Patent Publication Number: US-7718025-B2

Title: Method of forming folded-stack packaged device using progressive folding tool

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
   This is a Divisional Application of U.S. patent application Ser. No. 10/261,335, filed Sep. 30, 2002 now U.S. Pat. No. 7,089,984. This Divisional Application claims the benefit of the U.S. patent application Ser. No. 10/261,335. 

   BACKGROUND 
   1. Field 
   Embodiments of the invention relate to the field of packaging, and more specifically, to folded-stack packaging. 
   2. Background 
   Chip scale technology offers many advantages in electronics packaging. One emerging packaging technique in chip scale technology is micro ball grid array (μBGA) packaging. μBGA provides the smallest size, highest performance, and best reliability of currently available packages. Folded-stack (fs) μBGA further improves board density and reliability. 
   Existing techniques for folded-stack packaged devices are typically manual, requiring boat-to-boat handling from singulation process to cure process. These techniques have a number of drawbacks. First, the process is slow and cumbersome. The packaged device units are processed through a number of discrete operations: saw singulation, first boat handling, folding and adhere, second boat handling, folding and curing, and then traying. Second, it is expensive because several components (e.g., jigs) are needed. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may best be understood by referring to the following description and accompanying drawings that are used to illustrate embodiments of the invention. In the drawings: 
       FIG. 1  is a diagram illustrating a micro ball grid array (μBGA) packaging layout according one embodiment of the invention. 
       FIG. 2  is a diagram illustrating a folded-stack μBGA packaged device according to one embodiment of the invention. 
       FIG. 3  is a diagram illustrating a tool assembly according to one embodiment of the invention. 
       FIG. 4A  is a diagram illustrating a first folding phase for the μBGA packaged device according to one embodiment of the invention. 
       FIG. 4B  is a diagram illustrating continuation of the first folding phase for the μBGA packaged device according to one embodiment of the invention. 
       FIG. 4C  is a diagram illustrating a second folding phase for the μBGA packaged device according to one embodiment of the invention. 
       FIG. 4D  is a diagram illustrating continuation of the second folding phase for the μBGA packaged device according to one embodiment of the invention. 
       FIG. 5  is a flowchart illustrating a process to fold the μBGA packaged device according to one embodiment of the invention. 
       FIG. 6  is a diagram illustrating a packaging assembly line for folded-stack μBGA packaged devices according to one embodiment of the invention. 
       FIG. 7A  is a diagram illustrating a first unit trim station in the packaging assembly line shown in  FIG. 6  according to one embodiment of the invention. 
       FIG. 7B  is a diagram illustrating an adhesive application station in the packaging assembly line shown in  FIG. 6  according to one embodiment of the invention. 
       FIG. 7C  is a diagram illustrating a first folding and second unit trim station in the packaging assembly line shown in  FIG. 6  according to one embodiment of the invention. 
       FIG. 7D  is a diagram illustrating a second folding station in the packaging assembly line shown in  FIG. 6  according to one embodiment of the invention. 
       FIG. 7E  is a diagram illustrating a tape de-dambar and pick and place station in the packaging assembly line shown in  FIG. 6  according to one embodiment of the invention. 
       FIG. 8  is a diagram illustrating a top view of a packaging assembly line for folded-stack μBGA packaged devices according to one embodiment of the invention. 
       FIG. 9  is a flowchart illustrating an assembly process for folded-stack μBGA packaged devices according to one embodiment of the invention. 
   

   DESCRIPTION 
   An embodiment of the present invention includes a plunger, a heating element, and first and second arms. The plunger affixes a first unit to a second unit with adhesive. The first and second units are on a strip of a flexible tape. The strip is on a folding base unit. The folding base unit folds the first unit on top of the second unit. The heating element is attached to the plunger to cure the adhesive. The first and second arms are positioned on first and second sides of the plunger respectively, to secure the first and second units underneath the plunger. Another embodiment of the invention includes a first sub-assembly and a second sub-assembly. The first sub-assembly supports a first unit. The first sub-assembly, when activated, folds the first unit on top of a second unit. The first and second units are on a strip of a flexible tape. The second sub-assembly supports the second unit. 
