1. Deposition of Solder Paste into High Density Cavity Assemblies
Add Authors Names onto this slide.
Maybe a nice Photo too.
Fernando Coma Jeffrey Kennedy Thilo Sack
CELESTICA INC

2. Aggressive space reduction and density increase.
Why?
For any tech to be introduce into production, If we can not rework it is not possible to have high production.
Using these technologies it has been possible to increase density in PCBAs.
New fine grain allowed to reduce clearance around cavities – Is it too soon to talk about cavities?
NEED to Shorten the Below into bullet points you will recognize at a quick glance
Circuit functional density requirements continue to drive innovative approaches to high performance packaging.
Some new approaches include;
Aggressive space reduction
embedded solutions,
and those that offer some form of risk reduction and rework potential are now options that are being explored by customers.
Requirements for assembly of these types of packages necessitate:
the deposition of solder paste
and assembly of components into cavities of the substrates to gain z-axis density as well as area functional density.
Advances in the fabrication of PWB’s with cavities using newly developed laser micro-fabrication processes along with increased circuit pitch density of 50 micron lines and spaces permit new applications for high performance electronic substrates.
Novel techniques for depositing solder paste have been explored for a variety of cavity sizes and depths.
High density patterns like 0.4mm pitch package on package (PoP) , 0201, and 0402 discrete components, 0.3mm pitch WLCSP components have been used to study the cavity paste deposition and SMT assembly techniques.
Discussion will include the methods for paste deposition and results from trials using a custom designed test vehicle for each of these processes;
step stencil and slit squeegee,
dispensing,
and solder paste jetting,.
Benefits and challenges associated with each of the techniques will be presented and some summary and conclusions based on the experimental results. Eventually it will be shown how to integrate these new techniques in a standard SMT line and the effect it might provoke in a real production, talking about cycle time, flexibility, and quality.
This paper will present the test vehicle, overview of the associated paste deposition methods, challenges for each of the techniques and results of the trials with the two test vehicles. Additional discussion on the limits of each process as explored and some discussion on the DFx rules derived from the work and results.
CELESTICA INC
Presentation Name | Month 2

3. Evaluate the practical limits of integrating PWBs with cavities into a standard IPC 610 Class 3 process
GOAL: Evaluate limits of integrating cavities into pcbs
The motivation for the work is to acieve high density package assemble that meets IPC610 class 3 with rework capability for critical components.
The objective of this project was to evaluate the practical limits of integrating PWBs with cavities into a standard IPC 610 Class 3, high reliability SMT process with the end goal of defining a set of process and design guidelines for their use in future functional product.
CELESTICA INC
DEPOSITION OF SOLDER PASTE INTO HIGH DENSITY CAVITY ASSEMBLIES| Month 3

4. 3 Solder Deposition Techniques which were evaluated:
Step stencil
Dispensing
Explain the three tech:
step stencil = use standard paste printing machine with special stencil and squeegee standard solder paste
Dispensing = uses custom high accuracy solder paste dispensing head and various needles for dot size and special solder paste made for dispensing
Jetting = uses custom machine and solder paste formulated especially for the process
Jetting
CELESTICA INC
DEPOSITION OF SOLDER PASTE INTO HIGH DENSITY CAVITY ASSEMBLIES| Month 4

5. test vehicle
Design to evaluate the three technologies.
Size: 185x60 mm
CELESTICA INC
DEPOSITION OF SOLDER PASTE INTO HIGH DENSITY CAVITY ASSEMBLIES| Month 5

6. 17 cavities Cavity sizes (mm): 30 x 40 30 x 15 20 x 18 15 x 15 12 x 12
Explain why we have a variety of sizes = to determine any limitation of the 3 processes under evaluation. For example, We expected the smallest size cavity to present a challenge for the step stencil printing process. We had various patterns in the variety of sizes to evaluate if any DFx limitations existed.
Different cavity sizes used  Determine any limitation of the three different processes under evaluation
Components located on the board with different distance to the cavity edge, both in and out of the cavity  Looking for limitations
CELESTICA INC
DEPOSITION OF SOLDER PASTE INTO HIGH DENSITY CAVITY ASSEMBLIES| Month 6

