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IPC-4552 Proposal June 2001 KeV to avoid copp er 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 preci…

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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IPC-4552 Proposal June 2001

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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IPC-4552 Proposal June 2001

ENIG is an electroless nickel layer capped with a thin layer of immersion gold. It is a multifunctional surface finish,

applicable to soldering, aluminum wire bonding, press fit connections, and as a contact surface. The immersion

gold’s primary function is to protect the underlying nickel from oxidation/passivation so that it retains its integrity

for its intended use.

Industry Survey:

As a starting point for this specification the members of the committee were asked to submit typical values of ENIG

thickness, that they either recommended (supplier) or that, they were requested to supply by their customers. The

purpose was to find out what thickness of nickel and of gold were called out by designers and what board shops

were delivering, as well as what chemical suppliers were saying are the capabilities of their respective processes.

Figure 1 and 2 show the results of this survey.

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