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IPC-4552 Proposal June 2001 6. Rinsing The purpose of this step is to remove residual proc e ss chemicals from the PWB su rface after each chemical processing step. This may be achieved in a single or m ultiple rinse st …

IPC-4552 Proposal June 2001

APPENDIX 1 CHEMICAL DEFINITIONS

Electroless Process

This chemical process promotes continuous deposition of a metal onto the PWB surface through an oxidation-

reduction chemical reaction, without the use of an external electrical potential. A reducing agent, such as sodium

hypophosphite, donates electrons to the positively charged metal ions in solution, thereby reducing the metal and

promoting its deposition onto the catalyzed metal surfaces of the PWB. This reaction is considered autocatalytic

because it will continue to plate in the presence of source metal ions and a reducing agent until the board is removed

from the plating bath. The thickness of plated deposits varies according to temperature, chemical parameters and the

amount of time spent in the plating bath.

Immersion Process

This chemical process uses a chemical displacement reaction to deposit a metal layer onto the exposed metal surface

of the PWB. In this reaction, the base metal donates the electrons that reduce the positively charged metal ions in the

solution. Driven by the electrochemical potential difference, the metal ions in solution (e.g., gold ions in the ENIG

process) are deposited onto the surface of the board, simultaneously displacing ions of the surface metal (e.g., nickel

ions in the ENIG process) back into solution. This reaction is considered self-limiting, because once the surface

metal is plated, there is no longer a source of electrons and the reaction stops

APPENDIX 2 PROCESS SEQUENCE

1. Cleaner

The purpose of this step is to clean the copper surface in preparation for processing. The cleaner removes oxides,

and light surface contaminants and ensures that the copper surface will be in a condition to be uniformly micro-

etched. Vendor specifications of temperature, dwell time, agitation and bath chemical control should be followed.

2. Microetch

The purpose of this step is to micro-etch the copper and to produce a surface that may be uniformly catalyzed and

plated with good deposit adhesion. A variety of different etchant types may be used (for example: sodium persulfate,

peroxide/sulfuric). Vendor specifications of temperature, dwell time, agitation and bath chemical control should be

followed.

3. Catalyst

The purpose of this step is to deposit a material that is catalytic to electroless nickel plating on the copper surface.

The catalyst lowers the activation energy for nickel deposition and allows plating to initiate on the copper surface.

Examples of catalysts include palladium and ruthenium. Vendor specifications of temperature, dwell time, agitation

and bath chemical control should be followed.

4. Electroless Nickel

The purpose of this bath is to deposit the required thickness of electroless nickel on the catalyzed copper surface.

The nickel thickness should be adequate to create a diffusion barrier to copper migration, and also to serve as a

solderable surface depending on the intended application.

The nickel bath has a relatively high deposition rate and its active chemical components must be replenished and

maintained in balance on a continuous basis. Electroless nickel baths typically run at high temperatures and

extended dwell time to achieve the required deposit thickness. It is therefore important to ensure that compatible

PWB substrate and solder mask materials are used. Vendor specifications of temperature, dwell time, agitation, bath

loading and bath chemical control should be followed.

5. Immersion Gold

The purpose of this step is to deposit a thin continuous layer of immersion gold. The gold protects the electroless

nickel from oxidation or passivation, and also serves as a contact surface depending on the intended application.

This bath runs at relatively high temperatures and dwell times. Ensure compatibility of substrate and solder mask if

used. Vendor specifications of temperature, dwell time, agitation and bath chemical control should be followed.

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IPC-4552 Proposal June 2001

6. Rinsing

The purpose of this step is to remove residual process chemicals from the PWB surface after each chemical

processing step. This may be achieved in a single or multiple rinse steps. In some instances pre-dip and/or post-dip

process steps may also be required for optimum process performance. Vendor specifications of temperature, dwell

time, agitation and turn over rate should be followed.

7. Drying

The purpose of this step is to ensure the boards are completely dry. This may be achieved by online vertical or off

line horizontal drying. Offline horizontal drying should be preceded by a horizontal rinsing step and should be

dedicated to the ENIG process. The time and temperature have to be optimized to suit the type of product.

APPENDIX 3 Qualification of ENIG Process by the Board Supplier

During process qualification, the following aspects shall be required:

Solderabilty

Solderability shall meet the requirements of J-STD-003 Category 3 throughout the life of the nickel bath.

Thickness Distribution

Characterize thickness distribution from panel to panel and within the different features of the panel. Establish

acceptance criteria.

