FCBGA Package Warpage Definition Stage Project Raiyo Aspandiar – INTEL HDP User Group Member...

FCBGA Package Warpage Definition Stage Project

Raiyo Aspandiar – INTEL

HDP User Group Member MeetingHost: Shengyi Technology Co., Ltd.

And NERCECBMGuangdong, China

December 5, 2012Presented by Brian Smith

© HDP User Group International, Inc. 1


Purpose


Background

Package/board Warpage increasing trends •Driven by thinner package substrates and thinner die

Package/Board contacts getting smaller and closer

thereby reducing ability to overcome increased Warpage. Solder Joint Quality Impact of increasing Package

and Warpage.

With advent of lead free soldering, the assembly temperatures have increased and the warpage impact has been exacerbated.


Examples of Warpage Induced Defects

for Area Array Solder Joints

Stretched joint

Head on pillow

Head-on-Pillow Open

Die

Board

Non –Wet Open

Various Solder Joint Defects can occur during SMT Reflow Soldering due to Excessive BGA component and/or Board Warpage


Real Time Video of FCBGA Solder Joint Mechanism in Assembly Line Reflow Oven


Project Scope

- What is IN the Scope

Die

Package Substrate

Printed Circuit Board

Project Focus Area

FC BGA Package Substrate

Solder Paste Rheology Wetting Metallurgy Activator chemistry Volume printed on land

Package Termination Geometry (ball, pillar, column, etc) Metallurgy (SAC, low Ag SAC, BiSnAg, other)

FCBGA Package just as its entering the SMT Reflow Oven

Reflow Process Temperature –Time Profile Oven Atmosphere

Surface Finish OSP/ENIG/

ImAg/ENEPIG/ etc


Project Scope

Die

Package Substrate

Printed Circuit Board

FC BGA Package Substrate

Package Warpage Mitigation Laminate Type Stack-up Die Thickness Die Size

- What is OUT of the Scope

Package Warpage Measurement Metrologies Specifications

Board Warpage Mitigation Laminate Type Stack-up

Board Warpage Measurement Metrologies Specifications

Objective

8© HDP User Group International, Inc.

Establish a limit for dynamic package warpage

that can be mitigated during board assembly

without impacting solder joint quality

9

Project Goals

• Identify mitigation paths for solder joint yield loss caused during the SMT reflow soldering process specifically due to the excessive warpage of package and/or boards

• Evaluate these mitigation paths for their effectiveness in increasing solder joint yield despite high levels of package and/or board warpage

© HDP User Group International, Inc.

Technical Discussion

Potential Mitigation Paths

• Alternate solder paste and package solder ball metallurgies,

i.e., low temperature solders

• Alternate solder temp profiles (steep ramp, soak, spike reflow

profile, for instance)

• Alternate package termination geometries (instead of balls)

• Optimized solder paste printing geometries

• New solder paste formulations

• Tacky Fluxes, applied by dipping

Each mitigation path can comprise a separate project proposal or all can be combined into one project

10© HDP User Group International, Inc.

What has been Done to Date

• Project Scope Identified• Key Variables Identified to Control and Monitor

during SMT Reflow Solderingo Gap between Ball and Paste

• Researched various methods developed in the Industry to Simulate Major Solder Joint Defects such as HoP and NWOo Three methods selected for further Evaluation and

Development Modified Plexus’ Method of Adhesive and Solder Pre-

form under Package Cisco’s Rework Equipment Use Method Real Time observation of Solder Joint Formation

within in-line reflow oven© HDP User Group International, Inc. 11

15 -17

18-25

What is Planned For the Future

• Develop Metrology: Explore the feasibility of using Cisco Rework Method and Modified Plexus Adhesive Method for use with a full array package instead of a single solder ball at a time

• Develop Real Time Monitoring: Explore possibility of using real time monitoring of solder joint formation within reflow oven with high dynamic warpage components, and subsequently characterize the dynamic warpage of problematic packages already in use today

• Brainstorm Mitigation paths: for excessive warpage of package and/or boards induced SMT solder joint yield loss and select potential candidates for assessment

• Select Mitigation Path: Obtain Components and Design and Procure Boards for Evaluation of Mitigation Paths

• Assess Effectiveness of Mitigation Path: Design and Run Experiments to evaluate present capability of materials and processes and feasibility of the Warpage-induced Defects’ Mitigation Paths



Proposed Project Flow

Project Team is working on breaking down this project into 3 Phases


Team Members –to date

• Akrometrix

• Alcatel-Lucent

• Arlon

• ASE

• Celestica

• Ciena

• Cisco

• Curtiss-Wright

• Ericsson

• Flextronics

• Fujitsu

• Hitachi

• Hitachi-Chemical

• IBM

• Intel

• IST Group

• Juniper

• Multek

• Nihon-Superior

• Panasonic

• Phillips

• Plexus

• Rockwell

• Shengyi

• TTM Tech

• Ventec

Back-up


Temp

Effect of Typical Dynamic Warpage Characteristics of FCBGA packages on Solder Joint Formation

© HDP User Group International, Inc.16

Head-on-Pillow Open

Room Temperature

`Flat Package` Point

Start of Significant Pull Way of Ball from Board

Maximum Ball-Board Separation Point

Post Solder Ball Collapse Point

Post Solidification Point

Time

Ramp

up

soak

reflowCool down

The typical Dynamic Warpage of a FCBGA package entails For a corner solder joint…….contact first, then separation between the solder ball and the solder paste For a center solder joint……separation first, then contact between the solder ball and the solder paste

This contact + separation or vice versa sequence creates solder joint defects

Variation of Gap between Bottom of Solder Ball and Top of Solder Paste (when solid or molten)