   In the following description, numerous specific details are set forth. However, it is understood that embodiments of the invention may be practiced without these specific details. In other instances, well-known circuits, structures, and techniques have not been shown in order not to obscure the understanding of this description. 
   One embodiment of the invention may be described as a process which is usually depicted as a flowchart, a flow diagram, a structure diagram, or a block diagram. Although a flowchart may describe the operations as a sequential process, many of the operations can be performed in parallel or concurrently. In addition, the order of the operations may be re-arranged. A process is terminated when its operations are completed. A process may correspond to a method, a program, a procedure, etc. 
     FIG. 1  is a diagram illustrating a micro ball grid array (μBGA) packaging layout  100  according to one embodiment of the invention. The layout  100  includes a tape carrier  120 , and a flexible tape  130 . 
   The tape carrier  120  is typically a metal frame to carry a number of packaged devices affixed on the flexible tape  130 . The flexible tape  130  is a rectangular tape that can be flexibly folded. The flexible tape  130  may be a double-sided polyimide tape (e.g., Kapton or Upilex) having a thickness of about 10 μm to 75 μm. The packaged devices are affixed on the flexible tape  130  in a number of rows and columns. In one embodiment, there are three rows and N columns. Typically values of N are 16, 20, 24, 32, etc. 
   Each column corresponds to a strip of the flexible tape  130  and includes three units: the first unit  140   1 , the second unit  140   2 , and the third unit  140   3 . Each of the units includes a die affixed to a corresponding portion of the flexible tape  130 . For example, the first unit  140   1 , the second unit  140   2 , and the third unit  140   3  include a first die, a second die, and a third die, respectively, affixed to a first portion  135   1 , a second portion  135   2 , and a third portion  135   3 , respectively, of the strip. Each side of the flexible tape  130  may have a metal layer that has leads and/or interconnections between the die. The die may be any semiconductor chip or devices such as memory device, flash memory, static random access memory (SRAM), dynamic random access memory (DRAM), logic devices, processing elements, etc. The die are affixed to the flexible tape  130  with die-attaching materials based on silicon or epoxy. The die may also be lead-bonded and encapsulated. The die are attached to the strip by solder balls. 
   The packaged devices on the flexible tape  130  are packaged and processed according to traditional packaging techniques. In one embodiment, the packaging technique is the μBGA. The packaged devices on each strip or column are folded in a stacking manner to form a folded-stack μBGA. In the following description, for illustrative purposes, the packaged device will be referred to as a folded-stack μBGA. It is contemplated that the packaged device may use any suitable packaging techniques. 
     FIG. 2  is a diagram illustrating a folded-stack μBGA packaged device  200  according to one embodiment of the invention. 
   The folded-stack device  200  includes first, second, and third die  210   1 ,  210   2 , and  210   3 , respectively, and the flexible tape  130 . As discussed above, the first, second, and third die  210   1 ,  210   2 , and  210   3  are attached to the flexible tape  130  via solder balls  210 . The solder balls  210  are micro grid array of soldering materials attached to the pads on the flexible tape  130 . The flexible tape  130  is folded in two folds. In the first fold, the first die  210   1  is positioned such that its surface is affixed to the surface of the second die  210   2  by an adhesive. The affixed first and second die  210   1  and  210   2  form a partially folded unit. In the second fold, the third portion of the flexible tape  130  attaching to the third die  210   3  is folded on top of the partially folded unit. The surface of the third die  210   3  is affixed to the bottom side of the first portion of the flexible tape  130  by adhesive. 
   The folded-stack μBGA packaged device  200  as formed is compact and provides high density and high reliability on a printed circuit board. The process to fold the μBGA packaged device  200  is performed in a progressive manner in an assembly line that includes several stations. The folding process is efficiently performed by using a tool assembly for folding, affixing, and curing. 
