PWB Back Drilling Phase 2


 Phase 1 of the project showed that the shallow back-drill fails earlier relative to the medium and deep back-drill and proved the capability to electrically measure remaining stub lengths from a single ended trace on a single inner layer position.  Phase 2 will focus on the shallow back-drill but on a much thicker 30 layer PCB using two heating circuits and will evaluate the capability to electrically measure via stubs from various layers within the 30 layer PCB TV using differential signal traces.

Project stage: 
Lead company: 
PWB Interconnect Solutions

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Idea Information

Background: 

While back drilling of PWBs helps to remove signal distortion by removing via related stubs, reliability issues attributed to this practice appear to be on the rise. Both the number of back drilled vias and the variation of depths (any & all layers) are increasing.  Design rules are driven by electrical requirements, not necessarily based on PWB reliability data or fabrication capabilities. The capability of electrically measuring remaining stub length from all layers is critical

 

  •We learned in Phase 1 of the project that the shallow back-drill fails earlier relative to the medium and deep back-drill.  Phase 2 will focus on the shallow back-drill but on a much thicker 30 layer PCBs (compared to the phase 1 - 0 layer).

 •We also proved the capability to electrically measure remaining stub lengths from a single ended trace on a single inner layer position.  Phase 2 will evaluate this capability from various layers within the 30 layer PCB TV with differential signal traces.

 

 

Problem: 

 Both the number of back drilled vias and the variation of depths (any & all layers) are increasing on each board. 

 Reliability issues with back drilling is often related to an unique family of products (Design/Material/Process).

 Many family of products have thicker backplanes that are needed to interconnect signals.  Phase 2 explores the relative reliability of the thicker backplane-like PCBs’.

 The capability of electrically measuring remaining stub length from all layers is critical.

 Example of multiple backdrilled vias in a single board.

Definition Information

Goals / Benefits: 

 This project will explore the reason for back drilling related PWB failures and identify potential solutions and design/process guidelines to prevent future problems associated with this process along with verifying improved electrical measurements of stub lengths.

  •  ·         Quantify Back Drill reliability of shallow backdrills on thicker PCBs
  •  ·         Evaluate the effect of an adding a “sacrificial” pad on the layer right above the MNC layer.
  •  ·         Verify accuracy and resolution of electrical measurements of stub length on differential trace routing
  •  ·         Verify electrical measurements of stub lengths with both long and short via barrels at various depths within a board.
  •  ·         Verify electrical measurements on typical backplane structures

 

Benefits of Project:

·         This methodology will save the OEM/PWB fabricator money in post build inspection by providing a coupon design and test methodology to electrically test backdrill holes for reliability and conformance to specification during fabrication.

·         These design guidelines will help reduce development cost for redesigns and improve time to market by delivering a set of design parameters for HDP members that will guide OEM, ODM, and system designers in designing a more reliable back drill product.

High level objectives: 

  •    ·         Quantify Back Drill reliability of shallow backdrills on thicker PCBs
  •  ·         Evaluate the effect of an adding a “sacrificial” pad on the layer right above the MNC layer.
  •  ·         Verify accuracy and resolution of electrical measurements of stub length on differential trace routing
  •  ·         Verify electrical measurements of stub lengths with both long and short via barrels at various depths within a board.
  •  ·         Verify electrical measurements on typical backplane structures

Deliverables

          Publish a report documenting the reliability of shallow backdrilled designs for thicker PCBs.

          Publish a report documenting the accuracy and resolution of electrical measurements of stub length on differential trace routing and various stub lengths with both long and short via barrels

 

          Publish a report documenting the reliability of shallow backdrilled designs for thicker PCBs.

          Publish a report documenting the accuracy and resolution of electrical measurements of stub length on differential trace routing and various stub lengths with both long and short via barrels

 

Approach: 

 From the learnings of Phase 1:

 Evaluate only the shallow backdrills

 Thicker boards –4mm thick nominal –with a resin content more reflective to a 30 layer board

 Same basic coupon designs – but with some layer modifications and elimination of the medium and deep backdrills

 Additional backdrill coupon with a “sacrificial” pad on the layer right above the MNC layer.

 Only 4 coupon designs (0.8mm, 1mm and 2mm pitch – with the sacrificial pad version only at 1mm).

 6X reflow preconditioning –followed by 25-150C testing to fail (minimum 6 sample, but target 12)

Add additional STUBviewdifferential land structures with a via pattern reflective of typical backplane layout.
Verify electrical measurement of stub length with differential traces
Verify electrical measurements of stub lengths at various depths including
Short vias/long backdrills (e.g., L3 signal routing with backdrill from bottom upto L4)
Long vias with short backdrills (e.g., L26 signal routing with backdrill from bottom upto L27)
Verify stub measurements on typical backplane structures (larger thru holes -17.7mil drill)

Add additional STUBviewdifferential land structures with a via pattern reflective of typical backplane layout.

Verify electrical measurement of stub length with differential traces
Verify electrical measurements of stub lengths at various depths including
Short vias/long backdrills (e.g., L3 signal routing with backdrill from bottom upto L4)
Long vias with short backdrills (e.g., L26 signal routing with backdrill from bottom upto L27)
Verify stub measurements on typical backplane structures (larger thru holes -17.7mil drill)

 

Preliminary Schedule:

 

Flow Diagram: