b 1 4

Removal of Rosin and Resin based Solder Flux from electronic -=’

i

assemblies with N-Methylpyrrolidone/water mixtures. 6 963- PJF Bv William C. Walsh

BASF Corporation, Chemicals Division Parsippany, N J

7

. INTRO D U CTlO N

The use of the solvent, N-Methylpyrrolidone (NMP), in the integrated circuits manufacturing industry, has been established for many years.

NMP has a Kauri-Butanol value>300, and thus has the capability of solvating a very broad range of substances. In the integrated circuit fabrication process, NMP containing formulations are found to be most effective as photoresist (particularly POSITIVE resists) removal agents. The cured polymeric films have high crosslink densities, and require the use of very active solvent systems to remove them from the metal alloy substrates.

Also, because NMP forms complexes, to varying degrees, with unsaturated hydrocarbons, the aromatic based resist resins, tend to remain soluble in the NMP once they have been removed from the integrated circuit surfaces.

The abilities to both solvate resin systems, and to complex with unsaturated hydrocarbons make NMP a desirable solvent to be included in solder flux removal formulations.

DISCUSSION

The long established rosin based fluxes, have some unsaturation in the hydrocarbon back bone. Both natural and modified rosinous materials are soluble in NMP because the NMP complexes with the areas of unsaturation in the rosin.

The more recently used "water soluble" solder flux products usually have some oxygenated hydrocarbon backbones that are present in the form of low molecular weight chain polymers.

These polymers, by definition, are to some extent, soluble in water. They also are readily soluble in NMP.

The high solvent activity of pure NMP can lead to some potential problems of degradation of polymeric substrates and encapsulating compounds which are commonly used in the circuit fabrication industry. Even the FR-4 boards themselves, can be degraded when in contact with hot NMP for extended periods of time.

This potential problem, is easily overcome by blending water into the NMP. The high degree of hydrogen bonding that occurs between NMP and water, ties up a great amount of the electronic energy in the NMP that is free for solvating activity.

The NMP/water blends, do not have the energy to aggressively attack the polymeric substrates, but still have enough solvating capability to remove and solubilize the film of solder flux present on the board and components surfaces.

Two other properties of NMP make it an ideal component in flux removal/cleaning formulations:

a) High flash point: 91% (196") Since most flux removal is done at elevated temperatures, the high flash point of NMP

allows for greater degrees of safety when using heated baths (* The flash point of a 80% (by weight) NMPMO% water mixture is 11 8% (244°F))

b.) One of the most important characteristics of N-Methylpyrrolidone, that is often

N-Methylpyrrolidone has no tendency to aggressively react with, or attach to any of the metals overlooked, is the amotic nature of the NMP molecule.

found in the current list of metals or alloys employed in circuit assembly manufacture.

SOLUBILITY OF TYPE R* ROSIN FLUX IN NMP/H,O BLENDS

FLUX 97: TYPE R

NMP (% by weight)

50 60

80

100

Amount Rosin Soluble (Yo by weight)

78GF <0.1

0.1

0 > l o

100gF 120eF 140eF G 0.26 1 0.97 1 2.82 5.54 I >IO I -

160gF 1 0.20 I

- I

'Superior Flux Manufacturing Co. Cleveland, Ohio (Superior Flux No. 97)

SAFETY COMMENTS

NMP is a solvent, and like most solvents, it will de-Fatlde-oil the skin. When using NMP, make sure that proper NMP resistant Butyl or Neoprene rubber gloves are worn. Also, as with all solvents, goggles should be worn to protect the eyes from accidental splashes.

NMP has a flash point (196'F/91eC). When water is added to NMP the flash point increases (Example: 80/20 blend has a flash point of 244eF/118eC). Even with the high flash points of NMP and NMP/water blends, care should be taken to follow all appropriate rules and regulations governing the use of combustible solvents in cleaning baths.

FINAL COMMENTS

The continuing trends of both downsizing of assemblies and higher concentration density of discrete components on the smaller assemblies, will call for the changing or replacing of the current non CFC cleaning technologies. At some point in the future, even the minute amounts of residue left from no clean fluxes, may give rise to problems.

Water or water surfactant blends may not be the best available technologies for flux removal. However, with the inclusion of NMP, in the cleaning solution, to aggressively aid in both solvating and coupling the flux residue into the water, aqueous or semi aqueous cleaning technologies can be extended and enhanced.

The following pages show some data for various NMP/water blends. The sets of data are put forth to give the reader a better understanding of the ability for N-Methylpyrrolidone/water blends to aid in the removal of solder flux residue.

(Some data is presented on the use of Plurafac RA-40/NMP blends. Plurafac RA-40 is manufactured and marketed by BASF CORPORATION PERFORMANCE CHEMICALS GROUP.)

Step

Step 2:

Step 3:

Step 4:

PROCEDURE FOR CLEANING SOLDER FLUX FROM CIRCUIT ASSEMBLIES

I m me rse/S u b m e rg e asse m b I i es i n t o NMPNVater Blend e Spray under immersion bath (Agitated) e Bath Temperature 63% 0 Dwell Time In Bath 2 - 5 minutes

Immerse/Submerge assemblies into a second NMPMlater Blend e

e Bath Temper at u re 63% e Dwell time In Bath 1 minute

Spray under immersion bath (Agitated)

I m me rse/S u bm e rg e asse m b I i es in to Deionized Water Bath e Agitated Bath (spray under immersion)

0 Dwell Time In Bath 2 minutes 0 Bath Temperature 50e - 68% *

I

Dry Assemblies with Forced Hot Air

* Temperature of the water bath can be higher or lower; - If it is more economical to run hot air dryer at higher temp, then run bath at lower temp - Usually, more economical to dry higher temp water off of parts with air; (Quicker)

