4.3 The incoming inspection plan

Incoming Quality Control (IQC) acts as a critical financial and operational firewall for the entire EMS factory. Once a defective component is permanently soldered onto a PCB, the cost of resolving that issue increases by a factor of 10 for rework, and up to a factor of 100 if the board must be completely scrapped.

Quality cannot be achieved merely by inspecting products at the receiving dock; instead, the role of IQC is to thoughtfully verify that the supply chain has delivered the required quality. This plan defines the structured logic for safely accepting incoming materials into the warehouse.

Risk-based inspection logic

Inspecting 100% of every incoming reel of resistors is an inefficient use of quality resources. Instead, a risk-based approach is recommended to allocate IQC bandwidth where failure is most likely—and financially most costly.

Class A: high risk (critical / custom)

• Items: Bare PCBs, Custom Mechanicals (like Plastics and Metals), Complex Cables, Displays, Batteries, and safety-critical parts.
• Logic: These physical components naturally exhibit higher manufacturing variability, or they carry a severe impact on product safety and function if they fail.
• Guideline: An AQL 0.65 or 1.0 (Level II) plan should be utilized. Mandatory dimensional and detailed visual checks should be performed.

Class B: medium risk (active components)

• Items: Integrated Circuits (ICs), High-Pin-Count Connectors, LEDs, Transistors, and expensive active modules.
• Logic: While intrinsic silicon manufacturing defects from the component maker are rare, mishandling, packaging errors, MSL violations, or hidden ESD damage are far more common in the supply chain.
• Guideline: An AQL 1.0 (Level I) plan should be utilized. Focus must be placed heavily on verifying supplier labels, packaging integrity, and the physical condition of the fragile leads.

Class C: low risk (standard passives)

• Items: Standard Resistors, Capacitors, Screws, and Washers (Common commercial off-the-shelf catalog parts).
• Logic: The production processes for these passive components are highly automated worldwide and statistically very stable.
• Guideline: These should be placed on Dock-to-Stock (DTS) status. The dock team only needs to verify the Manufacturer Part Number and Quantity. Physical inspection is usually suspended unless flagged by subsequent process failures on the floor.

The golden rules of verification

Before ever opening a sealed reel or a boxed shipment, the inspector should perform the “Label vs. System” triangular check. The vast majority of IQC escapes are actually administrative mismatches, not physical component defects.

1. MPN Match: Does the Manufacturer Part Number on the physical label match the ERP system exactly? (For example, an ATMEGA328P-AU behaves differently than an ATMEGA328P-MU, and the letters matter).
2. AVL Verification: Is this specific manufacturer actively listed on the Approved Vendor List (AVL) for this particular project build?
3. Date Code Review: Is the component within the allowable age limit (typically < 2 years from date of manufacture)?
   • If the Date Code is > 2 years, it is best practice to temporarily block the material and request a fresh Solderability Test to ensure the leads haven’t oxidized.

Component-specific inspection criteria

These focused checks should be applied based on the specific component family being evaluated.

A. bare PCBs (the foundation)

The bare PCB is an incredibly complex component. If it is flawed, the entire PCBA built on top of it becomes expensive scrap.

• Warpage: The bare board must be placed on a verified, flat surface plate. Any natural bow and twist must be ≤ 0.75% of the diagonal dimension (unless IPC-6012 Class 3 dictates a tighter tolerance for the build).
• Solder Mask: A careful visual check for any signs of peeling or blistering mask must be performed.
• Pads/Finish: It must be verified that ENIG or HASL finishes are clean and not oxidized. Any notable discoloration or dullness is usually grounds for rejection or further evaluation.
• Dimensional: The critical housing cutouts and mounting holes must always be measured against the engineering drawing.

B. moisture sensitive devices (MSDs)

ICs and modules packaged in moisture barrier bags (MBB) are highly sensitive components if mismanaged during transit.

• Humidity Indicator Card (HIC): The outer cardboard box must be opened. The HIC must be checked through the bag (if the bag is transparent) or immediately upon carefully opening the seal.
  • If the 10% spot is pink, the internal environment is wet. The lot must be rejected or a proper, controlled bake cycle mandated immediately to recover them.
• Bag Integrity: A close inspection for punctures or loss of vacuum must be performed. If the aluminum bag does not tightly conform to the reels inside, the internal dry environment has been compromised.

