9 lead finish and ball attach

8/6/2019 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!

9 lead finish and ball attach

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