     FIG. 3  is a diagram illustrating a tool assembly  300  according to one embodiment of the invention. The tool assembly  300  includes a plunger assembly  310  and a folding base unit  360 . The tool assembly  300  operates on a strip of the flexible tape  130  (shown in  FIG. 1 ) which carries the first, second, and third units  140   1 ,  140   2 , and  140   3 , respectively. 
   The plunger assembly  310  includes a panel  315 , a plunger  320 , a heating element  330 , and two arms  342  and  344 . The panel  315  is a rectangular or square plate of suitable material such as metal. The panel  315  has a hole located in the middle to allow the plunger to travel up and down. The plunger  320  is a rod of sufficient length and is made of suitable material such as metal. The rod may be of any suitable shape such as cylinder. In the initial, or home position, the plunger  320  is retracted upward. The position shown in  FIG. 3  is when the plunger  320  is about half way down, or half way up. The plunger  320  may move upward and downward through the hole of the panel  315  under action of an activation unit (not shown). When forced to move downward, the plunger  320  affixes the first unit  140   1  to the second unit  140   2  with adhesive. The first unit  140   1  is folded to be on top of the second unit  140   2  by the folding base unit  360 . 
   The heating element  330  is attached to the plunger  320  at a distal end. The heating element  330  may also be integral to the plunger  320 . The heating element  330  is made of thermally conducting material such as metal or alloy. Heat may be generated by applying an electrical voltage across wires internal to the heating element  330 . The heating element  330  thermally cures the adhesive between the first and second units  140   1  and  140   2 . 
   The first and second arms  342  and  344  are positioned on the first and second sides of the plunger  320  around the panel  315  via first and second hinges  322  and  324 , respectively. They are used to align the first unit  140   1  when folded op top of the second unit  140   2  and when the third unit  140   3  is folded on top of the folded first unit  140   1 . The first and second arms  342  and  344  move around the first and second hinges  322  and  324 . The position shown in  FIG. 3  is an open position. In the initial, or home position, the first and second arms  342  and  344  are in a closed position when they are approximately vertical and directly facing to each other. A cam mechanism is used to mechanically connect the vertical movement of the plunger  320  and the swinging movement of the first and second arms  342  and  344 . When the plunger  320  extends downward, the first and second arms  342  and  344  swing open. When the plunger  320  retracts upward, they swing closed. When they move inward toward each other, they secure the first and second units  140   1  and  140   2  underneath the plunger  320 . This is typically done when the plunger  320  moves downward to press on the folded first unit  140   1  and the second unit  140   2 . The first and second arms  342  and  344  extend outward around the first and second hinges  322  and  324 , respectively, to release the first and second units  140   1  and  140   2  so that they can be transferred or moved to the next area. This can be done when the plunger  320  moves upward after affixing and curing the first and second units  140   1  and  140   2 . The first and second arms  342  and  344  may extend under some spring action that is activated when the plunger  320  moves upward. 
   The first and second arms  342  and  344  may also have first and second stoppers  352  and  354 , respectively, facing inward toward the plunger  320 . The first and second stoppers  352  and  354  act as a guide slot to secure units in place to be folded and affixed. For illustrative purposes, the second stopper  354  is shown to be above the first stopper  352 , just enough to hold down the folded units on the sides of the flexible tape and not touching the mold unit. The exact location of the stoppers  352  and  354  on the first and second arms  342  and  344 , respectively, depends on the thickness of the unit to be folded and affixed. 
