1Scope This test method provides a means to assess the propensity for surface electrochemical migration.This test method can be used to assess soldering materials and/or processes.
2Applicable Documents
2.1IPC
IPC-B-25Multipurpose Test Board
IPC-B-25A Multipurpose Test Board
IPC-6012A Qualification and Performance Specification for Rigid Printed Boards
IPC-9201Surface Insulation Resistance Handbook
2.1American Society for Testing and Materials(ASTM) ASTM D-257-93Standard Test Methods for DC Resistance or Conductance of Insulating Materials
3Test Specimens IPC-B-25(B or E pattern)or IPC-B-25A (D pattern)test boards shall be used,with conductor line widths and spacings of0.318mm[0.01250in].The method of manufacture should provide optimized conductor edge definition(refer to the Class2and3conductor width require-
ments in IPC-6012).The finished test boards should be untreated,bare copper,unless another surface finish is part of the evaluation.Figure1shows the IPC-B-25A test board;the D pattern is identical to the IPC-B-25B or E pattern.For pro-cess evaluation,the test pattern board should be made using the same substrate material as will be used in practice to duplicate actual working conditions.
4Equipment/Apparatus
4.1Test Chamber A temperature/humidity chamber capable of producing an environment of40°C±2°C[104±3.6°F],93%±2%RH,65°C±2°C[149±3.6°F],88.5%±3.5%RH,or85°C±2°C[185±3.6°F],88.5%±3.5%RH and allowing test boards to be electrically biased and mea-sured without being opened under these temperature and humidity conditions is used.
4.2Measuring Equipment High resistance measuring equipment,equivalent to that described in ASTM D-257-93,with a range up to1012ohm and capable of yielding an accu-racy of±5%at1010ohm with an applied potential of100VDC (10%tolerance);standard resistors should be used for routine calibration.
4.3Power Supply Equipment capable of providing10 VDC at100µA,with a10%tolerance,shall be used.
4.4Current-Limiting Resistors Use one106ohm resistor in each current path.This equates to three curr
ent-limiting resistors for each5-point comb pattern.Note that some test equipment has the current limiting resistors built into the test-ing system.
4.5Connecting Wire Use PTFE-insulated,solid-conductor,copper wire,or equivalent.(See IPC-9201Surface Insulation Resistance Handbook.)
IPC-26141-1
Figure1IPC-B-25A Test Board
Material in this Test Methods Manual was voluntarily established by Technical Committees of IPC.This material is advisory only
and its use or adaptation is entirely voluntary.IPC disclaims all liability of any kind as to the use,application,or adaptation of this
material.Users are also wholly responsible for protecting themselves against all claims or liabilities for patent infringement.
Equipment referenced is for the convenience of the user and does not imply endorsement by IPC.
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4.6Other Dedicated Fixtures Hardwiring is the default connection method.Other dedicated fixtures may be used, provided that the fixture does not change the resistance for more than0.1decade compared to a comparable hardwired system,when measured at the test conditions.
5Procedure
5.1Test Specimen Preparation
5.1.1In performing a material ,flux),all specimens are to be cleaned and dried using a process capable of yielding a minimum insulation resistance value of 4x1010ohm when tested at35°C,85%minimum RH after 24hours.If this test is being performed as a process qualifi-cation,additional pre-test processing is not allowed.
5.1.2A minimum of three test specimens cleaned per5.1.1 shall be used for controls.
5.1.3For liquid flux:
Apply the liquid flux to the entire surface of the test specimen by brushing liberal quantities of the flux onto the specimen,by floating the specimen comb side down on the liquid flux,or by dipping the specimen into the flux.The specimen shall be drained vertically for one minute with the fingers of the comb pattern vertical.Alternatively,flux may be applied by produc-tion application processes-spray,foam,or wave.The edge connector fingers should be protected from flux.
It is recommended that production wave soldering equipment be used for soldering the test specimens,with a preheat pro-file representative of production.A solder fountain may be used(not a solder pot),with a residence time similar to the residence time in a solder wave.Solder composition is usually 60%tin±5%,remainder is lead;for such alloys,the solder temperature shall be250°C±6°C[482±10.8°F].For alloys other than those with compositions near the tin-lead eutectic, the solder temperature will be compatible with the usual sol-dering temperature for the alloy used.
If any solder bridging occurs,that specimen shall be dis-carded.A minimum of three specimens from the sample group shall be tested.
