IPC J-STD-003B.pdf - 第33页

6 NOTES 6.1 Correction for Buoyancy For the wetting balance to obtain wetting force values that are comparable with one another , it is necessary to correct for the variability in test specimen sizes, particularly width …

5 EVALUATION AIDS

5.1 Evaluation Aids – Surface

As an aid to evaluation of

the test results (see Figure 5-1). This aid is to be used pri-

marily to illustrate types of defects rather than percentage

of area covered.

5.2 Evaluation Aids – For Class 3 Plated-Through

Holes

Proﬁle views of acceptable conditions are pre-

sented in Figure 4-5 for aid in visualizing all the common

conditions. The following are also acceptable conditions

for speciﬁc cases:

• Solderability acceptance for plated-through holes with

aspect ratios greater than 5:1 (board thickness: hole diam-

eter) shall be agreed upon by user and vendor.

• Depressed ﬁllets in holes are acceptable under the follow-

ing condition: the solder in partially ﬁlled holes must

exhibit a contact angle less than 90 ° relative to the hole

wall (see Figure 4-5 and 4-6).

• All holes less than 1.5 mm [0.0591 in] diameter shall

retain a solder plug after solidiﬁcation. Holes greater than

1.5 mm [0.0591 in] shall not be rejected for failure to

retain a full solder plug provided that the entire barrel of

the hole and the surface of the top land have been wetted

with solder (see Figures 4-5 and 4-6).

Preferred

Wetting

Small Amount

of Dewetting

Complete

Dewetting Nonwetting

IPC-003b-5-1

Figure 5-1 Aid to Evaluation

IPC J-STD-003B March 2007

Provided by IHS under license with IPC

Not for Resale

No reproduction or networking permitted without license from IHS

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6 NOTES

6.1 Correction for Buoyancy

For the wetting balance to

obtain wetting force values that are comparable with one

another, it is necessary to correct for the variability in test

specimen sizes, particularly width and thickness. This is

done by correcting for the volume of the sample immersed

in the solder. The following formula may be used to calcu-

late the buoyant force correction:

= ρ gV

where:

ρ = Density of solder at 235 °C (8.12 g/cm) for Sn60/

Pb40 Alloy

ρ = Density of solder at 255 °C (7.41 g/cm) for SAC305

Alloy

g = Acceleration of gravity (981 cm/sec

)

V = Immersed volume of the test specimen (cm

)

= width x thickness x immersion depth, for example.

When the buoyancy force is calculated, it should be used

to correct the zero axis. This correction is required to

obtain both the proper measurement of wetting times, as

well as wetting forces. All measurements of wetting times

and wetting forces must be made from the corrected zero

axis. In the case of an upright curve, the new corrected zero

axis will be below the instrumental zero.

6.2 Preheat If the board test specimen does not pass the

standard solderability test then a uniform preheat of a sec-

ond test specimen may be used to determine if design/

construction has impacted the solderability test (i.e., a thick

board with heavy internal ground planes). If this referee

test specimen passes, then testing with preheat shall be the

method of choice for future testing of test specimens hav-

ing that design/construction.

6.3 Baking The occurrence of outgassing, which may

result in blowholes, measling, blisters or delamination, may

be reduced by baking the printed board prior to soldering

to eliminate moisture or solvents. Other factors, such as

conveyor speed (for wave solder testing), solder tempera-

ture, contamination content, etc., may also cause defects

and, therefore, should be analyzed if problems occur. Test

specimens should be baked in a suitable oven to remove

any absorbed moisture. Temperature and time of baking is

to be determined on an individual basis.

The time between baking and solderability testing should

be kept to a minimum (not more than 24 hours) in order to

prevent re-absorption of water vapor into the laminate

structure. The actual time delay threshold is dependent

upon ambient temperature and humidity levels.

6.4 Prebaking Printed boards should be prebaked only if

prebaking is normally used as a production procedure.

Baking should be kept to a minimum, adhering to the pro-

duction procedure, to prevent excessive oxidation and

intermetallic growth.

6.5 Safety Note Care must be taken in both usage and

storage to keep ﬂammable solvents from sparks or ﬂames.

See the Material Safety Data Sheets (MSDS) for all sol-

vents. All chemicals shall be handled per appropriate data

sheets, and disposed of per local regulations.

6.6 Use of Nonactivated Flux This standard speciﬁes a

rosin-based ﬂux with a very speciﬁc quantity of activator.

The intent of requiring the use of a speciﬁc quantity of ﬂux

activator is to reduce the variability of test results that were

seen with pure rosin ﬂux, enable the solderability testing of

nontin component lead metallizations, and provide a realis-

tic solderability testing safety factor by keeping the amount

of activator both ﬁxed and less than that used for produc-

tion soldering. The beneﬁt of using this speciﬁed activated

solderability testing ﬂux composition was demonstrated by

extensive testing, as reported in the J-STD-002/003 Acti-

vated Solderability Test Flux Rationale Committee Letter.

