius) to 40° C. above the temperature at which the utilized solder alloy melts or reaches liquid us state. The melting temperature of most popular solder alloys utilized by the electronic industry ranges from 190° C. to 230° C.
The majority of consumer electronic products need to be rated, and indeed are, to operate at maximum temperatures that range from 50° C. to 90° C. Consequently, components that form part of every electronic product manufactured by reflow soldering process are required to survive temperatures, at least, 120° C. higher than the temperature level encountered during their most severe actual operation. Therefore, all electronic components must be unnecessarily temperature-overrated to tolerate or survive the soldering process. This requirement for high-temperature-exposure survival increases the cost of every component to be soldered to a PCB.
During the oven reflow soldering process, thermal shock (due to a fast heating rate) can crack certain components, in particular ceramic capacitors, increasing rejects and/or requiring costly rework. Fast heating of plastic IC packages could induce cracking when moisture absorbed inside said packages can turn into steam during a oven reflow soldering process causing the so called “pop-corn” effect that internally damage the IC package. Electrolytic capacitors are extremely sensitive to high temperature exposure. Laminated PCBs may become soft by extended exposure to heat. An increase in soldering process temperature can damage a PCB metal-plated through-holes or vias, by cracking their barrels due to differential thermal expansion between the PCB dielectric material and its barrels' plating metal. Warpage, or twisting of a PCB, increases with soldering temperature. Warpage can cause defective soldered joints because coplanarity of the mating surfaces is compromised.
Recently, electrically conductive adhesives are becoming increasingly prominent in electronics packaging applications in large part because their ability to provide electrical interconnection without the need to subject the component to the harsh high-temperature environment of a oven reflow soldering process. Heat sensitive components that could be damaged during reflow process are being electrically interconnected by conductive adhesives. This type of electrical interconnection is not as desirable as traditional soldered joints and in addition increases cost.
In conclusion, the cost of manufacturing electronic products utilizing PCBs can be reduced and the quality and reliability of said products improved, if a better soldering process could be created to replace the oven reflow
A new soldering process to be more effective than the prior art, should heat the entire PCB substrate and its soldering pads (or lands) but only heat a portion of the mating leads (or terminations) extending out from electronic component casings. Therefore, allowing said casings and its internal parts to remain relatively cold. Such a novel soldering process would permit the elimination of all the disadvantages enumerated above.
When this inventor realized the urgent and long-felt need to create means to efficiently solder electronic components to a PCB without the necessity for heating the whole PCB assembly, the objectives and purposes of this invention were inspired, leading him to the conception and the accomplishment of this invention.
The general objective of this invention is to provide the electronic manufacturing, or electronic packaging, industry with a new, safe, reliable, speedier, useful and, above all, a more economical process and means for soldering components to a PCB.
Because this invention only heats the component's leads or terminations to be joined by solder to a PCB while the rest of said component (namely its casing or housing) remains relatively cold, utilization of this invention will help to reduce manufactured-product cost because components rated to tolerate much lower temperature exposure (than now required by the oven reflow soldering process) cost less. This invention eliminates the need to de-moisturize certain components, for example the requirement set by the Joint Electronic Devices Engineering Council (JEDEC) to bake plastic BGAs at 125° C. for 24 hours prior to reflow is eliminated.
The invention can readily be integrated into conventional automated assembly equipment increasing their yield. This invention also allows to reduce the required manufacturing floor space since the traditional reflow oven is eliminated form the assembly line. Further objectives and advantages of the invention will become apparent from a consideration of the drawings and following descriptions.
Underlined numerals designate either an assembly or a group of parts. Notes in parentheses help to identify to which assembly the preceding part belongs, the working or function of the preceding part or the relationship of the preceding part to a method or process.
The parts cited in the following description are:
All throughout the specification, the abstract, the appended claims and figures some or all of the following acronyms or abbreviations are used;
This invention discloses a novel PCB that incorporates an electric heater embedded into its dielectric substrate. The heater, in due time, becomes the exclusive source of heat required to attain a self-reflowing soldering process rather than utilizing reflow oven equipment as customarily done nowadays by the electronic assembly industry. Multiple electronic components are simultaneously soldered to said PCB face when the embedded electric heater is supplied with an adequate electric current.
Specifically this invention; (1) improves the quality of electronic assemblies because the casing of the electronic components being soldered are not heated and, (2) reduces the cost of soldering electronic assemblies because it requires fewer manufacturing equipment and consumes less energy when compared to the prior art. As a result, electronic products assembled around this novel PCB should become of better quality, more reliable and its overall manufacturing cost reduced.
Now, aided by
Notice that the term—oven reflow soldering—refers to the customary process utilized by the prior art for obtaining soldered joints, consisting of baking a PCB assembly inside a reflow oven. The term—self-reflow soldering—refers to a process disclosed by this invention for obtaining soldered joints, consisting of supplying an electric current to a PCBwEEH instead of baking the PCBwEEH assembly inside a reflow oven.
