Printed circuit board cleaning quality inspection (1)

Quality requirements

1. Quality requirements of raw materials

1) Tin-lead solder

The chemical composition of press-working tin-lead solder must meet the requirements of GB/T 31311.

The chemical composition of cast tin-lead solder must meet the requirements of GB/T 8012.

2) flux

Regarding flux quality, it should be from appearance, physical stability and color of the flux, non-volatile content, viscosity and density, water extraction resistance value, halogen content, solid content, solderability, dryness, copper mirror corrosion, insulation resistance , ion pollution and other aspects of detection.

Such as no-clean liquid flux requirements:

1) Appearance: liquid is transparent, uniform, no impurities, precipitation, foreign matter and strong irritating odor;

2) Solid content: not more than 10%;

3) Halogen content: Halogen-free ion;

4) Solderability: The expansion rate is not less than 80%;

5) Copper mirror corrosion: The copper mirror should have no penetrating corrosion;

6) Insulation resistance: (after welding) is greater than 1*10Ω;

7) Ion pollution levels and requirements are in accordance with Table 1.

Table 1

Grade NaCl content, μg/cm2
1 <1.5
2 1.5-3.0
3 3.0-5.0

Another example is Rosin base liquid flux according to GB/T 9491 requirements:

1) Appearance: The quality of flux should be uniform, transparent, no precipitation or layering, no foreign matter;

2) Physical stability and color: The flux should be kept transparent and without delamination or precipitation; or the color of the R-type flux should not be deeper than the color gradation number 11 of the ferrochrome colorimeter, the color of the RMA type should not be deeper than the iron Drill colorimeter level number 13;

3) Non-volatile content: not less than 15%;

4) Viscosity and density: When the viscosity is 50C, it should be quickly inhaled by medical pipettes; when the density is 250C, it should be 0.80g/cm3-0.95g/cm3;

5) Water extraction resistance: The average resistivity of water extracts of R-type and RMA-type flux is not less than 1*105Ω.cm, and the average resistivity of RA-type flux is not less than 5*105Ω.cm;

6) Halogen content: R-type and RMA-type fluxes should not make the color of the silver chromate test paper white or light yellow, and the halogen content of the RA type flux should be 0.07%-0.20% or comply with the relevant regulations;

7) Solderability: The flux expansion rate R type should not be less than 75%, the RMA type should not be less than 80%, and the RA type should not be less than 90%;

8) Dryness: The flux residue surface should be non-viscous, chalky powder on the surface should be easily removed;

9) Copper mirror corrosion: RA type flux should be basically unchanged, RMA type flux should not make the copper film have penetrating corrosion;

10) Insulation resistance: The insulation resistance before and after welding should not be less than the value in Table 2.

Table 2

Flux Type Level 2
R, RMA 1*1012Ω 1*1011Ω
RA 1*1011Ω 1*1010Ω

IPC-SF-818 provides the surface insulation resistance of the flux as shown in Table 3.

2. Printed Board Cleaning Quality Requirements

At present, China's electronics industry has not formed a uniform cleaning quality specification for the printed circuit board as the final product. In the industrial standards that are more commonly used in developed countries, there are the following provisions on the cleaning quality of printed circuit boards.

1) J-STD-001B provides:

A, ionic contamination content: <1.56μgNaCl/cm2;

B, flux residue: Level <200 μg NaCl/cm2, Level 2 <100 μg NaCl/cm2, Level 3 <40 μg Na-Cl/cm2;

C, the average insulation resistance> 1 * 108 Ω, (log10) standard deviation <3.

2) The values ​​of IPC-SA-61 according to the process are shown in Table 4.

3) MIL-STD-2000A specifies an ionic contaminant content of <1.56 μg NaCl/cm2.

In addition, in the MIL-P-28809 specification, it is also stipulated that the resistivity of the cleaning or cleaning solution may be used as a criterion for the degree of cleaning. The resistivity of the cleaning solution is greater than 2*10<6> OMEGA.cm, which is otherwise clean. This method is suitable for monitoring the cleaning process.

Due to the appearance of various commercial surface ion contamination testers, the test results of different test systems are all different, but they are all higher than manual test results. Therefore, the concept of equivalent coefficient was proposed and the test results of different systems were comparable. (table 5).

table 3

grade

Flux Type Level 1 Level 2 Level 3 (500C, 90%RH, 7 days) (500C, 90%RH, 7 days) (500C, 90%RH, 7 days)
Low activity flux 100 100 100
Medium active flux 100 100 100
Active flux 100 100 100

Table 4

content

Process ion contaminant content flux residue process A <1.5μg NaCl/cm2 <217μg/plate process C <2.8μgNaCl/cm2 <2852μg/plate process D <9.4μgNaCl/cm2 <1481μg/plate average insulation resistance> 1*108Ω The standard deviation of (log10) is <3 >1*108Ω, and the standard deviation of (log10) is <3
Note: 1 Process A: PCB bare board - test;

2 Process C: bare boards - SMT - reflow - cleaning - testing;

3 Process D: bare boards - SMT - reflow - cleaning - wave soldering - cleaning - testing;

4 The test board is IPC-B-36.

table 5

Method equivalent instrument method "acceptable range", μgNaCl/cm2
MIL-P-28809
Backman
7.545/7.545=1 1.56
MIL-P-28809
Markson
7.545/7.545=1 1.56
Omega Meter 10.51/7.545=1.39 2.2
Lonograph 15.20/7.545=2.01 3.1
Lon Chaser 24.50/7.545=3.25 5.1

Surface ion contamination detection standard

1) Manual test standards: GB/T 4677.22, IPC-TM-650-2.3.25, MIL-STD-2000A.

2) Instrument test standards: IPC-TM-650-2.3.26, IPC-TM-650-2.3.26.1, IPC-TM-650-2.3.28, GB/T 4677.22, MIL-STD-2000A.

3) Flux residue test standards: IPC-TM-650-2.3.27, IPC-TM-650-2.3.27.1, IPC-TM-650-2.3.38, IPC-TM-650-2.3.39.