5.1.4For solder paste:
A squeegee or screen printer shall be used with a stencil imaged with the test pattern.It should be note
d that the Tel-cordia GR-78pattern requires a minimum stencil thickness of 0.20mm[7.9mil].Due to the fact that the minimum stencil thickness is often dependent on the pitch or trace width and spacing,a smaller stencil thickness may be used for fine fea-tures and shall be agreed upon between the tester and cus-tomer for the purpose of this test method.
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Reflow the printed specimens using convection,infrared,or vapor phase reflow equipment using a reflow profile represen-tative of production.Equivalent methods may be used if such equipment is not available.
If any solder bridging occurs,that specimen shall be dis-carded.The edge connector fingers should be protected from paste.
A minimum of three specimens from the sample group shall be tested.
5.1.5For flux-cored wires:
Using a hand soldering iron and the cored wire under test, carefully apply solder to the fingers of all comb patterns.The edge connector fingers should be protected from flux.
If any solder bridging occurs,that specimen shall be dis-carded.
A minimum of three specimens from the sample group shall be tested.Each circuit path will be tested for the presence of solder shorts using a resistance digital multimeter).
5.1.6Post solder cleaning shall be performed only when such cleaning is part of the production process used in the final assembly.
5.1.7When evaluating incoming board quality and/or final finishes,test specimens shall be used as received or as speci-fied by the end user.
5.1.8Attach test leads to the land areas of all patterns either by mechanical ,edge connectors,spring-loaded pins)or by hand soldering using Rosin(R)cored wire,using a shield to protect the test patterns from flux contamination dur-ing soldering;the flux shall not spread into the pattern area. Do not remove the flux.
5.2Test Procedure
5.2.1Place the terminated test specimens in a suitable rack that maintains the specimens at least2.5cm apart and such that the air flow is parallel to the direction of the test speci-mens in the chamber.For hardwiring,wires should be
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dressed from the bottom to prevent flux residues from the wire attachment from flowing onto the test patterns.With mechanical fixtures,fixtures should be to the side.Insert the limiting resistors in terminating leads1,3,and5of each pat-tern.
5.2.2Place the rack approximately in the center of the test chamber.Route the wires to the outside of the chamber; dress the wiring away from the test patterns.Ensure that drops of condensation cannot fall on the specimens.
5.2.3Close the chamber and allow all samples to stabilize for96hours at the specific temperature and humidity.After the96-hour stabilization period,the initial insulation resistance measurements shall be made using voltage in the range of45 VDC to100VDC.Due to polarity,measurements should be made between terminals1and2,3and2,3and4,and5and 4,at the specific temperature and humidity with the current limiting resistors placed in series with the test circuit.Termi-nals2and4shall be at one potential,and terminals1,3,and 5at the opposite potential.
5.2.4Connect the samples to the power supply with the current limiting resistors placed in series with the test circuit, and apply10VDC for the duration of the test.The test polar-ity shall be the same as the
measurement polarity used in section5.2.3.
5.2.5After500hours of applied bias(596hours total),dis-connect the power supply and repeat the measurements per 5.2.3with the specimens under test conditions.
5.3Data Handling The average(geometric mean)insula-tion resistance(IR
avg
)is calculated from:
IR avg=10[1NΣ1N log IR i ]
where,
N=number of test points(10minimum),
IR
i
=individual insulation resistance measurements
Where an assignable cause of low insulation resistance,which is properly attributable to the materials of construction or to the process used to produce the test board,can be found, then such a value can be excluded from calculating the aver-age.
Such assignable causes include:
•Contamination on the insulating surface of the board,such as debris,solder splints,or water droplets from the condi-tioning chamber
•Incompletely etched patterns that decrease the insulating space between conductors by an amount greater than that allowed in the appropriate design requirements drawing
•Scratched,cracked,or obviously damaged insulation between conductors
A minimum of10test measurements is required for the test to be valid.
5.4Visual Examination After completion of the test,the test specimens shall be removed from the test chamber and examined,with back-lighting,at10x magnification for evi-dence of electrochemical migration(filament growth),discol-oration,and corrosion.
Note:Localized electrochemical migration on one comb may be caused by a testing anomaly.
6Notes
6.1Reference Documents
6.1.1IPC-TR-476A Electrochemical Migration:Electrically Induced Failures in Printed Wiring Assemblies
6.1.2IPC-9201Surface Insulation Resistance Handbook 6.1.3Telcordia GR-78-CORE
6.2Specification of Test Conditions Users of this test method will need to specify one(1)of the three(3) temperature/humidity conditions called out in section  4.1. Note that IPC-TR-476A recommends using65°C,85%RH.
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