DIAGRAM OF BASIC CIRCUIT ASSEMBLY CLEANING PROCESS

N M PNVa te r NMP/Water Deionized Water

+ Rinse Rinse Station Station

+ + Cleaning Station (63°C) (63°C) (50 - 68°C)

Forced Hot Air Dryer

N M P N a t er Clean Up**

** NMPIWATER can be cleaned of flux rosidresin by vamunl distillation

* Flux sludgeMMP residue can be accuniulated via low temperature vacuum distillation or membrane filtration

SOLUBILITY OF TYPE RA" ROSIN FLUX IN NMP/H,O BLENDS

FLUX 100: TYPE RA

NMP (Yo by weight) Amount Rosin Soluble (% by weight)

60 I 0.1 I -

70 I 0.1 I 0.2 80 I 2.50 I 6.02

- 90 100 - I 'lo I

1 209F 0.1 0.2 1.42 >10

1 40QF

0.3

160°F

0.5

*Superior Flux Manufacturing Co. Cleveland, Ohio Activated Rosin Flux (Superior Flux No. 100)

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SOLUBILITY OF TYPE OA* WATER SOLUBLE FLUX IN NMP/H,O BLENDS

FLUX 30: TYPE OA

I NMP (Yo by weight) Amount soluble (Yo by weight)

70 >I 0

I

1 O09F

c

'Superior Flux Manufacturing Co. Cleveland, Ohio

An organic acid (OA) water soluble flux consisting of urea, glutamic acid activator, water (Superior Flux No. 30)

SOLUBILITY OF TYPE OA* FOAM FLUX* IN NMPNATER BLENDS

FLUX 91: TYPE OA FOAM FLUX

-

'Superior Flux Manufacturing Co. Cleveland, Ohio An organic acid, water soluble, non halide foam flux (Superior Flux No. 91)

SOLUBILITY OF TYPE OA CONCENTRATE* WATER SOLUBLE FLUX IN NMPWATER BLENDS

FLUX 430: TYPE OA CONCENTRATE

Superior Flux Manufacturing Co. Cleveland, Ohio An organic acid, water soluble, non halide flux concentrate (Superior Flux No. 430)

REGENERATION/RECYCLE OF SPENT NMPDVATER CLEANING SOLUTIONS

Filled with Rosin Flux: A) Dilute the solution to 50/50 (by weight) NMPNater

Agitate or shake vigorously; rosin becomes insoluble (floats to surface) Skim off rosin; energy efficient vacuum Distill water out, back to 80/20 (NMPNater)

B) Or Vacuum Distillation of Solution

Filled with Water Soluble Flux Vacuum distillation of solution

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FR-4 EPOXY SOAK TEST PROCEDURE

1. Immerse boards (totally) into the NMPNVater Bath: Agitated Bath

63% 2. Remove board from NMPMater bath after desired time

(5, 10, 20 minutes were tested) 3A. When appropriate* immerse board into agitated, 51 % Deion-

ized Water rinse bath for 2 minutes 3. After removing board from NMPNVater, or water, bath blow off

excess liquid with nitrogen "lace boards in oven set at 85°C for 20-30 minutes. After

'I stre removed from oven allow to cool 30 minutes before

Tests were run with and without water rinse for 70/30 blend

SOAK TEST: FR-4 EPOXY LAMINATE

5 minutes 10 minutes

70% NMP / 30% WATER

0.299 0.1 69 0.295 0.278

(Yo weight gained by board)

c

Dwell Time 1 in NMPNVater bath No Deionozed

water rinse 2 minute *

deionized water rinse

20 minutes 0.380

*After doing this experiment it was decided that there was a benefit to a water rinse, so the other tests carried out on 80/20 (NMP/water) and 100% NMP baths only were done with 2 minutes water rinse

SOAK TEST: FR-4 EPOXY LAMINATE

Dwell Time in NMPNater bath

80% NMP 120% WATER

2 minute deionized water rinse

("10 weight gained by board)

0.41 9

I 1 /’

SOAK TEST: FR-4 EPOXY LAMINATE /’

100% NMP

J ,/’ (% weight gained by board)

Dwell Time in NMP bath

10 minutes*

2 minute deionized

water rinse

5 minute deionized

water rinse

10 minute deionized

water rinse

(0.66)

(0.68)

(0.66) (0.64)

It was observed that small flecks of the boards were floating in the bath after the 10 minute mark. Boards were being dissolved. It is not recommended that 100°/~ NMP be used to clean FR-4 laminate boards

FIELD TEST* NMPNATER BLEND CLEANING

OF FR-4 EPOXY LAMINATE

BOARDS CLEANED: Fully assembled multi layer FR-4 Epoxy Laminate circuits (Alpha metals 620F RMA Rosin based flux)

PRODUCT USED: 79.6% (by weight) NMP

Plurafac RA-40 0.4 Cleaning Bath Unagitated, 55%

Water Rinse 2 minut,es, 60% Hot Air Dry

Deionized Water 20.0

Dwell Time 5 minutes

20 minutes

- -AFAB Redmond, Washington

SURFACE TENSIONS NMP + PLURAFAC RA-40 BLENDS

43.0

42.0

41 .O

40.0 -

39.0 -

38.0

37.0 -

( 2 P C)

36.5

I I 0.1 0.2 0.3 0.4 0.5 0.6

% Plurafac RA-40 (by weight)

SU

RFA

CE

TE

NS

ION

(D

Y N

ES

/CM

)