   The folding base unit  360  provides support for the flexible tape  130  and the first, second, and third units  140   1 ,  140   2 , and  140   3 . The plunger assembly  310  is typically positioned to be directly above the middle row, or the second unit  140   2 . The folding base unit  360  also folds the first unit  140   1  on top of the second unit  140   2  to form a partially folded unit in a first folding phase. In a second folding phase, the folding base unit  360  folds the third unit  140   3  on top of the partially folded unit. Note that when used in two separate phases, there may be two separate tool assemblies  200 , one for the first folding phase and one for the second folding phase. The tool assemblies  200  for the two phases are almost identical. The differences may include the dimensions, the location of the stoppers  352  and  354 , the travel length of the plunger  320  (which may be adjusted), and the orientation of the folding base unit  360 , etc. Conceptually, therefore, the folding base unit  360  may be considered as a unit to fold a “first unit” on top of a “second unit”. In the first folding phase, the “first unit” is the first unit  140   1  and the “second unit” is the second unit  140   2 . In the second folding phase, the “first unit” is the third unit  140   3  and the “second unit” is the partially folded unit including the first unit  140   1  affixed to the second unit  140   2 . 
   The folding base unit  360  includes first, second, and third sub-assemblies  370 ,  380 , and  390  to support the first, second, and third units  140   1 ,  140   2 , and  140   3 , respectively. These sub-assemblies may be interconnected together by some interconnection mechanism. Alternatively, they may be integrated together in a single unit. 
   The first sub-assembly  370 , when activated, folds the first unit on top of the second unit. As noted above, when the folding base unit  360  is used in the first folding phase, the first unit to be folded is the first unit  140   1  and the second unit is the second unit  140   2 . When the folding base unit  360  is used in the second folding phase, it folds the third unit  140   3  on top of the partially folded unit. It should also be noted that in the first folding phase, the correspondence between the sub-assemblies and the units on the flexible tape  130  is reverse from that in the second folding phase. In the first folding phase, the first, second, and third sub-assemblies  370 ,  380 , and  390  support the first, second, and third units  140   1 ,  140   2 , and  140   3 . In the second folding phase, the first and second sub-assemblies  370  and  380  support the third unit  140   3  and the partially folded unit (formed by the first unit  140   1  affixed to the second unit  140   2 ). Normally, in the second folding phase, the third sub-assembly  390  is not required. 
   The first sub-assembly  370  includes a block  372  and a rocking mechanism  375 . The block  372  is an angles block having a slanted surface to provide a resting position for the first unit  140   1  in the first folding phase (or the third unit  140   3  in the second folding phase). In the following discussion, for clarity, references will be made to the first unit and the second unit without reference numerals. It should be understood that when used in the first folding phase, the term “first unit” refers to the first unit  140   1  and the term “second unit” refers to the second unit  140   2 . When used in the second folding phase, the term “first unit” refers to the third unit  140   3  and the term “second unit” refers to the partially folded unit. The inclination or slope of the slanted surface depends on the dimensions of the first unit  140   1  and the mechanical characteristics of the rocking mechanism  375 . In general the slope of the slanted surface is such that the folding action performed by the rocking mechanism  375  is facilitated. The steeper the slope, the less force the rocking mechanism  375  is exerted on the first unit. The block  372  has a hollow or vacuum space beneath the slanted surface to provide housing for the rocking mechanism  375 . The rocking mechanism  375  includes a rocking lever  365  and a cam  367 . The rocking lever  365 , when activated, causes the cam  367  to move or rotate to push the first unit from the resting position to fold the first unit on top of the second unit. 
     FIG. 4A  is a diagram illustrating a first folding phase  400  for the μBGA packaged device according to one embodiment of the invention. The first folding phase  400  includes a first act  410 , a second act  420 , a third act  430 , a fourth act  440 , and a fifth act  450 . 
   In the first act  410 , the strip of the flexible tape  130  with the three units  140   1 ,  140   2 , and  140   3  is on the folding base unit  360 . In addition, the first unit  140   1  has been punched out. The plunger assembly  310  is positioned directly above the second sub-assembly  380 . A first adhesive  425  and a second adhesive  427  are dispensed on the surface of the second unit  140   2  and the surface of the third unit  140   3 , respectively. 
   In the second act  420 , the rocking lever  365  is activated to rotate the cam  367 . The cam  367  pushes the first unit  140   1  out of the resting position to be folded on top of the second unit  140   2 . The first and second arms  342  and  344  help keeping the first unit  140   1  being bent or folded aligned with the second unit  140   2 . The plunger assembly  310  then moves down. 