C. custom mechanics (enclosures / metal)

• Fit Check: A quick physical mating test with a validated Golden Sample counterpart must always be performed.
• Cosmetics: The surfaces must be inspected under standard, bright lighting (800-1000 lux) at roughly arm’s length (approx. 40-50cm).
  • Scratches: Generally, none are allowable on Class A (user-facing) aesthetic surfaces.
  • Flash/Burrs: These must not interfere with assembly fit, nor should they present any scratch or injury risk to the operator.
• Color: The plastic color must be compared directly against the signed “Limit Sample” or the officially specified Pantone swatch.

D. connectors & electro-mechanical

• Pin Condition: The connector must be inspected closely for bent pins or poor coplanarity. If surface-mount pins are not entirely flat and parallel, they will almost certainly cause open solder joints during the reflow soldering process.
• Plating: A check for obvious signs of tarnish, oxidation, or exposed base metal on the critical friction contact surfaces must be performed.

Sampling strategy (AQL)

Utilizing the ANSI/ASQ Z1.4 (ISO 2859-1) standard is recommended to maintain disciplined sampling.

• Normal Inspection: This level should be started at for all new vendors and new part numbers.
• Reduced Inspection: This level can be safely switched to if a vendor successfully passes 10 consecutive lots without issue.
• Tightened Inspection: Escalation to this level is necessary if 2 out of 5 consecutive lots are rejected.

Example Scenario:

Upon receiving 500 units of a Custom Plastic Case (Class A):

1. Reference the Table: A lot size of 281-500 maps to Sample Size Code H.
2. Determine Sample Size: Exactly 50 units must be carefully inspected.
3. Apply AQL 1.0 Criteria:
   • Accept: If 1 defect is found.
   • Reject: If 2 defects are found.
4. Action: If 2 non-conforming units are found, it is generally best to reject the entire shipment. Sorting parts in-house is strongly advised against unless explicitly authorized by management, and the sorting cost is formally charged back to the supplier.

Non-conformance management (MRB)

When a lot fails incoming inspection, it should be moved to the secured Material Review Board (MRB) cage and must be digitally segregated in the ERP system (marked as BLOCKED).

Standard Dispositions:

1. “RTV” (Return to Vendor): This is the standard, expected procedure for non-conforming material.
2. UAI (Use As Is): This disposition requires formal Engineering and Quality Approval. It is typically only used if the defect is purely cosmetic and has zero impact on form, fit, or function (e.g., an incorrect font used on an internal, hidden label).
3. Sort/Rework: In this scenario, the EMS performs sorting or rework at the supplier’s expense. This requires a written commercial agreement on hourly billing rates established before the extra work begins.

Cost of Error

Caution must be exercised when considering accepting a “marginal” lot to keep the production line running. Doing so effectively assumes ownership of the risk. If the yield subsequently drops at the SMT process due to those parts, the customer will likely not absorb the cost of the scrap. A formal concession from the appropriate engineering authority must always be secured before releasing marginal parts to the floor.

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Recap: Incoming Component Inspection Requirements

Component Class	Core Inspection Requirement	Key Tolerance / AQL	Action on Failure
Class A (High Risk)	Dimensional & detailed visual check.	AQL 0.65 or 1.0 (Level II).	Reject lot; RTV standard.
Class B (Medium Risk)	Verify labels, packaging, physical lead condition.	AQL 1.0 (Level I).	Reject lot; RTV standard.
Class C (Low Risk)	Verify MPN & quantity only.	Dock-to-Stock (DTS).	Suspend inspection unless flagged.
All Components	MPN exact match; Supplier on AVL; Date Code ≤ 2 years.	N/A.	Block material if mismatch or aged; request solderability test if >2yrs.
Bare PCB	Warpage check; visual finish; dimensional check.	Warpage ≤ 0.75% of diagonal.	Reject or evaluate further.
MSD (Moisture Sensitive)	Check HIC; inspect bag integrity.	HIC 10% spot must not be pink.	Reject lot or mandate controlled bake.
Custom Mechanicals	Fit check; cosmetic inspection; color match.	No scratches on Class A surfaces; match Pantone/Limit Sample.	Reject for fit/function/cosmetic failure.
Connectors	Inspect pin condition & plating.	Pins must be flat, parallel, not oxidized.	Reject for bent pins, poor coplanarity, or tarnish.
Sampling (ANSI/ASQ Z1.4)	Apply AQL per risk class.	Accept/Reject per sample plan (e.g., AQL 1.0: Accept 1, Reject 2).	Reject entire shipment if lot fails.