IPC-4556 印制板化学镍钯浸金（ENEPIG）规范ENG - 第23页

depending on the intended application. This bath runs at relatively high temperatures and dwell times. The user should ensure compatibility of substrate and solder mask, if used. V endor speciﬁcations for temperature, dw…

APPENDIX 1

Chemical Definitions and Process Sequence

Martin Bayes

Dow Electronic Materials

Chemical Definitions

Electroless Process – This chemical process promotes sustained deposition of a metal or metal alloy onto either a dielec-

tric or metallic PWB surface through an oxidation-reduction chemical reaction, without the application of an external elec-

trical potential. Reducing agents, such as sodium hypophosphite or sodium formate, react at catalytic surfaces to release elec-

trons, which immediately reduce the positively charged metal ions (e.g., nickel ions in ENIG and ENEPIG and palladium

ions in ENEPIG), promoting their deposition onto the PWB.

This type of reaction is described as ‘‘autocatalytic,’’ as the deposition process will continue even after the substrate is com-

pletely covered by a continuous layer of the plated deposit. The deposit thickness will therefore continue to rise in the pres-

ence of source metal ions and a reducing agent, until the board is removed from the plating bath. The thickness of plated

deposits will vary depending on the bath temperature, chemical parameters (such as solution pH) and the amount of time

spent in the plating bath.

Immersion Process – This chemical process uses a chemical displacement reaction to deposit a layer of a second metal onto

a base metal surface. In this reaction, the base metal dissolves, releasing the electrons that reduce the positively charged ions

of the second metal present in solution. Driven by the electrochemical potential difference, the metal ions in solution (e.g.,

gold ions in ENIG or ENEPIG process) are deposited onto the surface of the board, simultaneously displacing ions of the

surface metal into solution.

This type of reaction is described as ‘‘self-limiting’’ because, once the base metal is covered with a continuous layer of the

deposited metal, there is no longer a source of electrons and the reaction ceases.

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 surface will be in a condition allowing it to be uniformly

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

2. Microetch – The purpose of this step is to produce a surface that may be uniformly catalyzed and plated with good deposit

adhesion by removing some copper from the surface. A variety of different etchant types may be used (e.g., sodium per-

sulfate, peroxide/sulfuric). Vendor speciﬁcations for 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 sur-

face. Examples of metal catalysts include palladium and ruthenium (typically deposited by an immersion reaction with

the copper surface). Vendor speciﬁcations for temperature, dwell time, agitation and bath chemical control should be fol-

lowed.

4. Electroless Nickel – The purpose of this bath is to deposit the required thickness of nickel on the catalyzed copper sur-

face. The nickel thickness should be adequate to cover the copper with a substantially pore-free coating, to create a dif-

fusion barrier to copper migration, and also 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 maintained in balance

on a continuous basis, by addition of appropriate replenishment components. Electroless nickel baths typically run at high

temperatures and extended dwell times to achieve the required deposit thickness. It is therefore important to ensure that

compatible PWB substrate and solder mask materials are used. Vendor speciﬁcations for temperature, dwell time, agita-

tion and bath chemical control should be followed.

5. Electroless Palladium – The purpose of this bath is to deposit the required thickness of palladium onto the initial elec-

troless nickel deposit. The palladium thickness should be adequate to provide a surface with the desired solderability

and/or wire bonding characteristics, depending on the intended application. This bath runs at moderately high tempera-

tures. Dwell times will vary, depending on the required deposit thickness. Vendor speciﬁcations for temperature, dwell

time, agitation and bath chemical control should be followed.

6. Immersion Gold – The purpose of this step is to deposit a thin, continuous layer of gold. The gold protects the underly-

ing electroless nickel/electroless palladium layers from oxidation or passivation, and also serves as a contact surface,

IPC-4556 January 2013

depending on the intended application. This bath runs at relatively high temperatures and dwell times. The user should

ensure compatibility of substrate and solder mask, if used. Vendor speciﬁcations for temperature, dwell time, agitation

and bath chemical control should be followed.

7. Rinsing – The purpose of these steps is to remove residual process chemicals from the PWB surface after each chemi-

cal processing step. This may be achieved in either a single step or with multiple rinses. In some instances, pre-dip and/or

post-dip process steps may also be required for optimum process performance. Vendor speciﬁcations for temperature,

dwell time, agitation and turn-over rate should be followed.

