Circuit board reliability and micro-sectioning
Circuit board reliability and micro-sectioning
1.Abrasion Resistance
In circuit board engineering, it often refers to the wear resistance of solder-proof green paint. The test method is to use a 1 kg soft grinding wheel to rotate and rub 50 times on the IP-B-25 sample board that has been painted green. The comb-shaped circuit area must not be worn out to expose the copper (see Circuit Board Information Magazine Issue 54, P.70 for details). This is the wear resistance of the green paint. Some specifications also have requirements for the wear resistance of gold fingers. In addition, Abrasive refers to abrasives, such as pumice powder. Accelerated Test (Aging) is an accelerated test, which is also called accelerated aging test (Aging). For example, the tin melting, tin spraying or tin rolling process on the surface of the board can maintain the solderability of the board for a long time. High temperature and high humidity accelerated tests can be used to simulate the deterioration of the solderability of the board after aging to determine whether its quality is acceptable. This artificial accelerated aging test, also known as environmental testing, is intended to see how the finished circuit board (already painted green) performs in terms of weather resistance. The new “Circuit Board Solderability Specification” (ANSI/J-STD-003, full translation available in issue 57 of this journal) has new requirements, namely that high reliability class 3 circuit boards must undergo 8 hours of “steam aging” before the solderability test, which also belongs to this type of test.
2.Accuracy
Refers to the gap between the results of the production and the established goals. For example, how confident is the ability of the drilled hole to reach its “true position”?
3.Adhesion
Refers to the strength of the surface layer’s adhesion to the main body, such as green paint on the copper surface, or copper skin on the substrate surface, or the adhesion between the plating layer and the substrate.
4.Aging
It refers to the fact that products obtained through physical or chemical processes will gradually lose their original quality over time. This process of maturation or degradation is called “aging”. However, it has also been translated as “time reaction” in other academic fields.
5.Arc Resistance
It refers to the arc generated under high voltage and low current. When this arc passes through the surface of insulating materials, the material’s own resistance or tolerance to the arc is called “arc resistance”. The quality of its endurance depends on how long it can resist before being attacked and causing carbide to conduct electricity.
6.Bed-of-Nail Testing
When the board is subjected to open/short electrical testing, a fixture with fixed wiring is required. The placement of each probe must be coordinated with the position of the through hole or test pad on the board. Electrical testing is performed under a specified voltage, so it is also called “bed of nail testing”. The official name of this electrical test should be Continuity Test, which means “connectivity test”.
7.Beta Ray Backscatter
It uses the unstable characteristics of isotope atoms to emit β rays, which pass through a specific window and hit the coating sample to be measured. The Geiger counter in the tester detects the rays that bounce back from the window and converts them into thickness data. General gold layer thickness testers, such as UPA’s Micro-derm, operate on this principle.
8.Bond strength
It refers to the force (LB/in2) applied per unit area when the adjacent layers in the laminated board are forcibly separated in the opposite direction (not torn apart).
9.Breakdown Voltage
Among the various high voltages that cause the failure of the board insulation material (such as the base material or green paint), the lowest and most basic voltage that causes its degradation is the “breakdown voltage” or “collapse voltage” for short. Or it refers to the voltage that causes gas or vapor to reach ionization. As “thin boards” become more popular, the characteristics of this substrate will also be required to be stricter. This term is also often called Dielectric Withstanding Voltage.
10.Burn-In High Temperature Accelerated Aging Test
The finished electronic products are deliberately placed in high temperature for a period of time (such as 7 days) before shipment, and their functional degradation is continuously tested. This is an accelerated aging test, also known as a high temperature life test.
11.Chemical Resistance
In a broad sense, it refers to the tolerance or resistance of various substances to chemicals. In a narrow sense, it refers to the resistance or tolerance of the circuit board substrate to solvents or various chemicals in wet processes, as well as to flux, etc.
12.Cleanliness
It refers to the amount of residual ions in the finished board. Since the circuit board has gone through a variety of wet processes, if the cleaning is insufficient and the conductive ions are left, the insulation resistance of the board will be reduced, causing potential corrosion crisis of the board circuit, and even causing electron migration (Electromigration) problems between conductors (including between layers) under moisture and voltage. Therefore, the board must be thoroughly cleaned and dried before printing green paint to achieve the best cleanliness. According to the requirements of the US military specification MIL-P-55110E, the cleanliness of the board is expressed by the conductivity (Conductivity) of the immersion extract (75% isopropyl alcohol + 25% pure water), which must be lower than 2×10-6 mho and should be above 2×106 ohm to be considered qualified.
13.Comb Pattern Comb Pattern
It is a “multi-finger” interlaced dense circuit pattern, which can perform high voltage tests on the board cleanliness and green paint insulation.
14.Corner Crack
At the intersection of the copper wall of the through hole and the hole ring of the board, the internal stress (Inner Stress) of the copper plating layer is relatively large. When the through hole is subjected to severe thermal shock (such as tinning), the hole angle is greatly increased under the strong expansion and pulling in the Z direction. The countermeasures can be improved from the ductility of the copper plating process, or the thickness of the board can be reduced as much as possible to reduce the effect of Z expansion.
15.Crack
In PCB, it often refers to the copper foil or the copper plating of the through hole. When encountering the test of thermal stress, partial or complete fractures of various layers often occur, which is called Crack. Its detailed definition can be seen in Figure 7 of IPC-RB-276.
16.Delamination
It often refers to the separation between the metal layer and the resin layer of a multilayer board, and also refers to the separation between the layers of glass fiber cloth in a “laminated board”. The main reason is the lack of adhesion between each other, and the subsequent strong heat or external force of soldering causes separation.
17.Dimensional Stability
It refers to the changes in the length, width, and flatness of the board under the influence of temperature changes, humidity, chemical treatment, aging or external pressure, which is generally expressed as a percentage. When the board warps, the vertical deformation of the PCB board surface is the vertical highest point from the reference plane (such as a marble platform) minus the board thickness, or the height of the board floating is directly measured with a steel needle measuring the aperture. This deformation is used as the numerator, and the board length or diagonal length is used as the denominator. The percentage obtained is the representation of dimensional stability, commonly known as “dimensional stability”. This term also often refers to the shrinkage of the length and width of multilayer boards during manufacturing, especially after lamination, the inner layer shrinks the most, usually about 4/10,000 in the warp direction and about 3/10,000 in the weft direction.
18.Electric Strength
It refers to the highest potential gradient (i.e. voltage or potential difference) that an insulating material can withstand before it breaks down and leaks electricity. Its value is related to the thickness of the material and the test method. This term has other synonyms such as (1) Dielectric Strength (2) Dielectric Break Down (3) Dielectric Withstand Voltage, etc. The third one is more commonly used in formal terms in general specifications.
19.Entrapment
It refers to the inclusion of foreign matter or foreign objects that should not be there between the green paint and the board surface, or between the primary copper and the secondary copper. The former is caused by the incomplete cleaning of the board surface or the presence of foreign matter in the green paint. The latter may be the resist added to the surface of the primary copper. When the construction is found to be poor and it is to be “removed” and reprocessed, the residual resist may be left due to incomplete cleaning and be covered by the secondary copper. This situation most often occurs in the copper plating layer on the hole wall. In addition, when the plating solution is unclean, a small amount of charged solid particles will be plated on the cathode along with the current. This type of inclusion most often appears at the opening of the through hole. The following two figures show a typical copper-plated entrap.
