Testing Solder Mask for Assembly Process Compatib ility

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Testing Solder Mask for Assembly Process Compatibility

Bob Willis - Consultant

19th January 2016


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Testing Solder Mask for Assembly Process Compatibility

Bob Willis - Consultant

19th January 2016

IPC Standards & NPL Report

http://defectsdatabase.npl.co.uk

All NPL Reports can be download via the database free of charge


Table of Tests Covered in IPC-SM-840


Solder Mask Potential Problems

Old magazine ad from the 90’s on the issues associated with solder mask


Why Use Solder Mask on PCBsThe functions of a solder resist are to:

Reduce solder bridging and shortsReduce the volume of solder pickup

Reduce solder pot contamination with copperProtect circuitry from handling and the environment

Provide known dielectric characteristics between conductors and padsProvide a barrier against electrochemical dendritic growth

Provide a dielectric barrier between components over conductor lines

The majority of companies do not specify their mask. They often do not know what plant ismaking their boards or when any changes are made to the materials being used ………......

Solder Mask Design RequirementsSolder mask is used to reduce the possibility of pasteand solder shorts forming

The solder mask web width is designed based on thepitch of component/material capability

The position of the mask is determined by the printedboard supplier based on the material and processtolerances

Solder mask characteristics is the main reasons forsolder balls sticking to the surface of a board


We have yet to find a good repeatable method of testingsolder mask that works!!!!!!!!

It is generally believed that solder mask reduces solderand paste printing shorts

There is no definitive correlation between solder mask webwidths, resist to pad separation or web aperture sizes

There is limited data on the effects of resist and patternheights on defect printing, however it dose cause issues

Solder Mask Design Requirements

Solder Mask AssessmentSolder Mask Variables

ThicknessApplication methodsFinish (Gloss/Matt)Cure ConditionsSupplier product

Measurement TechniquesPencil HardnessContact AngleDie PenThicknessRoughnessMicro Hardness (indenter)SIRCapacitanceGlossReflectance

Process EvaluationReflow Solder ballingWave solderingUnderfill VoidingOutgassing




Solder Mask Test MethodsHARDNESS

Hardness of the samples was tested as per IPC-TM-650 2.4.27.2 (Reference 2), as called up bySM-840 (Reference 3)

The coated board under test was placed on a solid horizontal surface. Pencils taken from thehardness range 4B-3B-2B-HB-F-H-2H-3H-4H-5H-6H were held firmly against the solder mask at a45° angle (see Figure 3). The pencil was then pushed away from the operator with uniformdownward and forward pressure in a ¼ inch stroke. The lowestpencil hardness not to cut or gouge the solder mask was recorded

Open solder mask design referred to as a gang aperture or individual solder mask openings. Solder balls are very unlikely to attach to the laminate surface in wave or selective soldering

Solder Mask Web Design Options


Solder Mask Web Options

Solder Mask Web OptionsChip components options below 0402


Solder Mask Web OptionsChip components options below 0402

Repeatable Solder Ball Formation

Designer Solder Balls


Random Solder Ball Formation

Random solder balling on the bottom of the boardin air or nitrogen can be related to the amount offlux, incorrect pre-heat, too higher wave setting. Ineach case causing solder balls to be generated andpropelled towards the board surface

Solder Ball Surface Contact


Solder Ball Formation

Solder Ball Formation


High Speed Video Ball Formation


Solder balls

0

5

10

15

20

25

Solder balls

Aver age number of solde rballs per board

Soldermask/finish results with respect to solderballing

101 - 105 Elpemer Enteck

106 - 110 Elpemer HASL

111 - 115 Probimer 52

201 - 205 PSR4000 Sn/Pb

206 - 210 PSR4000 HASL

211 - 215 PSR4000 HASL Pumice

301 - 305 Imagecure Std HASL

306 - 310 Imagecure Glicoat

311 - 315 Imagecure Glicoat Pumice

Solder Balling Customer Test

Solder Mask Surface Pick-up


Solder Mask Surface

Gloss Solder Mask

Reference Electra Presentation


Roughness Profile - Gloss


Matt Solder Mask



Roughness Profile - Matt


Solder ball size

Minimum clearance

Solder ball quantity

Solder Balling Inspection Criteria


PPM Monitoring Ball Criteria Reference

High Speed Video Lead Separation


Outgassing During Wave Soldering

Outgassing During Wave Soldering

Running a sample board through a wave with outany flux indicates if there is volatile material in thesurface of the mask which can contribute to theproblem during soldering