In operation, during self-reflow soldering, an electric current of predetermined intensity is supplied to said leads 20-20 for a predetermined time duration in order to heat up said sets of soldering pads 12 to a predetermined temperature level sufficiently high to cause melting of any type of solder material (i.e. solder paste, SSD, flux-cored solid solder, dry solder powder, etc.) deposited on said sets of soldering pads 12. Said electric current is interrupted when it becomes necessary to allow solidification of said solder material in order to attain soldered joints.
Depending on the application it may be desirable that the heat transferred from heater 14 be directed, or concentrated, into predetermined areas of top face 22 as illustrated in
In other application it may be desirable to have embedded, on the same plane, two separate electric heaters as illustrated in
Still other application may require a single electric heater 14 capable of generating more intense heat underneath soldering pads and less intense heat over the remainder of the board as illustrated in
Notice that leads 20-20 and 26-26 don't have to extend out of PCBwEEH 10 substrate as depicted in
Although
This feature becomes useful when self-reflow soldering double-sided PCB assemblies populated by heavy components on both sides. First top face 22 is self-reflow soldered using heater 14 then the board is flipped, components are placed on bottom face 30 (now facing upward) and self-reflow soldered using heater 28. Notice that during operation when face 30 is being heated the temperature level on the opposite face 22 (now facing downward) is lower than the level required for soldered joints to remelt.
Notice that, although in the figures only surface mounted components are depicted, however, through-hole mounted components and straddle mounted components are equally soldered according to this invention.
From the above description the reader can appreciate the unobvious novelty disclosed, specifically the ability to eliminate the traditional reflow oven from a PCB soldering operation, a capital equipment cost reducing solution. With this invention PCB assemblies can be self-reflow soldered in an open-air environment. In addition, the invention offers other important advantages that are described below with the aid of
When oven reflow soldering a PCB assembly according to the prior art the entire assembly is heated including the casing (and its internal parts) of every electronic component being soldered. Because of this harsh heating cycle components must be rated to survive reflow temperatures thus unnecessarily increasing their cost. However, as explained in the next paragraph, when applying this invention only the leads being soldered to a pad are heated.
Referring to
Referring now specifically to the entirety of this invention, a simple embodiment intended for self-reflow soldering a PCB assembly is shown in
In operation, a predetermined electric current is supplied to leads 20-20 via feed thru 42-42. The current heats up embedded electric heater 14 causing the solder paste, or if applicable the SSD, covering each soldering pad 12 to melt into individual lumps. Once every molten solder lump wets a pad 12 and the corresponding mating lead (or termination) that is part of electronic components 32 the electric current is interrupted to allow solder solidification to take place.
Concurrently, once the solder paste, or if applicable the SSD, covering each soldering pad 12 is melted, workholder 54 is made to vibrate under the action of vibration inducer means 46. The vibrations are transmitted from workholder 54 to every molten solder lump consequently; (1) facilitating the venting of any entrapped gas or vapor resulting from the heating of the solder paste and (2) enhancing the wetting action of molten solder upon pads 12 and leads. Venting produces void-free solder joints. The vibration is ceased after supplied current is interrupted. Identical results are attained if the vibrations are substituted by an ultrasound field generated inside enclosure chamber 48.
Subsequently enclosure 48 is removed allowing the soldered PCBwEEH 10 to cool down either naturally or by a forced flow of ambient air impinging on its upper surface. Subsequently soldered PCBwEEH 10 is removed from work holder 40 restoring the device for the next self-reflow soldering operation.
Before presenting the next embodiment it is useful to explain, with the aid of
Notice that the term—self-reflow—refers to a process disclosed by this invention for attaining molten solder lumps on top of a PCBwEEH consisting of supplying and electric current to said PCBwEEH when its pads are covered with any type of solder material (i.e. solder paste, SSD, flux-cored solid solder, dry solder powder, etc.)
A third embodiment of this invention intended for automated self-reflow soldering PCB assemblies is shown in
In operation, one empty PCBwEEH (with its plurality of soldering pads facing upward) is sequentially furnished from the supply of empty PCBwEEH 56 to paste printing station 58 where an appropriate amount of solder paste is deposited over each soldering pad. The pasted PCBwEEH is then moved to self-reflow station 60 and placed at the top of stack 62, simultaneously from the bottom of the stack 62 an already self-reflowed PCBwEEH is moved into components placement station 64. As soon as a pasted PCBwEEH enters stack 62 it is supplied with a predetermined electric current from controllable electric current source 74. As the pasted PCBwEEH moves down the stack it is being continuously heated by said predetermined electric current which can be made variable in intensity as the stack moves down in order to attain a desirable heating rate of the solder paste. When a pasted PCBwEEH reaches the bottom of the stack all the solder paste has already melted forming a multiplicity of molten solder lumps covering said plurality of soldering pads facing upward.