   In the third act  430 , the plunger assembly  310  secures the partially folded unit with its two arms. The surface of the first unit  140   1  is in contact with the surface of the second unit  140   2 . The first adhesive  425  acts to glue the two units together to form a partially folded unit  435 . The plunger  320  of the plunger assembly  310  is then activated to move downward to apply sufficient force on the first unit  140   1  to affix the first unit  140   1  to the second unit  140   2 . The heating element  330  then generates heat to thermally cure the adhesive  425 . 
     FIG. 4B  is a diagram illustrating continuation of the first folding phase for the μBGA packaged device according to one embodiment of the invention. The continuation includes a fourth act  440  and a fifth act  450 . In the fourth act  440 , the plunger assembly  310  is moved upward while the plunger  320  is still down. In the fifth act  450 , the plunger  320  is retracted upward to the home position. This concludes the first folding phase. 
     FIG. 4C  is a diagram illustrating a second folding phase  455  for the μBGA packaged device according to one embodiment of the invention. The second folding phase  455  includes a first act  460 , a second act  470 , a third act  480 , a fourth act  490 , and a fifth act  495 . Note that in the second folding phase, the strip with the third unit  140   3  and the partially folded unit  435  has been transferred from the first folding phase to the folding base unit such that the third unit  140   3  rests on the slanted surface of the first sub-assembly  370  and the partially folded unit  435  is on the second sub-assembly  380 . The third unit  140   3  has been punched out. 
   In the first act  460 , the plunger assembly  310  is positioned directly above the second sub-assembly  380 . The second adhesive remains on the surface of the third unit  140   3 . In the second act  470 , the rocking lever  365  is activated to cause the cam  367  to rotate to push the third unit  140   3  toward the partially folded unit  435  such that the third unit  140   3  is folded on the bottom of the second portion of the strip to form a fully folded unit  475 . The plunger assembly  310  moves toward the partially folded unit  435 . In the third act  480 , the plunger assembly  310  secures the fully folded unit by the two arms. The plunger  320  then moves downward to exert sufficient force on the third unit  140   3  to affix the third unit  140   3  to the partially folded unit via the second adhesive  427 . The heating element  330  then generates heat to thermally cure the second adhesive  427  and the entire folded unit. After the third act  480 , the plunger assembly  310  moves upward and the two arms extend to release the fully folded unit. 
     FIG. 4D  is a diagram illustrating continuation of the second folding phase for the μBGA packaged device according to one embodiment of the invention. The continuation includes the fourth act  490  and the fifth act  495 . In the fourth act  490 , the plunger assembly  310  is moved upward while the plunger  320  is still down. In the fifth act  495 , the plunger  320  is retracted upward to the home position. This concludes the second folding phase. 
     FIG. 5  is a flowchart illustrating a process  500  to fold the μBGA packaged device according to one embodiment of the invention. Note that the process  500  is applicable for both the first folding phase and the second folding phase. Again, the terms “first unit” and “second unit” are interpreted accordingly as discussed above. 
   Upon START, the process  500  places the strip of the three units on the folding base unit secured by vacuum underneath the second unit with cavity clearance for the solder balls (Block  510 ). Next, the process  500  positions the plunger assembly directly above the second unit of the strip and moves the plunger assembly downward just above the second unit (Block  520 ). Then, the process  500  extends the rocker lever to push-fold the first unit, seated on the slanted or angled block, into the plunger assembly (Block  530 ). The first unit is to be folded on top of the second unit. 
   Then, the process  500  keeps the first unit being folded and aligned with the second unit by the two arms (Block  550 ). Next, the process  500  affixes the first unit to the second unit with the adhesive by extending the plunger down (Block  560 ). The plunger continues the folding motion on the first unit until the first unit is secured on top of the second unit. 