As a board passes through the solder waveoutgassing may occur, most often its related to theamount of volatile from the flux which can thenleads to solder balls, spitting or skipped surfacemount or test pads


Wave Solder Test Board

Fine pitch single sided test board designed to asses solder wetting, solder skips, solder balling &solder drainage from the surface of the mask

Solder Balling During Intrusive ReflowSolder paste printed and reflow on the surface ofsolder mask is common for through hole reflow. Insome cases the combination of paste and soldermask results in solder ball formation

If fairly simple to test some sample boards with aselected paste to look for balls or not. The problemoccurs when customers do not define the soldermask and their supplier


Voiding During Flip Chip UnderfillVolatile material can outgas from the surface ofsolder masks as they pass through reflow orthrough a selective or wave soldering process. Inthe case of underfill this leads to voids in theepoxy when cured reducing the benefits of theunderfill and possible reducing the productreliability

The videos show outgassing from the solder maskthrough the underfill and can be seen through theglass slides mounted over the PCB

This is a very simple test to demonstrate the issue

Solder Mask Wash Off’sSolder masks can retain cleaning materials in thesurface if a board is subjected to a wash off due topoor paste printing. Any board washed off shouldbe left for a time before reprinting. If not theevaporation of the cleaning material duringheating may impact correct reflow of the paste

The video shows poor reflow of the solder pastedue to the wash off of the sample board show.The solvent from the cleaning solution has causeda problem with the flux in the paste


Initial Production Test Results

Reflow of solder pasteReflow and cure of underfill

Wave soldering test patternsDye pen mask testing

Reflow of Solder Paste

Reflow oven set-up for correct profile

Solder paste printed with 0.008” stencil

Underfill dispensed on to test sites

Glass slides placed on underfill

Boards passed through reflowExamine for solder balling and voiding


Reflow Oven Parameters

• 5 Zone Convection Reflow• Total Zone Length of Oven• Zone 1 Length• Zone 2 Length• Zone 3 Length• Zone 4 Length• Zone 5 Length

Reflow of Solder Paste

Peak reflow temperature 227oC


Reflow and Cure of Underfill

Glass balls used as underfill stand off

Glass slides placed on underfillBoards reflowed/cured at 227oC peak




Solder balling was highlighted on samples

Voiding in underfill on selected samples

Voiding also visible under slides adjacent to non masked areas

0

2

4

6

8

10

12

14

16

30min @150C

60min @150C

30min @150C+ UV

30min @150C

60min @150C

30min @150C+ UV

So

lde

r B

alls

(C

op

pe

r R

ou

nd

)

Supplier 1

Supplier 2

Supplier 3


0

2

4

6

8

10

12

14

16

18

20

30min @150C

60min @150C

30min @150C+ UV

30min @150C

60min @150C

30min @150C+ UV

So

lde

r B

alls

(C

op

pe

r S

qu

are

)

Supplier 1

Supplier 2

Supplier 3

0

2

4

6

8

10

12

14

16

30min @150C

60min @150C

30min @150C+ UV

30min @150C

60min @150C

30min @150C+ UV

So

lde

r B

alls

(L

amin

ate

Ro

un

d)

Supplier 1

Supplier 2

Supplier 3


0

2

4

6

8

10

12

14

16

30min @150C

60min @150C

30min @150C+ UV

30min @150C

60min @150C

30min @150C+ UV

So

lde

r B

alls

(L

amin

ate

Sq

uar

e)

Supplier 1

Supplier 2

Supplier 3

Reflow and Cure of UnderfillSample Supplier Finish Cure Solder Balls Mask Voiding

A Coates Matt 30min @ 150C Minor No

B Coates Matt 60min @ 150C Minor Yes

C Coates Matt 30min @150C + UV No Yes

D Coates Gloss 30min @ 150C No Yes

E Coates Gloss 60min @ 150C No No

F Coates Gloss 30min @150C + UV No Yes

G Electra Matt 30min @ 150C Yes Yes

H Electra Matt 60min @ 150C Yes No

J Electra Matt 30min @150C + UV Minor No

K Electra Gloss 30min @ 150C No No

L Electra Gloss 60min @ 150C No No

M Electra Gloss 30min @150C + UV No No

N Shipley Matt 30min @ 150C No No

P Shipley Matt 60min @ 150C No Yes

Q Shipley Matt 30min @150C + UV Minor No

R Shipley Gloss 30min @ 150C No Yes

T Shipley Gloss 60min @ 150C No Yes

U Shipley Gloss 30min @150C + UV No No


Reflow and Cure of UnderfillSample Supplier Finish Cure Solder Balls Mask Voiding