Time

`Flat Package` Point

Start of Significant Pull Away of Ball from Board

Temp Maximum Ball-Board Separation Point Post Solder Ball

Collapse Point

Post Solidification PointGap in Center

joint of Package

Gap in Corner Joint of Package

Gap

Room Temperature

Sharp Decrease in Gap due to collapse of central

solder joints

Besides first contact , then separation for the corner ball, there is a sharp decrease in the gap as the solder balls melt and collapse in the reflow zone within the reflow soldering oven

This sharp drop and gap can facilitate coalescence of the molten solder ball and molten solder paste if paste flux is still active

Key Points

Any method of simulating BGA package warpage during reflow soldering needs to address the following to simulate the conditions for corner solder joints

Initial Contact of the BGA solder ball with the solder paste and subsequent separation before or during the soak zone or ramp regionSharp drop in the gap between the molten solder ball and molten paste when the solder balls melt and collapse in the reflow zone region

Any method of simulating BGA package warpage during reflow soldering needs to address the following to simulate the conditions for central solder joints

No contact between the BGA solder ball and solder paste before or during the ramp zone or soak zone until the flat package point is reachedContact developed between the solder ball and solder paste later in the soak zone


Simulation for the Corner and Outer Row Solder Joints of the BGA


Requirements• Initial Contact of the BGA solder ball with the solder paste and subsequent separation before

or during the soak zone or ramp region• Sharp drop in the gap between the molten solder ball and molten paste when the solder balls

melt and collapse in the reflow zone region

Gap < 0

Package

Adhesive

Board

Gap > 0

Room Temp ~20C

Soak Zone Temp ~160C

Board

Board

Reflow Zone Temp ~240C Gap < 0

Initial set up entails amount (thickness) of adhesive adjusted to ensure negative

gap between the solder balls and the paste;

SAC Solder Pre-form and Solder Paste or Flux

Expansion of the adhesive in its thickness direction raised the balls from

the paste and creates a positive gap

Melting of solder preforms in the reflow zone and the subsequent flow of the

molten solder into PTH will dramatically drop the stand-off height of

the package and eliminate the gap

The amount of gap and rate at which it is created in the ramp zone of the profile is controlled by the expansion coefficient of the adhesive in its thickness direction

One Proposal to Simulate Gap Variation between the BGA Solder Ball and the Solder Paste on the PCB Land


Gap > 0

Board

Adhesive

Solder Preform

BGA PackageBGA Solder Balls

Solder Paste

Function of Adhesive: To simulate the increase in the gap by changing its length with increasing temperature due to its coefficient of thermal expansion

Function of Solder Preform: To melt at reflow temperature and flow into the PTH it is placed over and simulate the sudden decrease in the gap that occurs when the balls collapse after becoming molten

Simulation for the Central Solder Joints of the BGA

Gap < 0

When the low temp solder pre-form melts balls lowered and touch SAC

solder paste

Balls and solder paste melt while in contact

Gap at RT > 0

Gap < 0

Package

Adhesive

Room Temp ~20C

Soak Zone Temp ~160C

Reflow Zone Temp ~240C

Low Temp Solder Pre-form and Solder Paste or Flux Initial set up entails a gap between the

solder balls and the paste; gap amount controlled by amount (thickness) of

adhesive

Requirements• No contact between the BGA solder ball and solder paste before or during the

ramp zone or soak zone until the flat package point is reached• Contact developed between the solder ball and solder paste later in the soak zone

The temperature at which the solder ball and paste make contact can be varied by using different solders with different melting points in the SAC profile soak zone

Requirements for Adhesive Under Component to Simulate the Gap between the Ball and the Paste during SMT Reflow

Soldering


Gap > 0

Board Expansion of the adhesive in its thickness direction raises the balls from

the paste and creates a positive gapMinimum Gap to simulate problematic

warpage is 150 microns

Adhesive

1) High Coefficient of Linear Expansion after Cure; 2) Sufficient Hardness/Modulus/Rigidity to push the package up

as the adhesive expands3) Survive temperatures up to 250C after cure4) Dispensable or screen printable

Key question: What is the range of Linear Expansion that is needed for the adhesive?

Cisco’s SRT Rework Equipment Method for Head-on-Pillow Testing


BGA rework equipment was also used to reflow the solder ball and the solder paste, while controlling the movement of the solder ball with respect to the solder paste on the pad.

The purpose of controlling the movement is to mimic the movement of the corner balls of a BGA as they warp during the reflow process.

Conditions for Cisco’s Method


C1

C2

C1: Initially there is no initial contact between the solder ball and the paste. Then at 200 ° C during the cooling down phase, the solder ball is pulled down slowly (1 mil/second) so that it makes contact with the solder paste.

C2: Initially there is no initial contact between the solder ball and paste. Then at 190 °C during the cooling down phase, the solder ball is pulled down slowly so that it makes contact with the solder paste. This condition is similar to C1 but there is less contact timebefore solidification.

Note: When initial contact is made between ball and paste, no H-o-P defects were formed

Photo’s of Cisco’s Method for Evaluating H-o-P defects


Before Solder Paste Reflow After Solder Paste Reflow

Aspects of Actual Reflow Soldering of high warpage BGAs NOT Simulated in this Test Lifting up of the solder ball from the solder paste during initial temperature

ramp (though this can be done with the equipment) Un-constrained Ball Collapse when the solder melts Temperature Profile simulated with hot air rework machine instead of a

reflow oven

Malcom RCA-1 Observation Monitor


Observes and Records the components on a PCB as it goes through the Reflow Oven

FCBGA Package Warpage Definition Stage Project Raiyo Aspandiar – INTEL HDP User Group Member...

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