   Then, the process  500  cures adhesive and the first and second units by the heating element attached to the tip of the plunger (Block  570 ). Next, the process  500  moves the plunger assembly upward to release the cured unit (Block  580 ). The folded first and second units are now ready to be transferred or moved to other area. Then, the process  500  moves the plunger upward to the home position (Block  590 ). The process  500  is then terminated. 
     FIG. 6  is a diagram illustrating a packaging assembly line  600  for folded-stack μBGA packaged devices according to one embodiment of the invention. The assembly line  600  includes a first unit trim station  620 , an adhesive application station  630 , a prefold, cure, and second trim station  640 , a final fold and cure station  650 , and a flex tape singulation and pick and place station  660 , a transport assembly  670 , and a lifting assembly  680 . 
   The transport assembly  670  rolls the tape carrier  120  through the stations for processing. The tape carrier  120  carries a number of strips of flexible tape as described in  FIG. 1 . The process is progressive or pipeline such that when one strip is processed by one station, another strip is processed at another station at the same time. The assembly line starts at the ON LOAD area where the packaged devices on the flexible tape are loaded into the tape carrier  120 . As the tape carrier  120  progresses through the stations, the packaged devices are processed and eventually become a folded-stack μBGA devices to be transferred or delivered to a tray at the OFF LOAD area. The process is efficient as the tape carrier  120  fully enters the stations, all the stations are busy processing each phase of the process. 
   The transport assembly  670  includes a rotator  672 , a guide rod  674 , a guide rail  675 , a transport arm  676 , and a transport finger  678 . The rotator  672  rotates to advance the guide rod  674  with screw type in a linear motion along the guide rail  675  to move the tape carrier  120  through the processing stations. The lifting assembly  680  moves the tape carrier  120  up and down to allow punching the strip out to be placed on the folding base unit  360 . The lifting assembly  680  has two rows of lifters/cylinders  682  and  684  placed along the stations and two rows of guide pins  686  and  688 . The number of lifters/cylinders and guide pins along each row depends on the space and processing requirements of the manufacturing plant. When the lifters/cylinders  682  and  684  are in extended, or up position, the tape carrier  120  is transported to the next station one unit pitch at a time via the transport finger  678 . When the lifters/cylinders  682  and  684  are in the home, or down position, the tape carrier  120  is not transported. During this time, the pre-cut, adhesive dispense, folding, and unit singulation are done simultaneously at all the processing stations. 
   The packaged devices on the tape carrier may be arranged as 3×N where N is the number of columns or strips. The number of rows may not be limited to three. Depending on the folding configuration, the tool assembly shown in  FIG. 3  may be modified to accommodate different arrangements. 
   The exact spacing of the stations depends on the particular set-up of the assembly line. Regardless of the spacing or distance between the stations, each strip on the tape carrier  120  goes through all 6 stations. Typically, the processing times of the stations are approximately equal so that the tape carrier  120  can be moved at regular or uniform speed. However, as is known by persons of ordinary skill, the transport assembly  670  can be controlled to have non-uniform speed. 
     FIG. 7A  is a diagram illustrating a first unit trim station  620  in the packaging assembly line shown in  FIG. 6  according to one embodiment of the invention. 
   In the first unit trim station  620 , the first unit of a column or strip is trimmed, cut, or punched out. The first unit corresponds to the first row. When the first unit is trimmed and punched out, it is bent downward with respect to the horizontal direction. The strip is then moved to the adhesive application station  630 . 
     FIG. 7B  is a diagram illustrating an adhesive application station  630  in the packaging assembly line shown in  FIG. 6  according to one embodiment of the invention. 
   In the adhesive application station  630 , the adhesives are dispensed on the surface of the second unit  140   2  and the third unit  140   3 . The first unit  140   1  is bent downward. The adhesive can be dispensed by an adhesive dispenser located above the units. Then, the tape carrier  120  moves the strip to the first folding and second unit trim station  640 . 
