Download - 9 Lead Finish and Ball Attach
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Leadfinish and Ball attach
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Lead Frame
Provides mechanical support to the die
during its assembly into a finished product
Consists of die paddle for the die attachmentand leads for external electrical connection to
the outside world.
Lead frames are constructed from flat sheetmetal either by stamping or etching.
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LeadfinishLeadfinish is the process of applying a coat of metal over
the leads of an IC to:1) protect the leads against corrosion2) protect the leads against abrasion
3) improve the solderability of the leads
4) improve the appearance of the leads
There are two widely used leadfinish techniques in thesemiconductor industry, namely, plating and
coating. Further, there are two types of plating, i.e.,pure metal plating such as tin plating and alloy platingsuch as tin/lead plating.
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Leadfinish - CoatingCoating is the process of depositing a filler metal (usually
solder) over a surface, achieving metallurgical bondingthrough surface wetting.
The filler metal should have a melting temperature below315 degrees Celsius for the process to be classified as
coating.
The driving force for a solder coating process is surfacetension, i.e., wetting of the surface to be coated by the
solder must be achieved.
A solder diffusion layer grows at the surface-solderinterface as solder spreads through the surface during
the coating process.
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Leadfinish Tin PlatingTin plating is a form of pure metal electroplating, which is
the process of depositing a coating of metal on a surfaceby passing a current through a conductive medium, orelectrolyte.
An electroplating system has four (4) components:1) the cathode, which is the surface to be coated;
2) the anode, which is the source of coating metal;
3) the electrolyte, the aqueous medium through which themetal ions from the anode transfer to the cathode; and
4) the power source, which supplies the current or energyneeded for the plating process.
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Leadfinish Tin PlatingThe cathode, which is the
material to be plated, isthe electrode whereelectrons are consumedor where 'reduction'occurs.
The anode, which serves assource material for theplating, is the electrodewhere oxidation occurs,i.e., where electrons andmetal ions are released.
The energy needed for theplating process to occur isknown as theelectrochemical potentialor voltage.
If the applied voltage is lessthan the electrochemicalpotential, the process ofplating will not occur.
Solder plating is a form of alloyplating.
Example of reduction:
Sn+2 + 2e- => Sn0
Example of oxidation:
Sn0 => Sn+2 + 2e-
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SolderabilitySolderability Testing pertains to the process of
evaluating the solderability of terminations (i.e.,component leads, terminals, wires, etc)
Industry standards for performing solderability testing include thefollowing:
1) Mil-Std-883 Method 2003 - "Solderability"2) IPC/JEDEC J-STD-002 - "Solderability Tests for Component
Leads, Terminations, Lugs, Terminals and Wires"3) IPC/JEDEC J-STD-003 - "Solderability Tests for Printed Boards"
4) JESD22-B1025) Part 21 of the IEC 60749
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SolderabilityThe solderability of a surface is defined by its solder wetting
characteristics.
Solder wetting pertains to the formation of a relatively uniform, smooth,and unbroken film of solder that exhibits excellent adherence on thesoldered surface.
Non-wetting, on the other hand, is the condition wherein the soldercoating has contacted the surface but did not adhere completely toit, causing the surface or a part thereof to be exposed.
Dewetting is the condition wherein the solder recedes after coating a
surface, creating irregular mounds of solder, but leaving behind noexposed areas.
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Solderability - TestsThe two most common solderability test
methods are:1) the Dip and Look Method2) Wetting Balance Analysis.
In both of these tests, the samplesundergo an accelerated 'aging'process before being tested forsolderability, to take into consideration
the natural aging effects of storageprior to board-mounting.
The Dip and Look Method, which is widelyused in process QA and reliabilitymonitoring, is a qualitative testprocess, i.e., judgment on whether asample passes or fails the test isbased on the physical and visualattributes that it exhibits.
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Solderability - TestsWetting balance analysis, on the
other hand, is a quantitative
test,
It measures the wetting forcesimposed by the molten solderon the test surface as it is
dipped into and held in thesolder bath as a function oftime and plotted.
The plot starts with the wetting
force being negative (non-wetcondition), which rises until itcrosses the zero axis ofwetting force, indicating thatwetting has occurred.
The time it takes for wetting tooccur is one parameter used to
assess solderability.
There are, however, noestablished industry-standardpass/fail criteria for wetting
balance analysis, which is whyit is used primarily as anengineering tool and not as aproduction monitor.
Wetting force depends on thedensity and surface tension ofthe solder.
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Wetting Balance
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Ball AttachBall grid arrays (BGA) one
of the most popularpackage technology
Advantages:
Higher I/O No leads to bend
Lower coplanarityproblems
Minimized handlingissues
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Ball AttachIn solder-ball attach, solder balls
and flux are placed on gold-plated pads located on the
substrate.
When heat is applied to the part,the solder balls adhere to thepads.
Leads or solder balls provide thefinal interconnect between thecomponent and the boardapplication in the end-useproduct.
Unlike leadframe packages,PBGAs use solder balls as theinterconnect path from thepackage to the printed circuit
board.
Instead of lead formingprocesses, solder balls areattached to the substrate.
Solder balls are attached byapplying a flux, placing theballs on the pads, andreflowing the PBGA.
The reflow process forms ametallurgical joint between thesolder ball and the substrateball pad.
Alignment is a key parameterduring ball placement to avoidmissing balls or solderbridging.
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Chip Scale Packaging (CSP)
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Flip-stacked CSP
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SEE YOU NEXT MEETING!