- What’s the issue?
Transmission of high frequency signals on PCBs requires skilled design techniques that determine the impedance (characteristic) on PCBs. Transmission will not be effective – or fail – if this isn’t done when signals have high frequency. The design element associated with this is not covered here: it is assumed that the design is good. Manufacturing problems can follow on from essentially good designs. This article addresses such issues and suggests means by which they may be avoided.
What PCB Fabrication Issues Lead to Issues with Controlled Impedance?
There are really only 2 things that determine the quality of a controlled impedance channel on a PCB: material and dimension. These are heavily nuanced with practical problems and detail, but it helps to focus on this during assessment. If the design is correct, dimensionally perfect, and made from the right materials, it will perform as designed. This, of course, is an ideal.
The key issues are:
- Incorrect layer stack – PCBs are made of layers of insulating material and copper. These all have thickness and material properties (most important is the insulating material – FR4/Rogers – whatever). In an established design, the manufacturing records are stable, so an error would suggest weak manufacturing control.
- Incorrect copper tracking or copper filling – as with the previous bullet, established designs that have good process routing should not suffer this problem. Manufacturing failures that are caused by this suggest weak process control.
- Board Connectors – it must not be overlooked that the signals pass through more mediums than the PCB (connectors, cables), and any/all of these can cause problems. In terms of PCBA supply, the two relevant processes are fabrication and assembly. Connectors cause mismatch anyway. A poorly fitted or incorrect connector can cause problems.
- Fabrication Quality – solder splashes, poorly fitted components or poor general PCB assembly can lead to issues. If high frequency PCB designs are being produced, the cheapest
What are the Key Actions to Mitigate Controlled Impedance Issues in Manufacture?
The following are the manufacturing issues to consider:
- Design: Make sure that the layer stack (definition of PCB layers – material, thickness) is fully defined and part of the design pack for the contract manufacturer.
- Material: characteristic impedance is critically dependent on material. Insist that a declaration of conformity is supplied with every batch of product. This declaration must include a specific statement of the material used for the insulating layers of the PCB, e.g., Rogers. It is also necessary to be aware that most PCB materials are not truly isometric – different properties depending on propagation direction (weave of glass fibre, e.g.). If not isometric (design engineers must determine), ensure orientation is defined and adhered to for panel orientation and panel population. Include this requirement in the declaration of conformity.
- Impedance measurement: it is sensible to put ‘test coupons’ on high frequency PCB designs (good CEMs have equipment for measuring controlled impedance). These are redundant parts of the PCB design that will measure as a given controlled impedance if the layers and materials are correct. Put test coupons on all relevant PCB designs and ensure that the measurements of controlled impedance from these are recorded on the declaration of conformity.
- Assembly: assembly of components onto PCBs to form PCBAs is often a process carried out by a different manufacturer than the PCB fabricator. This is of no consequence because the client should insist that the assembly house carries forward the declaration of conformity from the fabricator of the PCBs – give the assembler responsibility for compliance. If the cost isn’t considered a problem, functional testing of the completed PCBA can also confirm problems don’t exist. Also, to reduce issues it may pay to insist on a relatively high assembly standard (IPC class).
- Quality: ensure that a declaration of conformity is issued with each batch of product. This declaration must contain all the data required (see declaration of conformity statements in this article). Note the comment in the foregoing bullet – assembler of components made responsible for declaration of conformity of the PCB fabricator. Also consider what punitive and remedial responsibilities apply, e.g., product rejected if no declaration of conformity received with batch.
- Shipment: ensure full documentation (declaration of conformity) is shipped with every batch of product.
