Patent Application
20110132972
The present invention relates to a method for the manual reattachment of solder balls to area array devices using a solder preform, and more particularly to a rework process for reattaching solder balls onto area array devices which have been removed from a circuit board or require reattachment of different metal alloy solder balls.
In the rework of printed circuit boards, area array devices must from time to time be removed and replaced. In doing so, the removed devices may need to be refurbished and have new balls attached. There are also times when, due to the density of the interconnections, the complexity and high cost of the removed area array device, re-use of the device becomes an important cost consideration. In other cases due to customer requirements, area array devices may require solder balls made of a different alloy. As with a reworked component, in such a case, the alternate alloy solder balls have to be attached to the land areas of the area array device. In each of these cases, the process by which the solder balls are reattached to the device is an important consideration.
In a typical manual solder reballing process, the solder balls are attached on the component pads by screening flux paste or gel on the underside of the component. Preformed solder balls are then forced or shaken into multiple stencil apertures which are aligned with the component pads. The stencil allows the balls to drop into the paste or gel only where pads exist. When the stencil is removed, the balls are held in place by the paste or gel. The assembly is then sent through a reflow source such as an oven to allow the balls to reflow and form an intermettalic layer with the device pads. Upon completion of the heating, the balls are attached metallurgically to the copper pads on the substrate, completing the connection of the integrated circuit package to the printed circuit board. Alternatively, balls having a higher melt temperature solder may be bonded to the pads with the addition of a higher reflow temperature solder paste. These techniques require highly experienced technicians with great dexterity to obtain a reasonable yield in a reasonable period of time.
In other manual methods such as described by Swamyet et al (U.S. Pat. No. 5,392,980), a combination of mechanical fixtures are used to remove components. In the Swamyet method, the device to be reworked is held in an inverted position in the fixture with a stencil overlaid on the device. Solder paste is then pressed through the stencil apertures and the device is reflowed. The properties of the eutectic solder minimize its surface area causing round solder “bumps” to form and balls to remain on the bottom of the device. In the Alghouli method (U.S. Pat. No. 6,769,596), mechanical fixtures are used to hold the solder balls, align the stencil over the device and hold the device into the fixture. In both approaches, extensive tooling and/or fixtures are required in order to reattach the balls onto the device. Thus, these approaches are uneconomical for low quantity area array device rework or where a quick turnaround time is important.
Alternately, a water-soluble paper preform, the manufacturing method for making which is described in Cairncross et al (U.S. Pat. No. 5,356,751), can be used for manual reballing. The paper preform is punched with hole patterns and balls are imbedded such that they are aligned with the pad patterns of the device to be reballed. This type of preform has sped up and simplified the reballing process. However, this technique is plagued by issues associated with the transport and handling of the preforms during which the balls become loose. Moreover, the non-planar nature of the paper preform at times does not allow all balls to adhere to the device pads. There is also limited availability of such preforms for fine-pitched, small ball diameter devices. Lastly, users are inconvenienced by the cleaning of the paper remnants from the preform after reflow and removal.
Accordingly, there continues to be a need for a faster, more reliable method for manually replacing solder balls. The known processes, particularly as the size of the solder balls continues to shrink and the number of I/O on a package grows, require a person of very high skill level. A process by which less skilled rework technicians could rework the devices would be advantageous.
The present invention comprises a manual rework process for area array packages which either have been previously removed from a printed circuit board assembly or require the reattachment of a solder ball made of a different alloy. The method uses a single-use solder preform comprising a polyimide fixture with an integral adhesive backing which holds the replacement solder balls in place at the correct position. After the balls have been reflowed and mechanically attached to the device being reballed, the entire preform is “peeled” away.
The solder preform generally includes three elements. The first element is a relatively thick fixture having apertures corresponding to the area array pattern of the device being reworked. The fixture can be made of a polyimide material. The second element of the preform is a high temperature tape, which can also be polyimide-based. The tape is attached to the bottom surface such that the adhesive of the tape faces into and forms the bottom of the apertures of the fixture. The last element of the preform is the solder balls that fit into the apertures of the fixture.
A further aspect of the present invention comprises a method by which the above-described preform can be used for reworking an eutectic area array package. This method includes removing the solder balls from the existing device followed by site preparation. Paste flux, designed to withstand the rework process times of area array devices, is then applied to the bottom of the devices. Next, the device is aligned and set on top of the single use preform. The device is then reflowed using a time-temperature profile consistent with the alloy of the solder balls. After reflow, the preform is peeled off leaving the solder balls attached to the pads of the device. Finally, the device is cleaned and inspected.
