Patent Application
20100147928
In the rework of printed circuit boards, bottom termination leadless devices such as QFNs, LGAs and MLFs must from time to time be removed and replaced by new devices on a populated PCB. Alternatively, removed devices may need to be recaptured and be re-attached. In both cases the attachment/reattachment of the leadless device is important.
In a typical leadless device rework process the pads on the board are paste printed, the device placed and the device reflowed. After component removal, the sites on the PCB are “dressed”, that is the solder is removed from the pads with a solder removal tool or more commonly with solder wick. This is followed by a cleaning process. A miniature metal stencil replicating both the patterns and the thickness of that used in the initial manufacturing process is used to define the area on the PCB where solder paste is to be applied. The solder paste is “rolled” through the apertures in the stencil using a handheld squeegee. Upon stencil removal, the replacement device is placed onto the correct area and reflowed. Typically a split vision system, found on higher end rework equipment, is used to accurately place the device onto the PCB and selectively reflow the device.
In another leadless device rework process, the pads on the device are paste printed and solder “bumps” created, the pads on the PCB fluxed, the device placed and reflowed. In this method, the replacement device, whether new or existing, has the solder pads “bumped” using either a dispense system or via a miniature stencil. After the site on the PCB is “dressed”, the pad locations are fluxed and the device is placed by hand or with a split vision rework system and reflowed into place.
In a third method, after the ground slug of the device is “bumped” and the device is “tacked down” to the board, each of the peripheral pads are then hand-soldered into place. After the device is removed, the bottom side pads of the device have the solder removed and cleaned as in the previous methods. The ground connection then has enough solder flowed onto it such that the overall height of the device off the board is the same as it was prior to its removal. The device is flipped over and the leadless device is “tacked” into place with a hot air source, typically a heat gun. Assuming that there is enough room between the IO pads of the device and the board, someone skilled in hand soldering can then make these solder connections.
In even another method, programmable dispensing equipment is used to apply solder paste selectively on the various pads on the printed circuit board and the device is then placed and reflowed. In particular, the PCB site location is first prepared as previously described. A programmable solder paste dispensing system is then used to dispense solder paste onto the pads on the PCB. This is followed by the placement of the leadless device package onto the PCB with a split vision rework system. Lastly, the device is reflowed according to the solder manufacturers' specifications.
While there are several methods for the removal and replacement of leadless devices, there continues to be the need for a faster, simpler method for the replacement of leadless devices that does not require the use of either highly skilled soldering technicians or high-end rework equipment. The use of these leadless devices, especially as the density of the boards upon which these package types are found increases, continues to proliferate. Therefore, a process by which the less skilled rework technicians could rework the devices would be advantageous.
An exemplary manual rework process for leadless device packages according to the invention which allows for the replacement of such devices is described herein and shown in the Figures. The process uses a matched pair of polyimide stencils to allow a solder “bumped” leadless device to fit into the apertures of a stay-in-place polyimide stencil affixed to a printed circuit board (PCB) or flex circuit. As used herein, the term printed circuit board or PCB is intended to include both printed circuit boards and flex circuits.
First, a leadless bottom terminated package device is removed from a PCB with a heat source such as a hot air gun, rework station or other heat source. Both the device as well as the PCB lands are then dressed such that excess solder is removed from the lands and cleaned. A polyimide stencil designed to fit over the pads of the leadless device (based on the mechanical device specifications or the actual pad dimensions) is placed onto the bottom side of the device. The adhesive on the stencil is then activated by pushing the stencil onto the bottom of the device. Solder paste is then “rolled” into the apertures. The device is then reflowed.
Upon cooling, the stencil is removed and the bottom of the device is then cleaned. The matched polyimide stencil with apertures designed to accept the “bumps” on the bottom of the device is then aligned, placed and adhered onto the PCB. Solder paste is “rolled” into these apertures with a miniature squeegee and the surface of the stencil is cleaned to remove any solder paste from the top surface. The “bumped” device is then “fitted” into the apertures and the device is reflowed using a heat source.