A Coates Matt 30min @ 150C Minor No

B Coates Matt 60min @ 150C Minor Yes

C Coates Matt 30min @150C + UV No Yes

D Coates Gloss 30min @ 150C No Yes

E Coates Gloss 60min @ 150C No No

F Coates Gloss 30min @150C + UV No Yes

G Electra Matt 30min @ 150C Yes Yes

H Electra Matt 60min @ 150C Yes No

J Electra Matt 30min @150C + UV Minor No

K Electra Gloss 30min @ 150C No No

L Electra Gloss 60min @ 150C No No

M Electra Gloss 30min @150C + UV No No

N Shipley Matt 30min @ 150C No No

P Shipley Matt 60min @ 150C No Yes

Q Shipley Matt 30min @150C + UV Minor No

R Shipley Gloss 30min @ 150C No Yes

T Shipley Gloss 60min @ 150C No Yes

U Shipley Gloss 30min @150C + UV No No


Solder Ball Size Measured between 0.002” & 0.003”



Video of Solder paste reflow on hot plate 230oC no balling

0

0.02

0.04

0.06

0.08

0.1

0.12

30min @150C

60min @150C

30min @150C+ UV

30min @150C

60min @150C

30min @150C+ UV

Are

a o

f Fl

ux

Sp

read

Supplier 1

Supplier 2

Supplier 3


0

0.02

0.04

0.06

0.08

0.1

0.12

30m

in @

150

C

60m

in @

150

C

30m

in @

150C

+ U

V

30m

in @

150

C

60m

in @

150

C

30m

in @

150C

+ U

V

30m

in @

150

C

60m

in @

150

C

30m

in @

150C

+ U

V

30m

in @

150

C

60m

in @

150

C

30m

in @

150C

+ U

V

30m

in @

150

C

60m

in @

150

C

30m

in @

150C

+ U

V

30m

in @

150

C

60m

in @

150

C

30m

in @

150C

+ U

V

Are

a o

f F

lux

Sp

rea

d

0

2

4

6

8

10

12

14

16

Area of Spread

AV.square balls

Av

. No

. of S

old

er B

alls

on

Sq

ua

re P

ad

s

M ATT GLOSS

Supplier 1 Supplier 2 Supplier 3 Supplier 1 Supplier 2 Supplier 3

0

0.5

1

1.5

2

2.5

3

3.5

30min @150C

60min @150C

30min @150C+ UV

30min @150C

60min @150C

30min @150C+ UV

Vo

idin

g in

Un

de

rfill

Supplier 1

Supplier 2

Supplier 3




Video of Underfill Reflow on hot plate 230oC no voiding


Wave Soldering Test Patterns

Adhesive dispensed on to boardComponents placed/adhesive cured 120oCBoards wave soldered with no clean fluxSpray fluxerConveyor Speed 1m/minSingle vibrating wave contact 40mm

Wave Soldering Test Patterns

A B C

D E F

Solder Ball Size Measured between 0.002” & 0.004”


0

10

20

30

40

50

60

30min @ 150C 60min @ 150C 30min @150C+ UV

30min @ 150C 60min @ 150C 30min @150C+ UV

So

lde

r B

all C

ou

nt

Supplier 1

Supplier 2

Supplier 3

MATT GLOSS

Dye Pen Mask Testing

Test board tested with Dye Pens

Each of the pens were usedTwo pen stripes prior to test strip

Test based on de-wetting in 2 & 5 sec




Video of dye pen testing, there are 90min of this!!!!



Video of dye pen testing


Video of dye pen testing



34

36

38

40

42

44

46

48

30min @ 150C 60min @ 150C 30min @150C+ UV

30min @ 150C 60min @ 150C 30min @150C+ UV

Supplier 1

Supplier 2

Supplier 3

MA TT GLOSS

Solder Mask AssessmentSolder Mask Variables

ThicknessApplication methodsFinish (Gloss/Matt)Cure ConditionsSupplier product

Measurement TechniquesPencil HardnessContact AngleDie PenThicknessRoughnessMicro Hardness (indenter)SIRCapacitanceGlossReflectance

Process EvaluationReflow Solder ballingWave solderingUnderfill VoidingOutgassing






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To book visit www.npl.co.uk/ei

Testing Solder Mask for Assembly Process Compatib ility

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