PCB Technical - Practice and Thinking of SMT Whole Process Quality Control

Practice and Thinking of SMT Whole Process Quality Control
1 SMT process control characteristics
The focus of SMT process control is mainly 5P: PCB printed board, Parts, Paste solder paste, Placement placement, Profile curve, that is, material control + printing design (template design + printing control) + patch control + temperature curve design .

1.1 Material control

PCB (Printed circuit board, printed circuit board) is the most important material. After receiving the material, it is necessary to check the manufacturer’s COC (Certification of Completion) certificate (including metallographic section report, pad solderability report, cleanliness test), Flatness test, etc.), pre-drying is required before assembly to remove moisture in the board, and the expiration date of the PCB must be paid attention to.

The components need to pay attention to the solderability of the pins and remove the gold treatment. For components that require secondary screening, coplanarity should also be paid attention to, because the secondary screening process is susceptible to clamping deformation of the test fixture, resulting in poor coplanarity.

1.2 Printing design

Before printing, it is necessary to design a template, determine the opening of the pad and the thickness of the template, and determine the processing method of the template. Generally, a stainless steel laser cutting + inner wall electropolishing template is selected. When the template is delivered, the tension should be tested, which should be 40-50 N. The tension should also be tested before each use to ensure that it is not less than 30 N after repeated use.

On the premise that the template is determined, the factors that affect the printing quality are: solder paste quality and printing process parameters. Attention should be paid to the testing and verification of solder paste before use, the re-temperature during use, stirring, and the limitation of use time.

The pressure of the squeegee in the process parameters is related to the size of the squeegee, which is required to be 1.97 ~ 2.76 N/cm. The printing speed is related to the composition of the solder paste and the pitch of the printed pads.

1.3 Patch control

The chip needs to check the coplanarity of the component pins, generally set 0.08～0.12 mm.

The mounting pressure is generally controlled so that the depth of the component leads pressed into the solder paste is at least half the thickness of the pin. It should be noted that for fragile glass package devices, the mounting pressure needs to be lowered to avoid damage to the device body.

1.4 Temperature curve design

The reflow soldering temperature curve requires actual temperature testing for each different type of PCBA (Printed Circuit Board Assembly). A good temperature curve, the maximum heat capacity and the minimum heat capacity at the end of the preheating temperature converge, also That is, the temperature of the entire board reaches thermal equilibrium.

The temperature curve setting is related to the PCBA size and number of layers, component specifications, pin plating, and solder specifications.

2. NADCAP AC7120 audit requirements

NADCAP is a certification system for special engineering management systems for national aerospace and defense contractors or authorized projects. AC7120 is a standard for auditing printed circuit board components, covering all processes of the printed circuit board assembly manufacturing process, such as CAD ( Computer -Aided Design) data, programming, component placement, AOI (Automated Optical Inspection) or X-ray machine inspection, wave soldering, reflow soldering, manual soldering, assembly, cleaning, coating and sealing, Final test (flying probe, needle bed, boundary test, functional test), rework, etc.

AC7120 audit is mainly divided into two parts: document audit, mainly to check whether the working procedures and specifications meet the requirements of the audit clauses, and whether they are complete and sufficient; the effectiveness of the implementation is mainly to check whether the effective implementation and compliance with the requirements of the documents are conducted through model making (Customer requirements, industry requirements).

2.1 Basic audit requirements

Although AC7120 is an audit requirement for the process compliance of the printed board assembly manufacturing process, there are some basic requirements that can be universally used for other production line control, such as:

a) Personnel requirements: IPC for operators and inspectors, proof of vision inspection; familiarity with various specifications and positions; work as required.

b) Material requirements: specified in the design drawings; in compliance with ESD (Electro-Static discharge)/MSD (moisture sensitive device) control; use expiration date, opening expiration date, subcontract expiration date identification and control; entering the institute Acceptance required; periodic testing in use.

c) Equipment requirements: maintenance checklist; authorized personnel to operate the equipment; configuration of various monitoring devices; test and save every shift;

d) Process requirements: meeting international or industry standards; confirmation and testing after each process; procedure control; rationality of process methods;

e) Environmental requirements: temperature, humidity, ESD requirements in EPA (Electrostatic Protected Area), noise, illuminance, and cleanliness.