8. Drying – The purpose of this step is to ensure the boards are completely dry. This may be achieved by use of either in-line

vertical, or off-line horizontal drying. Off-line horizontal drying should be preceded by a horizontal rinsing step and

should be dedicated to the boards from the ENIG/ENEPIG processes. The time and temperature should be optimized to

suit the type of product.

9. ENIG/ENEPIG Process Combination – Due to the very substantial overlap between the process ﬂows for production of

ENEPIG and ENIG, single process lines can be designed to be capable of producing either product type, by use of dif-

ferent line software control programs.

January 2013 IPC-4556

APPENDIX 2

Round Robin Test Summary

George Milad

Uyemura International Corporation

Objective To determine the appropriate speciﬁcation limits for electroless palladium in ENEPIG for soldering and wire

bonding applications.

Supplier Participation Six suppliers namely: Atotech, Cookson Electronics, Dow Electronics Materials, MacDermid,

OMG and UIC Uyemura, committed to providing samples of the required ﬁnish for the Round Robin testing:

Thickness Measurement Industry capability to accurately measure the thickness of palladium in thickness ranges desired

to be applied to the test vehicle (TV) values was questioned by members of the sub-committee. Prior to the preparation of

the samples for the round robin testing of solderability and wire bonding performance, it was decided to obtain an ENEPIG

test sample to be sent to interested participants with measuring equipment to determine industry capabilities to reproducibly

measure thickness values. The test sample had 16 pads designated for measurement and recorded values were sent to Gerard

O’Brien for compilation. The results are provided in the speciﬁc Appendix 3, authored by Gerard O’Brien. Participants in

this study were able to use this data to conﬁrm their own measurement capability and to identify any need for changes in

their measurement protocols.

Solderabilty and Solder Joint Reliability Testing

The Test Coupon The ‘‘W Coupon’’ described in the IPC-6012 document was used for this phase of the Round Robin. The

coupon is 16.5 cm X 14.0 cm [6.5 in X 5.5 in]. It has 18 sub-units as shown above, as well as 2 detachable BGA coupons.

All submitted samples will be subjected to the stress or conditioning for 8 hours at 72 °C [162 °F] and 85% RH.

Thickness of the ENEPIG to be Tested After additional discussions, the thicknesses of the multiple layers to be tested

were agreed upon and all suppliers targeted the desired thicknesses.

• Electroless Nickel (EN): 5 µm±1µm

• Electroless Palladium (EP): 0.1 µm, 0.2 µm, 0.3 µm and 0.5 µm*

*(from only one supplier).

• Immersion Gold (IG): Supplier preferred immersion gold process (one supplier chose to supply a second ENEPIG sample

set with a very thin immersion gold layer).

All combinations will be tested as follows:

• Soldering with eutectic solder

• Soldering with Lead Free SAC305 alloy

Sample Size Each supplier agreed to provide 6 ‘‘W Coupons’’ per thickness iteration. All samples were shipped to Gerard

O’Brien for coding, prior to being sent to the test locations, to ensure anonymity of the supplier. Gerard will also maintain

a record of the actual thickness plated on each set of coupons, based on measurements made at a single (referee) test

location.

Solderability/Wettability This was evaluated using ‘‘Wetting Balance’’ testing as well as Spread test coupons. Wetting

Balance tests were carried out by G. O’Brien of ST and S Group (see APPENDIX 5). Solder spread testing was conducted

by Brian Madsen of Continental Automotive Systems (see APPENDIX 6).

Solder Joint Reliability Ball Shear testing on the BGA portion of the coupon was used to make this assessment. The testing

was carried out at Rockwell Collins by David Hillman. Both force and fracture mode were recorded. Cross sections of rep-

resentative samples were prepared to examine the composition of the intermetallic compound (IMC) and its conﬁguration.

The data is reported by David Hillman in APPENDIX 7.

Wire Bonding Evaluation

Wire Bonding Coupon The test coupon design was provide by Horst Clauberg of Kulick & Sofa. James Monarchio at TTM

manufactured the test coupons. Jim supplied sets of test coupons at 2 different levels of surface roughness. One to be par-

ticularly coarse (RA ~ 380), utilizing mechanical scrubbing. The surface roughness was measure by proﬁlometrry at Enthone

by Karl Wengenroth.

IPC-4556 January 2013