The Complete Guide to FPC Inspection & Quality Control: Your One-Stop Solution for Acceptance

General Provisions

Scope : This standard applies to the acceptance of all Surface-Mount Technology (SMT) assembly products on Flexible Printed Circuits (FPC).

Inspection Environment :

Illumination : Adequate lighting of at least 500 Lux at a distance of 30-40 cm from the product under cool white fluorescent light.
Visual Acuity : Inspectors must have a corrected visual acuity of 20/20 or better.
ESD Protection : All handling must be performed on an Electrostatic Discharge (ESD) protected workstation with operators wearing a grounded wrist strap.
Sampling Plan : Generally based on MIL-STD-105E (ANS/ASQ Z1.4) Level II normal, single sampling plan. Acceptable Quality Limit (AQL) shall be mutually agreed upon with the customer (common AQL for visual defects is 0.65%, for critical defects is 0.4%).

Reference Standards :

IPC-A-610 : Acceptability of Electronic Assemblies "Acceptable" class is the minimum requirement.
IPC-J-STD-001 : Requirements for Soldered Electrical and Electronic Assemblies .
Customer-Specific Requirements (CSR) : Customer specifications shall take precedence when provided.

2. Detailed Inspection Criteria

2.1 Visual Inspection

Criteria	Acceptable Standard	Defective Standard
Component Placement	1. All components correct as per BOM and assembly drawing (value, size, P/N). 2. Polarity and orientation correct. 3. Markings legible. No physical damage, cracks, or oxidation.	1. Wrong part, missing part, wrong orientation, or reversed polarity. 2. Cracked, chipped, or damaged component body. 3. Severe oxidation or contamination on terminations.
Solder Quality	1. Solder joints are smooth, bright, and concave with good wetting. 2. Solder covers ≥75% of the termination or pad. 3. No cold solder joints, non-wetting, or dewetting. 4. No solder bridges (shorts) between adjacent leads/pads. 5. Minimal, non-activated, non-ionic flux residue (if no-clean process). Residue should be thin, transparent, and non-tacky.	1. Cold solder joint, non-wetting, dewetting (dull, grainy, porous appearance). 2. Solder bridges between any conductors. 3. Solder balls (especially under BGAs, QFNs), particularly any >0.13mm in diameter. 4. Solder icicles or spikes. 5. Excessive, corrosive, or tacky flux residue.
FPC Condition	1. Gold fingers are clean, bright, free from scratches, oxidation, solder splash, or contamination. 2. Coverlay (Cover Coat) is intact with no blistering, delamination, or tears. 3. Pads are intact with no lifting or peeling. 4. FPC is not permanently deformed or creased (must lie flat in fixture).	1. Gold fingers are scratched (exposing nickel/copper), oxidized, contaminated, or tarnished. 2. Coverlay is blistered, delaminated, or torn, exposing underlying copper. 3. Lifted or missing pads. 4. Sharp creases, folds, or permanent deformation of the FPC.
Adhesive/Shielding	1. Adhesive (if used) is properly cured and adequately secures components without contaminating pads or contacts. 2. Shielding cans are seated flat and soldered securely on all sides.	1. Uncured or insufficient adhesive. 2. Adhesive contamination on solderable surfaces. 3. Shielding can is warped, lifted, or improperly soldered.

2.2 Electrical Performance Test

 

Criteria	Acceptable Standard	Defective Standard
Continuity (ICT/FCT)	1. 100% pass on In-Circuit Test (ICT) and/or Functional Test (FCT). 2. All networks correct—no opens or shorts.	1. Test failure: opens or shorts. 2. Component values (R, L, C) outside specified tolerance. 3. Integrated circuits (ICs) fail functional test.
Insulation Resistance	Meets product specification. Typically ≥100MΩ @ specified voltage (e.g., DC500V) between high-voltage nets.	Insulation resistance below specified requirement.
Withstanding Voltage (Hipot)	Meets product specification. No breakdown or arcing at specified test voltage (e.g., AC1500V or DC500V for 60 sec).	Electrical breakdown, excessive leakage current, or arcing during test.

 

2.3 Dimensional & Mechanical Inspection

 

Criteria	Acceptable Standard	Defective Standard
Component Height	All components conform to the assembled height restriction, especially if housed in an enclosure.	Component height causes mechanical interference with the housing or other parts.
Component Misalignment	1. Chip Components (C, R, L): Misalignment ≤50% of component width or pad width (whichever is less), with good wetting. 2. ICs (QFP, BGA, QFN): Misalignment ≤25% lead/terminal width. All leads must have contact with the land.	1. Misalignment causes reduced electrical clearance or risk of shorting. 2. Terminals of bottom-terminated components (BTCs) like BGAs are off the pad.
Tombstoning	None.

2.4 X-Ray Inspection (For BGAs, QFNs, and hidden joints)

Purpose : To inspect the integrity of hidden solder joints.

Acceptable Standard :

Solder joints are continuous and well-formed.
Voiding is < 25% of the joint area per IPC-7095 (may be stricter, e.g., <10%, for high-reliability applications).
No cracks or voids at the interface.
No solder balls bridging between joints.

Defective Standard :

Voids exceed the acceptable area limit.
Solder bridges between joints.
Missing or incomplete solder joints.
Cracks within the solder joint.
Significant misalignment.

Note: Handling Precautions : Due to their flexible nature, FPCs must be handled with extreme care. Always hold the board by its edges, avoiding contact with gold fingers, exposed pads, and components. Customer-Specific Requirements (CSR) always take precedence over this general guideline. Note: Handling Precautions : Due to their flexible nature, FPCs must be handled with extreme care. Always hold the board by its edges, avoiding contact with gold fingers, exposed pads, and components. Customer-Specific Requirements (CSR) always take precedence over this general guideline.