Compatibility

Due to the chemically aggressive nature of the ENIG process, verification of material compatibility is very

important. Verify that the process is compatible with pre- and with post-process steps and material. Solder

mask/legend ink are good example of process or material compatibility concerns.

Processability

Follow vendor instructions for process control to include; temperature/dwell time, frequency of analysis, dump and

remake schedules, controllers etc. The use of automatic controllers for the nickel bath is a must for production

facilities.

APPENDIX 4 Recommendation For Thickness Measurement

General recommendations for accurately measuring the thickness of ENIG on copper clad epoxy laminate,

(Au/Ni/Cu/epoxy) using Energy Dispersive X-ray Fluorescence (EDS-XRF) are as follows. Contact your XRF

supplier for compliance with these recommendations or other specific procedures.

Standards

The measured thickness value for the standard should match the labeled, NIST-traceable value, to within 5% for the

top layer (Au) and 10% for the second layer (Ni). A minimum of one thickness standard should be measured at the

same time as the sample lot. Choose a standard that is close to the expected thickness to be measured. For the most

accurate results, the standard should match the base material of the sample.

Beam Size

The size of the X-ray beam should be at least 30% smaller than the area of the samples measured. Larger beam sizes

could give erroneous higher thickness values for Au. This is a result of interference from the Br in the base material.

Energy Range (Spectral Line)

The energy range or spectral line associated with Au must be carefully selected to avoid overlap from the Cu layer

line and from the Br line in the epoxy laminate. EDS-XRF instruments with solid-state detector typically do not

have overlap issues, and can measure Au using the Au L spectral line. Pin Diode Detector instruments have a small

overlap between the bromine (Br) K spectral line and the Au L line. To avoid Br interference use the Au L line from

9.4 KeV to 10.1 KeV, for the measurement. Gas Proportional Counters, have overlap both between the copper (Cu)

K line and the Au L line and between the Au L line and the Br K line. Select the Au L line from 10.5 KeV to 12.7

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KeV to avoid copper interference and use a numerical filtering technique to remove overlap from the Br.

Measurement time

Measurement time affects the accuracy of the calibration and the precision of the subsequent measurements. For

high levels of accuracy, the instrument should be calibrated using long calibration measurement times, typically 60-

seconds lifetime.

Precision of measurement results can be determined by measuring one location on a sample or standard many times

in succession. The standard deviation of a set of measurements decreases (the precision improves) as a function of

the square root of the measurement time, so that a 40 second measurement will typically have ½ the standard

deviation (be twice as precise) as a 10 second measurement.

APPENDIX 5

Standard Developments Efforts of Electroless Nickel

Immersion Gold IPC-4552

George Milad Shipley Ronal

Gerard O’Brien Photocircuits

Introduction:

Electroless nickel/Immersion gold (ENIG) has been in use in the industry since the early 1990s. As the product gets

more sophisticated with finer pitch, smaller features and thinner substrates, Hot Air Solder Leveling (HASL) was

quickly reaching its limitations. ENIG offered greater latitude over HASL, and continues to gain market share with

product evolution.

ENIG is now widely accepted as a solderable surface with superior and shelf life. The ENIG finish has excellent

contact surface resistance properties and is wire bondable. ENIG is also easy to inspect, it indirectly strengthens the

plated through hole and is lead free.

In spite of a few bumps along the road ENIG continues to gain market share at the expense of other HASL

alternatives. The primary defect that was associated with ENIG was the occasional but rare occurrence of “Black

Pad”. Black pad gained its name from the fact that when the defect was encountered it manifested itself in solder

joint failure at the nickel/solder interface. The surface of the nickel at the interface looked darker or black. A series

of studies were published on the cause of the Black pad. The general understanding to date is that it is the result of

excessive corrosion of the nickel surface during the immersion gold deposition step. The problem is contained

through good process control to achieve corrosion resistant nickel morphology and through minimizing the

corrosion/dwell time in the gold bath.

As time went by, an industry consensus was evolving, defining the required thickness for Nickel and for Gold.

However in the absence of set guidelines some outliners were occasionally called out. Requests for thicker gold (in

excess of 5µin) from an immersion process is both impractical and a source of potential problems. The need to

understand and define this process for the industry became very timely.

The IPC Plating Subcommittee 4-14 took on the task of creating an IPC specification that could be called out by

designers, manufacturers and buyers [Original Equipment Manufacturer (OEM) and assembler or Electronic

Manufacturing Services (EMS)]. The project attracted participation from a representative cross section of the

industry. The active members were compromised of OEMs, CEMs, board manufacturers, chemical suppliers and

others.

The first order of business was to define the process and set the scope of the specification. There was a lot of

agreement on that.

Process definition:

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