     FIG. 7C  is a diagram illustrating a first folding and second unit trim station  640  in the packaging assembly line shown in  FIG. 6  according to one embodiment of the invention. Note that the term “pre-fold” and “final fold” folding phase may be replaced by “first folding phase” or “second folding phase”. 
   In the first folding and second unit trim station  640 , the strip is processed by the tool assembly as shown in  FIG. 4A . The first unit  140   1  is folded on top of the second unit  140   2 . The plunger unit affixes the first unit  140   1  to the second unit  1402  with the adhesive dispensed on the surface of the second unit  140   2 . The plunger unit then cures the adhesive. The affixed first and second units  140   1  and  140   2  form the partially folded unit  435 . At the same time, the third unit  140   3  is trimmed, cut, and punched out. The strip is then moved to the second folding station  650 . 
     FIG. 7D  is a diagram illustrating a second folding station  650  in the packaging assembly line shown in  FIG. 6  according to one embodiment of the invention. 
   In the second folding station  650 , the strip is processed by the tool assembly as shown in  FIG. 4B . The third unit  140   3  is then folded on top of the partially folded unit. The plunger assembly then affixes the third unit  140   3  to the partially folded unit  435  with the adhesive dispensed on the surface of the third unit  140   3 . The plunger assembly then cures the adhesive. The affixed third unit  140   3  and the partially folded unit form a fully folded unit  710 . Then, the strip with the fully folded unit  710  is moved to the flex tape singulation and pick and place station  660 . 
     FIG. 7E  is a diagram illustrating a flex tape singulation and pick and place station  660  in the packaging assembly line shown in  FIG. 6  according to one embodiment of the invention. 
   The flex tape singulation and pick and place station  660  removes or detaches the flexible tape from the fully folded unit to form a folded-stack packaged device. The pick and place station then picks the folded-stack package device and transfers to the tray at the OFF LOAD area. 
     FIG. 8  is a diagram illustrating a top view of a packaging assembly line for folded-stack μBGA packaged devices according to one embodiment of the invention. 
   As seen from the top, at any time, the six stations process six strips or columns  810 ,  820 ,  830 ,  840 ,  850 , and  860  on the flexible tape on the tape carrier. The strip  810  is processed by the first unit trim station  620  as shown in  FIG. 7A . The strip  820  is processed by the adhesive application station  630  as shown in  FIG. 7B . The strip  830  is processed by the first folding and second unit trim station  640  as shown in  FIG. 7C . The strip  840  is processed by the second folding station  650  as shown in  FIG. 7D . The strip  850  is processed by the flex tape singulation and pick and place station  660  as shown in  FIG. 7E . At the end of the assembly line, the final folded-stack packaged device located in the second row is removed and transferred to the OFF LOAD area. 
     FIG. 9  is a flowchart illustrating an assembly process  900  for folded-stack μBGA packaged devices according to one embodiment of the invention. 
   Upon START, the process  900  cuts and punches the first unit from the strip of the flexible tape (Block  910 ). Next, the process  900  applies adhesive to the second and third units (Block  920 ). Then, the process  900  folds the first unit on top of the second unit, affixes the first unit to the second unit with adhesive, and thermally cures the adhesive and the first and second units (Block  930 ). The affixed first and second units form a partially folded unit. 
   Next, the process  900  cuts and punches the third unit from the strip (Block  940 ). Then, the process  900  folds the third unit on top of the partially folded unit, affixes the third unit to the partially folded unit with adhesive, and thermally cures the third unit and the partially folded unit. The cured third unit and partially folded unit form a fully folded unit (Block  950 ). Next, the process  900  detaches the fully folded unit from the flexible tape using singulation punch, activates the pick and place mechanism to deliver the folded stack packaged device in tray in the off-load area (Block  960 ). The process  900  is then terminated. 
   While the invention has been described in terms of several embodiments, those of ordinary skill in the art will recognize that the invention is not limited to the embodiments described, but can be practiced with modification and alteration within the spirit and scope of the appended claims. The description is thus to be regarded as illustrative instead of limiting.