12 Flexible Printed Circuit Boards
14.2 Testing for Quality Control
Testing for quality control is conducted in the following stages:
a Pre-production Testing: It includes visual and dimensional examination, micro-sectioning, tests for plating adhesion, terminal pull, warp and twist, water absorption, solderability, plated through-hole structure, thermal shock, thermal stress, interconnection stress, moisture and insulation resistance, dielectric strength and current breakdown.
a Production Testing: Testing is done to a minimum level during the day-to-day production phase provided the quality is maintained. Under the normal conditions, the following tests are performed:
a Visual and dimensional inspection;
a Micro-sectioning in one plane;
a Plating adhesion, moisture and insulation resistance tests on the test coupons; and a Circuitry tests on the boards.
For effective sampling tests, destructive tests are done every week or after every five thousand board, whichever occurs first. The following tests are done:
a Visual and dimensional inspection;
a Micro-sectioning in the principal planes; and
a Tests for terminal pull, water absorption, solderability, plated through-hole structure, thermal shock, thermal stress, interconnection resistance, dielectric strength, current-carrying capacity etc.
If failures occur in any one or more parameter(s), normal production is resumed only after the defect is detected and corrective action taken.
a Final Testing: Final testing is very crucial for the testing of multi-layer boards. The two most critical requirements which must be tested are:
a Micro-sectioning of one test coupon from each production panel; and a Electrical circuitry testing of the finished boards.
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In multi-layer boards, the mechanical integrity of the plated through-holes is of the utmost importance.
14.2.1 Characteristics for Testing of Quality Assurance
As explained in the previous section, a number of characteristics are required to be tested to ensure proper quality assurance. Table 14.1 shows all such requirements. In this Table 14.1, the following define the various levels:
a LR = Commercial boards, with limited requirements of tests;
a HR = High reliability requirements;
a MIL = Military specifications; and
a 1,2,3 = Three levels of quality designated by IPC.
Table 14.1 Characteristics for Testing Various Levels of Quality (after Coombs, 1988)
Requirements LR 1 2 3 MIL HR
Circuitry electrical test (100 %) ÷ ÷
Current-carrying capacity ÷ ÷ ÷ ÷
Dielectric strength ÷ ÷ ÷ ÷ ÷
Etch-back ÷
Flammability ÷ ÷ ÷ ÷
Insulation resistance ÷ ÷ ÷ ÷ ÷
Internal shorts ÷ ÷ ÷ ÷ ÷
Mechanical shock ÷
Micro-sectioning ÷ ÷
Moisture resistance ÷ ÷ ÷
Outgassing ÷
Plating adhesion ÷ ÷ ÷ ÷ ÷
PTH structure ÷ ÷
Solderability ÷ ÷ ÷
Terminal pull ÷ ÷ ÷ ÷ ÷
Thermal shock ÷ ÷
Thermal stress ÷ ÷
Traceability ÷
Vibration ÷
(Contd.)
Table 14.1 (Contd.)
Requirements LR 1 2 3 MIL HR
Visual and dimensional characteristics ÷ ÷ ÷ ÷ ÷ ÷
Warp and twist ÷ ÷ ÷ ÷
Water absorption ÷
The quality of the final printed circuit board (PCB) depends upon the cumulative quality of many successive fabrication steps. The entire process, beginning with raw material and tooling through the final inspection and test, may incorporate as many as forty or more process steps, each with its own opportunity for defects. The cumulative impact of an uncontrolled operation can be devastating on both the quality and reliability of the products (Watts, 1993).
For this reason, a growing number of companies are electing to invest in a balanced quality assurance (QA) programme, which typically combines incoming inspections, in-process product checks and process controls with quality conformance and reliability assurance tests.
The first three serve as preventive measures for minimizing the value added and probability of defects. Quality conformance tests act as screens, filtering out escapes from the preventive measures in place. The reliability evaluations expose latent or hidden defects that cannot be detected by any other means.
The PCB manufacturer should include all types of tests in an overall quality programme. These tests should be tailored to match resources, process capability and customer requirements.
14.2.2 Designing a QA Programme
Given the number of process steps in a typical PCB fabrication line, a step-by-step evaluation programme for monitoring product quality at every significant step is both impractical and impossible.
The competitive company will find the right optimum of process control to supplement product inspection (Table 14.1) during the fabrication process.
An optimum quality assurance programme is achieved best by prioritizing the value of inspection at each process step. The final choice of evaluation steps can be based on any number of criteria but should take into consideration the impact of quality as it relates to the following:
a Circuit functionality;
a Process capability;
a Circuit long-term reliability;
a Containment of defects;
a Re-work capability;
a Customer requirements;
a Subsequent processing (assembly); and a Product cost.
From the standpoint of process control, not enough can be said about the importance of a well- equipped and well-staffed control lab. Circuit fabrication relies upon chemical consistency in 75
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per cent of its processing steps. If a manufacturer is really serious about preventing defects through process control, investing in a control lab is as fundamental as the PCB fabrication process itself.
Although most chemical analyses can be performed by using a basic titration test, more control is achieved with cyclic voltametric stripping (CVS), atomic absorption, UV visible spectroscopes and gas chromatographic analysis.
14.2.3 Incoming QA
Despite the numerous raw materials that feed a PCB fabrication process, very few are actually inspected on-site by the manufacturer. Again, it is both impractical and unnecessary to do so. Decades of development work has resulted in very uniform chemistries, resists, laminates and drills. Stiff competition within the supplier sector has also resulted in products that are continually upgraded to keep pace with ever-changing customer demand.
Manufacturers with available laboratory facilities can perform relatively quick tests on laminates and prepregs to monitor the consistency in flow, cure and resin content of materials entering their processes. A visual audit of copper clad laminates and prepregs is sufficient to sort out the lots with gross defects.
14.2.4 Traceability
For all boards used in high reliability applications/products, it is necessary to establish and maintain a traceability programme. This is required because of the possibility of latent defects which may appear at a later stage and affect the reliability of the product. The traceability programme starts at the lamination panel level and continues throughout the process of flow upto the serial number of the equipment in which it is used. Similarly, a record of the rejected boards is also useful to identify the weak links in quality control in the process.