Hyper corrosion testing of ENIG per the requirements of the IPC 4552A

Hyper corrosion testing of ENIG per the requirements of the IPC 4552A

 

The issue with hyper corrosion of ENIG and its acceptance or rejection has been an issue since the adoption of this surface finish. The End User of the part has attempted to reject even a single occurrence of hyper corrosion while the chemical supply base has argued that extensive levels of corrosion are still acceptable and will produce a reliable solder joint. It has been the experience of S T and S that the truth lies somewhere between these two extremes and as such pushed for the adoption of a corrosion level acceptance scale by the IPC.

The issue of an acceptable level of hyper corrosion with ENIG has finally been addressed with the release of the new IPC 4552A in August  of 2017.  The key items of this new acceptance scale are as follows:

  1. The inspection will be performed at 1000X optically. If it cannot be seen at 1000X then it is not rejectable. We have seen ENIG examined in a SEM at 40,000X and rejected for a single corrosion spike. This is outside the scope of the IPC 4552A and would require special contract requirements and agreement between the supplier and user (AABUS).
  2. There are 3 levels of corrosion detailing automatic acceptance, automatic rejection and a level that further testing is required to prove acceptance or rejection of the sample.
  3. Stripping of gold using a chemical process (cyanide or iodide based) and evaluating the sample in a “top down” orientation is not an acceptable sample for acceptance or rejection, it must be in a vertical orientation cross section. If corrosion is suspected when viewed in a top down evaluation then a sample must be taken without gold stripping and evaluated as a cross section.
  4. The samples for evaluation should typically not have been etched for copper structure/plating evaluation but if so etched they can still be evaluated.
  5. Samples for evaluation are typically the outgoing QC cross sections and “special” coupons are not necessary
  6. Previously prepared cross sections can be submitted for hyper corrosion evaluation at S T and S or unpotted coupons can be submitted for evaluation. It is noted that some coupon clean up maybe necessary prior to evaluation. It is important that good edge retention of the plated surface be maintained by the potting compound, failure to do so will make inspection and documentation very difficult.

 

 

3.6.1.2  Hyper-Corrosion Rating Level Table

The ratings shall be per Table 3-4:

Table 3-4  Three Levels of Hyper-Corrosion Observed Using Optical Microscopy

Hyper-Corrosion Level

Visual Observations at 1000X Using Optical Microscopy

Disposition

Level 1

Less than ten spike type defects observed on most pads[1]. Corrosion spike depth is < 1 micron and no more than 20% of the nickel deposit thickness

Acceptable - This activity will not degrade solder joint integrity.

 

Level 2

More than a ten spike type defects and some spreader/spike defects on most pads observed. At this activity level, more than 99% of the solder surface has not degraded. Corrosion spike depth is < 2 microns and no more than 40% of the nickel deposit thickness

AABUS - Will require extra analysis and testing to ensure acceptability.

Level 3

More than ten spreader/spike defects and/or some area black band defects on most pads observed. Corrosion spike depth is ≥ 2.0 microns or ≥ 40% of the nickel deposit thickness, whichever is greater

Rejectable -  This level of hyper-corrosion will degrade solder joint integrity.

 

[1]A pad is defined as any surface that ENIG has been plated to. This could be a surface mount feature, a fiducial or an annular ring and may be on a specific coupon design or a portion of an actual circuit that is being evaluated in cross section

 

 

Images and requirements from IPC 4552A

3.6.1.1.1   Normal ENIG Free of Hyper Corrosion Defects

These are seen in Figures 3-17 and 3-18.

Figure 3-17:  Defect Free ENIG Deposit - Knee of Hole.

Figure 3-18: Defect Free ENIG Deposit - SMT Feature

 

 

3.6.1.1.2   Acceptable Conditions with Some Evidence of Hyper-Corrosion

These are rated as a Level 1 Hyper-Corrosion in both Figures 3-19 and 3-20.

Figure 3-19: Level 1 Hyper-Corrosion - SMT Feature.

Figure 3-20: Level 1 Hyper-Corrosion - Knee of Hole

 

3.6.1.1.3   Acceptable Conditions with End User Sign-Off/Acceptance

The following Figures 3-21 and 3-22 are rated as Level 2 Hyper-Corrosion.

Figure 3-21: Level 2 Hyper-Corrosion - Knee of Hole.

Figure 3-22: Level 2 Hyper-Corrosion - SMT Feature.

 

 

3.6.1.1.4   Rejectable Conditions

The following Figures 3-23 and 3-24 show a Level 3 Hyper-Corrosion.

Figure 3-23: Level 3 Hyper-Corrosion - Knee of Hole 

Figure 3-24: Level 3 Hyper-Corrosion - SMT Feature.

 

 

Images from a IPC4552A hyper corrosion evaluation performed at S T and S Group Inc.

Figure 1: level 1 hyper corrosion present on this sample.

Figure 2: level 1 hyper corrosion present on this sample.

Figure 3: there are more than 10 occurrence of corrosion in this section – level 2 hyper corrosion – additional testing needed.

Figure 4: there are more than 10 occurrences of corrosion in this section – level 2 hyper corrosion – additional testing needed.

Figure 5: less than 10 occurrences of hyper corrosion – level 1

Figure 6: less than 10 occurrences of hyper corrosion – level 1

Figure 7: there are multiple issues with this deposit including reduced thickness. The penetration of the resulting hyper corrosion penetrates more than 40% of the nickel thickness and extensive occurrence of corrosion – level 3 hyper corrosion.