The Reliability and Risk Assessment Branch of the NASA Goddard Space Flight Center received and analyzed 882 PCB coupon submissions from PCB suppliers. The top ten suppliers sent 638 samples, in which 260 non-conformances were detected. The issues were classified into 21 distinct non-conformances categories.

In this article we have listed the top 5 problem, their causes, their impact, and how to correct them.

1. Inner layer separations/inclusions

PCB manufacturing inner layer seperations

A. Overview

Plated through-holes are the conductive connection between the inner layers of a PCB. Any deficiency in the control of the manufacturing steps in fabricating through-holes can result in defects.

Why is this important?
An open circuit failure caused by inner-layer separation can cause intermittent electrical open or complete open after the board undergoes thermal processes (reflow, wave soldering, or rework).

B. Causes

Defects caused by Inclusion/Debris

This fault results from contaminant material/debris in an area, which interferes with the interconnect bond. Debris could be created by drill, desmear, micro-etch, and electroless copper plating processes and includes:

  • Drill debris
  • Desmear
  • Dielectric filler powder

Defects caused by Separation type

In this defect type, the inner copper layer separates from the plated through hole barrel during thermal exposure and is usually detectable only after the board undergoes thermal processes. Separation could be caused by:

  • improper lamination
  • inadequate coverage of inner layer oxide
  • pre-lamination bake cycle does not totally remove moisture
  • defective prepreg and/or laminate
  • faulty electroless copper process
  • post-electroless copper cleaning residues

Prevention

The two contributing factors to Inner layer separation & /inclusions
are:

  1. PCB manufacturing processes
  2. Raw materials
  • PCB fabricators should:
  • have an effective & consistent drilling process
  • reduce the resin content in the stack up
  • good desmear process
  • provide an adequate copper border for support and resin venting

It is more probable for some materials with inorganic fillers and more chemically resistant resin systems to have debris issues.

2. Separation/inclusions between plating layers

PCB manufacturing plating layer separation

A. Overview

Plating separation (the separation between a plating layer and copper foil) is another type of interconnect defect. When the weak adhesion between the electroless copper and the inner layer copper or between electroless copper and the electrolytic copper undergoes thermal processes, interconnect defects could appear.

Why is this important?
This type of failure gives rise to intermittent electrical open or complete opens due to mechanical or thermal stresses.

B. Causes

  • Incomplete wrap plating
  • Overly-aggressive cleaning process
  • Insufficient cleaning

Prevention

  • Adjust plating parameters
  • Optimize cleaning processes

3. Copper wicking above 2.0 mil

PCB Copper Wicking

A. Overview

Wicking occurs when the copper from a plated-through hole (PTH) extends along the glass fiber fabric.

Why is this important?
This results in intermittent electrical shorts or complete shorts due to bias driven migration of copper towards non-common conductors.

B. Causes

  • Dull drill bits or broken drill bits that cause a crack in the laminate
  • Incompatible laminate material
  • Insufficient glass etch
  • Weak glass to organic prepreg adhesion

Prevention

  • Optimize desmear parameters
  • Improve drilling operation (feed and speed)
  • Ensure sufficient resin wet-out of glass fibers (siloxane treatment).

4. Internal Annular Ring Less Than 2.0 mil

PCB Internal Annular Ring

A. Overview

This occurs, when the inner layer copper pad (measured from the hole wall plating to its outer most length) is less than 2 mils.

Why is this important?
When the board undergoes a thermal process (reflow, wave soldering or rework), inner layer breakouts causing intermittent electrical or complete open behavior.

B. Causes

  • Drilled-hole pattern not matching the lands on the internal layers (mis-registration)
  • Lamination process
  • Pre-lamination treatments that involve scrubbing or bending may stretch the thin laminate, which will then shrink after it is etched and baked dry
  • Application of specification or drawing notes

Prevention

  • Better material selection of laminate, improved cleanliness, and reduction in the amount of volatiles
  • Confirm whether or not it is operator error
  • Update drawing notes to bring the notes in line with current industry maturity levels

5. ENIG Less than Minimum Requirement

A. Overview

Electroless nickel and/or immersion gold plating thickness (ENIG) is less than the minimum requirements (118 micro-inches for Ni and 2 micro-inches for Au).

Why is this important?

Solderability may become uneven across all pads; this means reduction in solderability, especially for tiny parts such as BGA and FlipChip.

B. Causes

  • Improper cleaning of surfaces
  • Improper or inadequate rinsing
  • Chemical and pH parameters not being followed
  • Bath temperature too low
  • Oil or inhibiting film on copper surface

Prevention

  • Re-clean copper using chemical cleaners or mechanical cleaning
  • Scrubbing – implement micro-etch step to improve cleaning
  • Improve rinsing (flow, agitation and water quality)
  • Raise bath temperature per supplier specifications
  • Readjust to supplier operational parameters