In some cases, the board side stencil may not be required and the device pads or other electrical contact pads can be simply “bumped” as described above, flux applied to the PCB and the device reflowed. In such cases, no board stencil is necessary.
The present invention comprises a manual rework method for leadless device packages 10, such as shown in
The mated pair of stencils 12, 14 can be cut from a single polyimide element that can have several unique characteristics. In particular, the stencils can be an approximately 0.20 to approximately 0.55 mm thick polyimide material with an adhesive backing. The polymide material can be configured with properties that enable it to be laser machinable and to withstand reflow temperatures (e.g., very good thermal stability properties). The adhesive can be pressure-sensitive. The adhesive can be configured to exhibit excellent adhesive properties when activated and adhered to a PCB laminate material even through extended reflow profiles where the temperatures can exceed 270° C. for several minutes.
The first of the stencils 12 is designed to be used in selectively applying solder paste 18 onto the bottom side of the device 10. To this end, the stencil 12 has a pattern of apertures that matches the pattern of the contact pads or areas on the device 10. For example, the leadless device 10 is first removed and the pads or contact areas 16 properly dressed (see
The second of the stencils 14 has apertures designed to match the pattern of contacts or pads 24 on the printed circuit board 11 and into which the “bumps” 22 on the device 10 can fit (see
The stay-in-place or printed circuit board stencil 14 and/or the leadless device stencil 12 can have several characteristics including one or more of the following:
Another aspect of the invention lies in the fabrication of the matched set of stencils 12, 14. The fabrication method can consist of laser cutting holes into a high temperature non-wettable sheet backed with a (polyimide) thermo pressure adhesive sheet in order to create the correct aperture pattern. Due to the ever-declining pad sizes and increasing IO density, a tool with this type of cutting hole accuracy may be required.
One advantage of the invention is that the placement of the leadless device package 10 is simplified as the user can “feel” the device settle into the apertures of the stencil 14 that is permanently affixed to the printed circuit board 11. The “bumps” 22 created in the process allow users to tactilely fit the bumped device into the corresponding apertures of the stencil 14 on the board 11. This allows the user to hand place the devices without the use of high end rework equipment equipped with a split vision system.
Another advantage of the invention is that the stay-in-place stencil 14 can act as a means for repairing damaged solder mask areas on the printed circuit board 11. There are cases, where upon removal of the device package from the printed circuit board assembly the mask is damaged. Standard procedures call for the mask to be repaired by highly skilled repair technicians working by hand underneath a microscope to “touch up” and repair the mask. This can be a time consuming and lengthy process. The stay-in-place stencil 14 can act as a replacement mask that eliminates the need to repair each of the individual areas by hand.
Yet another advantage of the invention is that the stencils 12, 14 provide a built-in spacer which prevents excessive solder paste from collapse while providing a minimum spacing between the device 10 and the PCB 11.
Another advantage of the invention is that the stencil 14 on the printed circuit board 11 can act as an insulating barrier between solder connection points in that the stay-in-place stencil providing a mask between adjacent solder connections thereby increasing placement reliability.
Another advantage of the invention is that the stenciling technique can enable a technician to rework a warped device or a device that will be placed onto a warped PCB. Since the stencil is held in intimate contact with the PCB or device when the solder paste or flux is applied, any lack of co-planarity in the surface can be overcome.
Another advantage of the invention is that the non-wettable permanently adhered stencils can maintain high surface insulation resistance values as the stencil material can act as a dielectric media between the pads.
A manual method for reattachment of leadless device packages, more commonly described as QFNs, LGAs and MLF package types, is disclosed. This process allows for devices to be reused after they have been removed from the PCB or for the placement of a new device onto a populated PCB.
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 | |
|---|---|---|---|
| 61121358 | Dec 2008 | US |