7. 3 cavity depths Cavities fabricated by AT&S (patented process)
Factors when choosing depth:
Match the common thicknesses of SMT components
Available pre-preg used in the stack-up of the PWB
Depths:
0 (TOP surface)
150 um
300 um
The design points were decided by:
150 micron depth to use if we buy the extra-thin discrete packages.
300 micron depth used if we use standard thickness discrete packages or we have QFN or 0.3mm pitch CSP in a cavity.
CELESTICA INC
DEPOSITION OF SOLDER PASTE INTO HIGH DENSITY CAVITY ASSEMBLIES| Month 7

8. 4 component types
TYPE (inches/metric)
Density / Pitch
01005 (0402)
390 um
0201 (0603)
500 um
0402 (1005)
1 mm
CSP
300 um
Components selected to push the capabilities of each process and to expose if there are any weakness/limitation of any one of those 3 processes under evaluation.
For chip components, “density” is the minimum spacing between devices while for CSP’s, “pitch” if the device I/O minimum pitch evaluated.
CELESTICA INC
DEPOSITION OF SOLDER PASTE INTO HIGH DENSITY CAVITY ASSEMBLIES| Month 8

9. DFx RULES
Note the edge to cavity distance is varied to expose any limitation of any of the processes.
Note the daisy chain pattern to validate solder joint reliability
Note the small pad size of the 0.3mm pitch CSP part especially compared to the 0402 pad size.
CELESTICA INC
DEPOSITION OF SOLDER PASTE INTO HIGH DENSITY CAVITY ASSEMBLIES| Month 9

10. 1st – step stencil
Side wall should be uniform. Note slope of cavity sidewall – this must be minimized in order to have greatest cavity component density.
Alignment during stencil manufacturing process is crucial as stencil cavity side wall should be uniform.
Laser JOB (welding process) not used due it was not possible having all 17 cavities in the same stencil with all aligned to each other. That process was able to support up to 4 cavities on a CCA pattern.
Challenges: Stencil manufacturing is key for this technology.
Density into cavities is depending on stencil design.
CELESTICA INC
DEPOSITION OF SOLDER PASTE INTO HIGH DENSITY CAVITY ASSEMBLIES| Month 10

11. Std. 390 SS on Left and Fine Grain SS on Right
stencil material used
Fine grain stainless steel used for stencil
Recent material Break through for stencil fabrication – new smaller grain foil permits better etch resolution.
Newly developed fine grain stainless steel material for the fabrication of the stencil.
Std. 390 SS on Left and Fine Grain SS on Right
CELESTICA INC
DEPOSITION OF SOLDER PASTE INTO HIGH DENSITY CAVITY ASSEMBLIES| Month

12. stencil
fabrication
RED CIRCLE: Alignment is crucial and dependent on wall angle (both stencil angle and PCB angle).
Comment: Stencil fabrication is critical as stencil should touch the PBC both in and out of the cavity. Match the cavity.
We have done different stencils in order to validate the size of the cavity in all direction including Z.
CELESTICA INC
DEPOSITION OF SOLDER PASTE INTO HIGH DENSITY CAVITY ASSEMBLIES| Month 12

13. squeegees Challenges: Alignment between blade and stencil.
Cuts are needed to allow blade go into the cavities.
Blade support used to increase pressure into cavities.
Alignment --- Set up
Pressure increased using back support blade. More work required to define what is needed for various cavity sizes and depths. Force=pressure/area is important consideration for achieving multiple cavity with clean wipe post-print.
Set up consideration: next slide
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DEPOSITION OF SOLDER PASTE INTO HIGH DENSITY CAVITY ASSEMBLIES| Month 13

14. blade alignment: stencil & PCB
Set up considerations
Alignment between STENCIL / BLADES / PCB
Alignment between Stencil and Blades: Screen printer MOMENTUM used: Blade alignment done manually  Loose screws – alignment – tight screws
Alignment between PCB and Stencil: Machine – fiducial location.
CELESTICA INC
DEPOSITION OF SOLDER PASTE INTO HIGH DENSITY CAVITY ASSEMBLIES| Month

15. screen printer cycle
Screen printer video

16. blade support
Using backside squeegee blade support improves print performance significantly
However each cavity requires different amount of backside support
Without blade support
With blade support
The deeper the cavity the more support is needed.
Force per area.
Message  There are more work to do to know the force needed to have a clean surface.
FUTURE WORK
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DEPOSITION OF SOLDER PASTE INTO HIGH DENSITY CAVITY ASSEMBLIES| Month