6.7 Solder Contact The solder applied during the solder-

ability test must contact a feature in order for that feature

to be considered for evaluation. Small features surrounded

by a thick solder mask may prevent solder contact.

March 2007 IPC J-STD-003B

Provided by IHS under license with IPC

Not for Resale

No reproduction or networking permitted without license from IHS

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APPENDIX A

Calculation of Maximum Theoretical

Force for a Rectangular Cross-Section

Maximum theoretical force for the test board with a ground

plane surface is calculated using the procedure of Klein

Wassink.

The maximum force, in units of milliNewtons

(mN), is deﬁned as:

Force (Max. Theoretical) = (γ) (P) (cosine β) - (d)(g)(V) =

[0.4P - 0.08V] mN

where:

P = The perimeter of the test specimen in millimeters, i.e.,

the length in millimeters of the solder/printed board or

coupon pad (or hole)/air interface as measured at

maximum depth of immersion.

V = The volume in cubic millimeters of the test specimen

that resides below the solder/board air interface as

measured at the maximum depth of immersion.

γ = Surface tension of solder = 0.4 mN/mm

γ = Surface tension of Pbfree solder = 0.5 mN/mm

α = Immersion angle of the board to the horizontal surface,

i.e., α =45°

β = Wetting angle of solder to the board under optimal

conditions, i.e., β = 0, Therefore the cosine β =1

d = Density of solder at 235 °C, = 8120 kg/m

for Sn60/

Pb40 Alloy

d = Density of solder at 255 °C = 7410 kg/m

for SAC305

Alloy

g = Gravitational constant = 9.8 x 10

mm/s

Periphery and volumes Perimeter and volumes are to be

calculated using the nominal values provided by the test

board supplier and the angles and depths of immersion as

described in the speciﬁcation above. The TOTAL perimeter

(the length in millimeters of all of the solder/coupon or

coupon/pad (or hole)/air interfaces on the test coupon being

immersed (e.g., if there are ﬁve pads being immersed, then

the sum of the widths of the ﬁve pads parallel to the solder

surface) is to be used. For the immersion volume, use the

volume of the portion of the test coupon pushed below the

surface of the solder and NOT the entire volume of the

whole test coupon, is to be used in this calculation. Where:

For Example:

For a tin/lead solder alloy:

Width of coupon = 0.4 mm, Length = 9.2 mm, P = wetting

perimeter = 10 mm, Immersion depth=D=0.2mm

Hence for a dip at a 90 ° angle:

V = Total volume immersed = (10 - (2 x 0.4)) mm x

0.2 mm x 0.4 mm = 0.736 mm

Therefore, the maximum theoretical wetting force is:

Maximum Force = (γ) (P) (cosine b) - (d) (g) (V) =

(0.4 mN/mm x 10 mm x cosine 0) - (8.12 x 10-6 kg/mm

x 9.8 x 103 mm/s

x 0.736 mm

) = 3.94 mN

Finally, for a 10 mm perimeter, ideal wetting force per mil-

limeter of perimeter for our sample is 0.394 mN/mm. From

Table 4.5 (or 4.6) the force measured on a test specimen in

the ‘‘preferred’’ class must be close to 0.394 mN/mm. (It

CANNOT be greater than 0.394 mN/mm.)

Theoretical force calculations are difficult for test speci-

mens with other than pads that come to the edge of the

specimen. Therefore, the best way to use the wetting bal-

ance test method is to separately set up a control value for

a ‘‘Best Possible’’ sample; and other test pieces will be

compared to this value for establishing either an acceptable

or a reject criterion.

Second Example:

The calculations for the same sample dipped into the same

solder at a 45 angle

For a tin/lead solder alloy:

Width of coupon = 0.4 mm, Length = 9.2 mm, Immersion

depth=D=0.2mm,P=wetting perimeter = 10 mm

Hence for a dip at a 45 ° angle:

V = Total volume immersed = 0.5 x 9.2 mm x 0.283 mm

x 0.283 mm = 0.368 mm

(The 0.5 accounts for the fact that you are only dipping at

a 45 ° angle). Remember the area of a right angle triangle

is one half times the length of the two sides that are not the

hypotenuse.) Still assuming perfect wetting (wetting angle

= 0 °) Cosine of0°=1

Therefore, the maximum theoretical wetting force is:

Maximum Force = (γ) (P) (cosine b) - (d) (g) (V) =

(0.4 mN/mm x 10 mm x cosine 0) - (8.12 x 10

-6

kg/mm

x9.8x10

mm/s

x 0.368 mm

) = 3.97 mN

Therefore, again for a 10 mm perimeter, ideal wetting force

per millimeter of perimeter for our sample 0.397 mN/mm,

slightly higher than in the previous example because the

buoyancy correction is only half the size.

2. R.J. Klein Wassink, ‘‘Soldering in Electronics,’’ 2nd Edition, Electrochemical Publications, Ayr, Scotland, 1989, pp 308-309

IPC J-STD-003B March 2007

Provided by IHS under license with IPC

Not for Resale

No reproduction or networking permitted without license from IHS

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