These required documents will be reviewed by the auditor during the audit and will be verified item by item during the on-site audit.

2.2 Quality control requirements

2.2.1 Pay attention to the quality system

AC7120 professional audit is always based on the implementation of the unit's quality management system requirements, which specifically involves: quality manuals, procedures and document control, personnel qualifications, environmental requirements, material control, supplier control, first article appraisal, non-conforming product management and corrective measures, QA (Quality Assurance, quality assurance) personnel, internal audit effectiveness, combined with process audit requirements will be audited, and require implementation evidence.

2.2.2 Pay attention to the effectiveness of corrective measures

Pay attention to the effective implementation of corrective measures for non-conformities in the previous review process and the verification of the results. If the corrective measures are not effectively implemented and maintained for a long time, the AC7120 professional audit will issue 2 serious non-conformities: one is a duplicate problem, and the other is that the system cannot maintain the effectiveness of the corrective measures.

2.2.3 Pay attention to traceability

Any material must be traceable to the original manufacturer’s batch number, especially the COC must be attached to the first article report. All materials on site must have sub-packaging labels, indicating: material name and specification, original packaging validity period, batch number, opening validity period, subcontractor, and the material batch number must be transplanted with the work order. The files of all products are kept permanently, and all on-site production records are kept for more than 5 years.

2.3 Process professional requirements

2.3.1 ESD control

Static control is first of all the control of the EPA area: the hardware facilities in the area (floor, table mats, chairs, etc. must be made of anti-static materials), equipment grounding, and other objects should be made of anti-static materials (turnover boxes, trash cans, tweezers, Suction pen). If there is no condition to use anti-static materials, an ion fan should be configured during operation.

Static control of ESD materials, anti-static pockets, wear anti-static wrist straps when taking and putting them away.

Static control of personnel, daily inspection of anti-static shoes and wrist testing.

2.3.2 MSD control

Humidity-sensitive devices need to identify the level, dry according to the original factory prompts before assembly, and follow the assembly and use within the life of the workshop.

2.3.3 PCBA cleaning

Because PCBA involves many processes, there is still some time to finalize the assembly after the first side is soldered. For manufacturers who deliver the complete machine, PCBA needs to be cleaned after single board debugging, and PCBA is often cleaned after more than 8 hours after soldering. At this time, the cleaning effect can no longer remove the flux on the PCBA most effectively, so NADCAP AC7120 stipulates that the PCBA needs to be cleaned within 8 hours after soldering. If water cleaning or semi-aqueous cleaning is used, a drying time of 4 hours will be required, which will greatly restrict the process cycle of the production line, so vapor phase cleaning can be considered.

2.3.4 Rework and repair control

In the industry standard, there is only a limit on the number of rework and repairs, but NADCAP AC7120 emphasizes that rework is not allowed, and customer opinions must be obtained for repairs.

Process professional requirements mainly review the effectiveness of the on-site execution, which is also the focus of the AC7120 audit. During the on-site audit, attention will be paid to whether each regulation has the correct traceability, whether the execution in the process is accurate and in place, whether the records are complete, and whether the operators can Obtain current and effective guidance documents. All stipulated requirements are the promises of the organization and must be fulfilled in the process.

3. Practice and Thinking

After many times of NADCAP on-site audits and internal audits, I feel that there are still some deviations in understanding and implementation in the implementation of some management and control requirements, so I summarized a few experience sharing.

3.1 Validity of documents

When compiling a separate implementation document for the time being, in addition to your own test and experience formation requirements, you also need to follow standards and pay attention to customer requirements. However, sometimes there are differences and inconsistencies between industry standards. This is a detailed verification to verify these differences and inconsistencies. Coordinate, adopt international common norms, terminology, and requirements instead of just using them.