17. trials Blade support to create pressure
Blade behavior across the entire width of the board
Transference into cavities [01005 (0402) / 0402 (1005) / CSP apertures]
Data showed in the graph are Normalized data --- not absolute data.
Data can be used to compare behavior of different component type or behavior of same component place on different locations (in / out of the cavity)
Next slide: Pictures of some solder joint.
Chip 0402 & 0201 volume
CSP Inside vs Outside
of Cavity
CELESTICA INC
DEPOSITION OF SOLDER PASTE INTO HIGH DENSITY CAVITY ASSEMBLIES| Month 17

18. results
Solder paste transference for different apertures and cavity depths
Solder joint meeting IPC class 3, in and out of the cavity.

19. step stencil summary
Possibility of soldering all chip component types into cavities [includes down to (0402) sized chips]
Able to accommodate different depth cavities, limitations comes from stencil manufacturing.
Low cycle time as standard screen printer is used.
More work required to support different depths and apertures in the same stencil.
Solder joints meet IPC 610 class 3.
TO BE COMPLETED
DEPOSITION OF SOLDER PASTE INTO HIGH DENSITY CAVITY ASSEMBLIES| Month
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20. 2nd - dispensing
Micro piston used dispenses a single shot per pump cycle.
Able to dispense dots for (0402) components.
Programmable to dispense at any height.
Requires the use of special solder pastes designed for dispensing.
Custom dispense head made for solder paste dispensing.
Incremental capital beyond solder paste printer – affects total cost of ownership/product pricing
CELESTICA INC
DEPOSITION OF SOLDER PASTE INTO HIGH DENSITY CAVITY ASSEMBLIES| Month 20

21. dedicated solder paste
Dedicated solder paste is required for this process as the head needs small particles, high flux content and special formulation to do the dispensing
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DEPOSITION OF SOLDER PASTE INTO HIGH DENSITY CAVITY ASSEMBLIES| Month

22. dispensing machine cycle
Dispensing video

23. 1 dot for each 01005 (0402) 5 dots for each 0201 (0603)
Parameters defined to dispense solder paste for (0402) component.
Same parameters were used to dispense multiple dots for large parts.
0402 (1005) size pads needed up to 60 single dots.
Multiple head nozzle could reduce overall cycle time – must be considered when evaluating which process to select for making each CCA.
CELESTICA INC
DEPOSITION OF SOLDER PASTE INTO HIGH DENSITY CAVITY ASSEMBLIES| Month 23

24. dispensing summary
Possibility of soldering all chip component types into cavities [includes down to (0402) sized chips]
Able to accommodate different depth cavities without any restrictions.
For production it is ideal to choose a machine that supports multiple dispense heads to reduce overall cycle time.
Cycle time depends on the needle used to create dispensed dot sizes.
CELESTICA INC
DEPOSITION OF SOLDER PASTE INTO HIGH DENSITY CAVITY ASSEMBLIES| Month 24

25. 3rd - jetting
Requires the use of special solder pastes designed for jetting.
Minimum dot size able to be dispensed is 300um diameter [0201 components, (0603)] Impacts ability to handle smallest parts.
Max. dispense height limits
Custom Jetting machine made for solder paste dispensing/jetting like ink jet cartridge……..
Incremental capital beyond solder paste printer – affects total cost of ownership/product pricing
CELESTICA INC
DEPOSITION OF SOLDER PASTE INTO HIGH DENSITY CAVITY ASSEMBLIES| Month 25

26. dedicated solder paste
Dedicated solder paste is required for this process as the head needs small particles, high flux content and special formulation to do the jetting
Solder pastes are qualified for use by the Jetting machine manufacturer.
TYPE
Type 5
Flux Content
(wt%)
Metal Percent
(wt%)
CELESTICA INC
DEPOSITION OF SOLDER PASTE INTO HIGH DENSITY CAVITY ASSEMBLIES| Month 26