According to another aspect of the invention, a method for the manual reworking of high temperature alloy solder balled devices is provided. This method also comprises removing the existing solder balls from the device followed by site preparation and cleaning. The solder preform is loaded with the high temperature alloy solder balls and then affixed on a heat-resistant planar plate. Next, solder paste is printed on the pads of the device to be reworked. The preform is then mechanically matched to and aligned with the preprinted pads of the device and placed onto the device. The device is then reflowed using a time-temperature profile consistent with the solder balls being used. After reflow, the solder preform is “peeled off” leaving the solder balls attached to the pads of the device. Lastly, the device is cleaned and inspected.
Another aspect of the invention comprises a method for fabricating the solder preform. The method includes the step of laser ablating apertures into a polyimide sheet in order to create the correct aperture pattern. After the fixture is fabricated, an engineered polyimide tape having a release liner and liquid absorption properties is affixed with the adhesive side facing into the apertures. The solder balls are then loaded onto the apertures. The solder balls are pressed into place via a planar plate which extends generally parallel to the surface supporting the underlying fixture. The balls are then inspected to ensure that their uppermost points all fall within a single, common plane.
Referring now more particularly to the drawings, an exemplary fixture 100 used in the preform of the present invention to locate and hold a plurality of solder balls in place is shown in
As shown in
The adhesive layer 202 should be formulated to enable the tape 204 to adhere to the bottom side of the fixture 100 and to retain the solder balls in their respective apertures 104 even when the preform is inverted. Additionally, the adhesive layer 202 should be able to maintain its adhesion properties even at the elevated temperatures of the reflow process. Likewise, the carrier 203 also should be thermally stable to at least 275° C. for 90 seconds in order to be able to withstand the reflow cycles. The tape 204 also should be capable of absorbing liquids such as paste fluxes.
The final preform assembly 300 with the plurality of solder balls 301 inserted is shown in
The fixture 100 preferably has a thickness 103 such that at least 60% of the diameter of the solder balls 301 is covered when the balls are in the apertures 104. The apertures 104 of the fixture 100 should be configured such that the solder balls 301 can be freely placed into the apertures 104. However, the fit between the solder balls 301 and the apertures 104 should not be so loose that the balls are off-center relative to the pads on the device being reballed when the preform is used. According to one embodiment, the apertures 104 can have a generally trapezoidal shaped profile that helps ensure that the solder balls 301 are readily held in place in the fixture 100. With apertures 104 having such a configuration, the solder balls 301 are loaded into the fixture 100 from the larger side of a trapezoid. The apertures 104 can be shaped in this fashion due to the nature of the laser ablating process.
The solder balls 301 can be manufactured to industry standard sizes and tolerances and are available in a variety of different alloy types. As noted, during the loading of solder balls 301 into the preform it is important that the uppermost points of the solder balls 301 relative to the surface of the fixture 100 be generally arranged in a single plane and remain so even after shipping to the end user. This helps ensure that upon their attachment to the pads of the device being reballed, each of the solder balls 301 is equidistant from the bottom of the respective ball to the bottom of the device.
An exemplary method for fabricating the solder preform 300 is shown in the flowchart of
Next, the device 402 and the solder preform 300 can be aligned as shown in
Preparation of the device to be manually reballed is shown in
Next, the solder preform 300 is aligned to the device is shown in
From the foregoing, a manual method for reattachment of solder balls for area array devices is disclosed. This process allows for devices to be reused after they have been removed from the PCB or for the replacement of the solder alloy. An advantage of the present invention is that the amount of dexterity required for the manual reballing process is less than other known techniques. This is particularly true when compared to a manual stencil process using an overlay stencil. Such a process can be very tedious given that the solder balls can have diameters of 0.38 mm (15 mils) or less. Such small solder balls are difficult to handle and up to several thousand solder balls need to be loaded into the stencil.
A further advantage of the invention is that the time required for the manual reballing of the device is greatly reduced as compared to the manual stenciling method. The time necessary for the user to load the solder balls into the stencil is eliminated as the solder balls are preloaded into the solder preform. Another advantage of the invention is that all the design and fabrication of the fixtures and stencils is borne by the manufacturer of the preform, instead of being done by the end user. The manufacturer of the solder preforms ensures that the correct stencil geometries and thicknesses are engineered, not the end user. This saves the end user the engineering time required for developing the fixtures and stencils.
All references, including publications, patent applications, and patents, cited herein are hereby incorporated by reference to the same extent as if each reference were individually and specifically indicated to be incorporated by reference and were set forth in its entirety herein.
The use of the terms “a” and “an” and “the” and similar referents in the context of describing the invention (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms “comprising,” “having,” “including,” and “containing” are to be construed as open-ended terms (i.e., meaning “including, but not limited to,”) unless otherwise noted. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention.
Preferred embodiments of this invention are described herein, including the best mode known to the inventors for carrying out the invention. Variations of those preferred embodiments may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventors expect skilled artisans to employ such variations as appropriate, and the inventors intend for the invention to be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context.
| Number | Date | Country | |
|---|---|---|---|
| 60934862 | Jun 2007 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 12138901 | Jun 2008 | US |
| Child | 13022226 | US |