For the forms that need to be filled by the operator, whether it is a work order in the product dossier or a form filled out due to administrative needs, there must be a source. All forms must have a form number and a form version number, which can be traced to specific document regulations.

3.2 Personnel's position is competent

Generally, the process or procedure documents stipulate that operators and inspectors need to obtain job certificates, and the dynamic management of personnel competence must form an effective system. These personnel need to be inspected and evaluated regularly or annually on the spot, inquiring and witnessing their operations:

a) Employees describe what their positions are and require each employee to clearly know the positions and responsibilities they are engaged in;

b) Perform the actual operation again, requiring each employee's operation to be complete and correct;

c) Whether employees can inquire about the supporting documents of the operation, and each employee is required to know clearly what the valid SOP is based on and where to obtain it.

3.3 FAI First Article Inspection

When implementing the requirements of the first article inspection documents, pay attention to whether each process and the final inspection are qualified, and whether there is any deviation in the whole process. In addition, the following three requirements need to be focused on:

a) The quality certificate of the materials, to ensure the traceability of the quality of the incoming materials, the materials include outsourcing parts, PCB and other outsourcing parts;

b) FAI (first article inspection) of parts, such as FAI report of PCB and chassis;

c) Special process and characteristic size testing. These features that need to be tested need to be clearly specified in the drawings, so that the design, process and execution can be fully realized to ensure product compliance.

3.4 Isolation

In order to prevent unintended use, the items that need to be isolated on site are: unqualified or problematic pending items, expired items, and extras. Usually the program documents are relatively rough, and only the requirements for identification and isolation are put forward, and there are no specific implementation steps. It needs to be further clarified during on-site execution:

a) Who is responsible for making the isolation request;

b) The scope of quarantined products (work-in-process, finished products), whether the ex-factory products need to be recovered;

c) Isolate the departments that require the issuance to avoid missing execution steps;

d) Decision makers.

3.5 6s control of production line

It does not require a real environmental monitoring system to measure a piece of data to characterize whether the production line environment is up to standard, but to shape a good working environment through the overall 6s:

a) How to control pollutants in the environment and minimize FOD (Foreign Object Debris);

b) Whether employees wash their hands when entering and leaving the production line to ensure that it is harmless to the product and harmless to themselves;

c) The cleanliness of the work surface and ground;

d) The cleanliness of the exhaust pipes of wave soldering and reflow soldering equipment;

e) The cleanliness of the bottom and periphery of containers such as anti-static bags and boxes;

f) The cleanliness of the inside of the refrigerator, flux pens, and chemical reagents.

3.6 Risk control

Risks are everywhere, and may come from misoperation, non-execution, or damage. Effective precautions must be taken to prevent risks from occurring. There are many possible risks in the SMT production line. It is necessary to implement the intrinsic safety of the equipment and the automatic transformation of the production line to reduce or resolve the risks. In combination with the current reality, automatic and intelligent means are adopted.

a) For the temperature and humidity control of the production line, the best practice is to use 24-hour precision temperature and humidity air-conditioning control, once exceeding the standard, the alarm will be immediately; the slightly practice is the 24-hour needle-type line drawing real-time recording method to ensure uninterrupted monitoring; the worst method is man-made The way of checking the watch every 2 hours lacks real-time monitoring. Once the standard is exceeded, taking measures has already had an impact on the product.

b) ESD detection for employees, the best practice is to set up an access control system, using gates and infrared alarms to prohibit failed inspections; a little practice is to only check without an alarm system; the worst practice is to not check when employees enter and exit the production line every day.
3.7 Statistical requirements
There are statistics everywhere in the process. Statistics are used to observe fluctuations for better improvement, and to maintain good processes, operations, and methods. The organization shall formulate the application requirements of statistical technology, stipulate the responsible unit, statistical scope, statistical frequency, post-statistical analysis, and measures to be taken. Statistics are not just product data. The factors that can be counted by SMT production line are:
a) Product quality data such as PCBA welding quality, PCBA coating thickness, and ion concentration after PCBA cleaning;