Determination of pre/post process surface roughness and comparison with copper film

adhesion on multiple polymer substrates

Prepared by Martin AguileraWork supported by the Packaging Research Center at Georgia Tech

Motivation

EU Directive Restrict lead-based solder by July 1, 2006

Companies given exceptions

Effect on manufacturingHigher solder temperaturesCTE considerationLayer adhesion

Scope of Project

Board polymer researchBoards provided by Company A for chemical copper coating Boards returned to Company A for adhesion testing (Peel Strength Test)

Board surface roughnessTested before copper coating allowing comparison to peel strength

Scope of Project

Two types of roughness:Micro-roughness (~100 Å)Macro-roughness (>1000 Å)

Examine which type of roughness is most affected by chemical roughening process

Correlation between types of roughness for each board and peel strength

To determine which, if either, is the dominant mode for adhesion

Adhesion Theory

Two main modes of adhesionChemical

Based on oxide formation

Mechanical Based on surface energy reduction

Adhesion Theory

Epoxy chemical structure

Oxygen group allows for oxide formation and chemical bond

Adhesion is based on surface area

Generic epoxy

Copper film

Adhesion Theory

Mechanical AdhesionSurface Energy

Asymmetry of chemical bondsAdditional layer relieves surface energy (bonding)

Adhesion is based on surface roughness and area

Expected Results

Chemical process is expected to increase micro-roughness more due to scale of reaction

Both types of adhesion depend on surface areaBoards with a higher surface roughness will have a higher peel strength

Both types of roughness increase surface areaIt is unclear which type of roughness will correlate with peel strength better

Methods: Sample Preparation

4 types of 5x5” boards (2 per type)1 board of each type set aside

Placed in Atotech Permanganate Etch

1x5” section cut Set aside as post process1x4” section set to Company A

4x5” boards were plated with electroless copper film (~1µm)

Boards were plated with electrolytic copper (~18µm)

4x5” Full Plating

1x4” Post Process

1x1” Post Process

5x5” Full Board

Methods: Sample Preparation

Dupont photoresist film was applied and developed according to peel test pattern

Copper was etched off and samples were sent back to Company A for testing

Methods: Sample Testing

All un-plated board surface roughness measurements made with Veeco DekTak 3030

Surfaces were taped to platform and leveled

DekTak function used to determine surface roughness

Deviation from a linear approximation between two selected points

Methods: Sample Testing

Micro-roughness was determined over a 5 micron range

Macro-roughness was determined over a 315 micron range

5 locations were averaged to determined the board roughness

7 trials were performed at each location

Methods: Peel Test

Samples of plated board and post-process boards were sent to Company A

Peel Strength Tests were performedMacro-scale surface roughness was performed over a 215x315 micron area using AFM

Company A returned data for analysis

Results:Effect on macro-roughness

91-51 91-50 120-1 120-2Before Etch (A) 1039.13 526.65 1682.38 1401.38After Etch (A) 1890 3855.33 7696.38 11762Percent Change 81.9% 632.0% 357.5% 739.3%

Roughness increases by a varied amount

Polymer 91-51 shows a lack of significant increase in macro-roughness

Polymer 120-2 has greatest end macro-roughness

Macro Roughness

0

2000

4000

6000

8000

10000

12000

14000

120-2 120-1 91-50 91-51

Polymer

Pre-Process Post-Process

Results: Effect on micro-roughness

Micro Roughness

0

50

100

150

200

250

300

120-2 120-1 91-50 91-51

Polymer

Pre-Process Post-Process

91-51 91-50 120-1 120-2Before Etch (A) 88.56 93.54 102.96 109.18After Etch (A) 147.48 138.9 180.08 238.18Percent Change 66.5% 48.5% 74.9% 118.2%

Smaller percentage in micro-roughness

Similar increases across polymers

Polymer 120-2 has greatest end micro-roughness

Results:Correlation to Peel StrengthPeel Strength

0

0.5

1

1.5

2

2.5

100 125 150 175 200 225 250

Micro-roughness (x10-10m)

91-50

120-1

120-2

91-51

Peel Strength

0

0.5

1

1.5

2

2.5

0 2500 5000 7500 10000 12500

Macro-roughness (x10-10m)

91-50

120-1

120-2

91-51Macro (A) Micro (A) Peel Strength (lbf/in)

120-2 8990 238.18 1.908120-1 7680 180.08 2.17691-50 2510 138.9 1.6691-51 1890 147.48 0.469

Peel strength shows a stronger correlation with macro-roughness than micro-roughness

Polymer 91-51 shows significantly smaller peel strength

Polymers 120-1 & 2 have greatest peel strength

Error

Board 91-51Deciding board for resultsOriginally had a decrease in macro-roughness with etchingContained two distinct roughness sections

Measuring techniquesAFM vs. DekTakSurface roughness vs. surface area

Future Work

Six more samples are being tested in the next few weeks

Improvement of micro/macro definitions to line up with current theory

Investigate whether micro or macro-roughness has a larger effect on surface area

Effect on Classroom: Lesson

Lesson based on understanding statistical significance of data

Students will learn statistical conceptsStandard DeviationConfidence IntervalsSample Sizing

Students will learn to use Excel

Effect on Classroom: Lesson

Data collected as a class will be used to establish formulae and procedures

Measuring height using a 12” ruler Use varying values to motivate question of how we know when to stop taking data

Effect on Classroom: Lesson

In class lab Students collect dataStudents hand calculate values for small set of dataStudents use Excel to calculate values for a larger set of data from lab

Two assessmentsSmall worksheet with questions after classroom workExcel spreadsheet with data to evaluate and electronic submission

ReferencesRF Saraf, JM Roldan, T Derderian, “Tailoring the surface morphology of polymide for –improved adhesion,” IBM Journal of Research Development, vol. 30, no. 4 pp. 441-455, 1994

Wikipedia, “Epoxy”, http://en.wikiedia.org/wiki/Epoxy, (accessed July 21st, 2006)

CP Wong, “Chemical and Mechanical Adhesion Mechanisms of Sputter-Deposited Metal on Epoxy Dielectric for High Density Interconnect Printed Circuit Boards,” IEEE Transactions on Components of Packaging Technologies, vol. 24, no. 3, pp. 416-424, 2001

Wikipedia, “Surface Roughness”, http://en.wikipedia.org/wiki/Surface_energy, (accessed July 21st, 2006)

DE Packham, “The mechanical Theory of Adhesion – a seventy year perspective and its current status” http://staff.bath.ac.uk/mssdep/dep70yrs.htm, (accessed July 21st, 2006)

Questions