27. jetting machine cycle
Jetting video

28. 3 heights tested
Ideal dispense height above the board surface is 650um, although increasing distance up to 850um does not impact accuracy.
Establishing datum to effectively jet into the cavities is critical (reference points)
Assessed impact of dispensing into cavities from heights > 650um on deposit consistency
Talk about Min and max values for z-height and jetting process.
Standard height distance 650um  From the tops surface to the ejector.
Using cavities it is not needed to increase this distance (650um) as the machine allows to modify the head height reducing the height as much as the cavity depth. Therefore for a cavity depth of 300um the distance from the ejector to the cavity would be 650 and the distance from the ejector to the top surface would be 350um.
Parameter should be changed depending on the product to run.
Dispense height works for 950 um between head and pad (Second trial process 2). It means machine can jet into a cavity 850 um depth without problems, distance between head and top layer equal to 100 um  not possible do that after screen printer.
I think we should find a way to say that machine have some limits and those limits are depending on the process to be followed.
CELESTICA INC
DEPOSITION OF SOLDER PASTE INTO HIGH DENSITY CAVITY ASSEMBLIES| Month 28

29. jetting height vs positional accuracy
Comp 0201 (0603)
Cavity: 150um
Dispense height: 650um
Make a note about top side components and how one might reduce the z-height/top side component clearnace by waiting to assemble those after doing the cavity parts.
Parameters should be modified depending on:
Product to run (number of components in and out of the cavity)  Jetting machine jet only comp into cavities Screen printer can help with the top components.
Depth cavity  If it is needed head height can be modified to increase cavity depth  Solder paste on the top should be place after.
Cycle time needed  Screen printer can help with comp on surface.
Some of these considerations might work for dispensing also.
Comp 0201 (0603)
Cavity: 150um
Dispense height: 1800um
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30. State which component this is for and explain the statistics and what they mean for process repeatability – many of the audience are not familiar with why that is important.
Explain that you also have similar data for 51nm – this is a wide range of volume both of which delivered good class 3 solder joints. 30

31. flexibility
Tell the audience What are the key points for flexibility?
- Speed to prototype
Cost avoidance for tooling – stencil not needed
Ability to make modifications to the process based on first piece part analysis = program flexibility
Time to prepare to build
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32. CLASS 3
Results
CELESTICA INC
Presentation Name | Month 32

33. jetting summary
Machine is capable for both processes (two head heights) and for different volumes (minimum 6.8 nl and maximum 51nl)
IPC 610 Class 3 solder joints are achievable using jetting process for both 0402 (1005) and 0201 (0603) components
Jetting machine is supposed to feed a pick and place machine mounting 40,000 components per hour. This data depends on the geometry of the PCB to be produced.
Dispense strategy for each package can easily be controlled via simple software changes
Standard jetting height is 650um, but working with the supplier this distance could be adapted to something less
CELESTICA INC
DEPOSITION OF SOLDER PASTE INTO HIGH DENSITY CAVITY ASSEMBLIES| Month

34. summary
Transferring solder paste into high density cavities is a process that can be done with high quality and capable of achieving IPC 610 class 3 standard solder joints.
The most appropriate and cost effective method to use will depend on the final product configuration since each method did have its own limitations in terms of capability to support certain design features or cycle time
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DEPOSITION OF SOLDER PASTE INTO HIGH DENSITY CAVITY ASSEMBLIES| Month

35. recommended deposition method based on package type
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DEPOSITION OF SOLDER PASTE INTO HIGH DENSITY CAVITY ASSEMBLIES| Month 35

36. process decision matrix
Several factors must be considered to integrate the most appropriate solder deposition method depending on product design and desired manufacturing setup
Complexity:
Number of cavities, number of depth, size of the cavities, clearance around the cavity edge, component size…
Explain the trade-offs from the slide chart:
Capital cost and effect of product pricing
Flexibility
Cycle time
Time to build/complete build

37. Acknowledgements
Jeff Kennedy, Thilo Sack, Vicenta Jorge, Samuel Plasencia, Javier Canillas, Miguel Sanchez (Celestica), Euripides BOB Partners: Thales TCS & TGS, AT&S.
Mydata / AB Electronics (Jetting)
GPD (Dispensing)
Great Lakes Engineering / Pantur (Stencils)
Indium (Solder Paste)
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DEPOSITION OF SOLDER PASTE INTO HIGH DENSITY CAVITY ASSEMBLIES| Month

38. Questions? CELESTICA INC
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