One PCBwEEH with said multiplicity of molten solder lumps is moved from the bottom of stack 62 into components placement station 64 where pick-and-place machine 66 sequentially places electronic components in predetermined locations such as that their leads become in deep contact with a predetermined set of molten solder lumps. During components placement a predetermined current is once again supplied from controllable electric current source 74 to PCBwEEH in order to prevent premature solidification of said molten solder lumps. Once the last electronic component is placed the current is interrupted to allow solder solidification to take place thus forming soldered joints. Finally the soldered PCBwEEH assembly is moved to cooling station 70 where it is cooled down by said cooling gas stream 72 impinging on its face.
During continuous operation while one empty PCBwEEH is furnished from the supply of empty PCBwEEHs 56 a completed soldered assembly emerges at cooling station 70. In the process described above the pick-and-place machine 66 can be replaced by a manual method of placing electronic components without departing from the spirit and scope of this invention.
Stack 62 can be contained inside a stack enclosure 78 in order to prevent heat losses and to contain vapors and/or gases generated during operation that may cause occupational health hazards and/or safety risks. Stack enclosure 78 includes means not shown to remove, in an environmentally safe manner, said vapor and/or gases, and also includes controllable inlet means and outlet means not shown for supplying an inert gas environs 80 (such as Nitrogen, Argon, etc.) in order to prevent oxidation off said multiplicity of molten solder lumps covering said plurality of soldering pads facing upward. Similarly, the placement of components by pick-and-place machine 66 can be performed inside an inert gas atmosphere 68 to prevent oxidation of said multiplicity of molten solder lumps.
The number of pasted PCBwEEHs to be included into stack 62 can be approximately determined by dividing the self-reflow time required for properly achieving said multiplicity of molten solder lumps by the cycle-time utilized by automated pick-and-place machine 66 to place on a PCBwEEH all the electronic components required. The optimum self-reflow time required for forming molten solder lumps depends on the particular application and must be determined empirically, however, as a guide it should be assumed to be in the order of a few minutes. This rather long heating cycle is imposed by the need to prevent boiling of the flux contained into the solder paste, flux boiling results in paste splatter. On the other hand, the cycle-time utilized by automated pick-and-place machine 66 is in general a fraction of one minute. Consequently, stack 62 is required to avoid that pick-and-place machine 66 would have to stay idle, between components placement cycles, wastefully waiting for the arrival of a new board covered with a multiplicity of molten solder lumps.
Notice that—solid solder deposit (SSD)—refers to a relatively thick layer of solid solder metallurgically bonded over the soldering pads of a bare-PCB. Bare-PCB refers to a PCB that would require deposition of solder paste prior to components placements for a subsequent reflow soldering operation. And—solid solder deposit-printed circuit board (SSD-PCB)—refers to a PCB with its soldering pads covered by SSDs. Therefore, a SSD-PCB provides by itself, in solid form, adequate amounts of solder fused to its soldering pads for reflow soldering components on it.
When the supply of empty PCBwEEHs 56 consist of SSD-PCBs all their pads are already covered with a SSD. Therefore there is no need to deposit solder paste because the solder is provided in solid form fused to the pads. In this case the solder paste printing station 58 is bypassed, see in
When a PCBwEEH of the SSD-PCB type is utilized (since there is not possibility for splatter), the optimum cycle-time required for forming a multiplicity of molten solder lumps could be about equal or even shorter than the cycle-time utilized by automated pick-and-place machine 66 to place all the electronic components required on a PCBwEEH. In this specific case it would not be necessary to include stack 62 into self-reflow station 60 since it will be possible to quickly heat only one board at the time.
Notice that after a soldered PCBwEEH is completed its embedded electric heater remains inside its substrate. Should, after testing the assembly for proper functionality it is detected a damaged electronic component or defective soldered joints, rework may be required. In this case the embedded electric heater becomes very useful for performing a safe repair operation.
The component to be re-soldered or removed is temporary covered with an air shield 84 as shown in
Repairing a PCB according to the prior art requires that the entire component to be removed or re-soldered be heated by a localized stream of hot air, a harsh process equivalent to oven reflow soldering. Therefore a PCBwEEH offers a harmless repair process.
Accordingly, the reader should notice that this invention is a truly innovative one that provides the electronic packaging industry with; (a) a new, safe and reliable PCBwEEH capable of self-reflow soldering operation and, (b) novel and useful processes for assembling electronic components.
Since the disclosed PCBwEEH and its associated processes do not require the cooperation of an external heat source, i.e. a reflow oven, as the prior art does, the utilization of this invention offers the following advantages when compared against the prior art:
Although the above description contains many specificities, these should not be construed as limiting the scope of the invention but as merely providing illustrations of some of the presently preferred embodiments of this invention. Any compositions or methods which are functionally equivalent are within the scope of this invention. Indeed, from the forgoing description, various other variations and structural changes will become apparent to those skilled in the art without departing from the spirit and scope of this invention.
Accordingly, the scope of this invention should be determined by the appended claims and their legal equivalents, rather than by the embodiment illustrated.
