The role of electronics manufacturing services (EMS) companies continues to extend beyond mere manufacturing argues TTems Ltd, as it recently took on a manufacturing challenge during a major contract for an aerospace-defence customer.
The prevalence of customers using complex electronic packaging technologies in their designs brings new manufacturing challenges. Simon Titman, commercial manager at TTems comments: “Customers today want to work closely with their EMS provider throughout the life-cycles of their products, looking for guidance on design for manufacture and test (DFM and DFT), issues relating to component sourcing and availability, and logistics services.”
Recent experience in supplying a range of complex multi-layer flexi-rigid
PCB assemblies to a major UK based aerospace-defence customer illustrates
the level of partnership required from EMS providers, when quality level had
to exceed the most stringent workmanship standard IPC-A-610 Revision D Class3.
Michael Satherley, process engineering manager at TTems takes on the story
“Each of the manufactured items is a number of 8-layer rigid PCBs interconnected
by flexible circuits. The company is contracted to manufacture 4 types of
these PCB assemblies (PCBA) at a rate of six hundred per month for mission
critical systems, so they’ve got to function with maximum reliability.”
“Each of the four rigid multi-layer PCBAs is populated on both sides
by surface mount devices,” continues Satherley. “Some of the SMDs
are custom devices. Most are fine pitch, and one in particular, an accelerometer,
is relatively large and heavy, and demands great accuracy at the soldering
process.”
The first challenge of this programme was the fabrication of the bare flexi-rigid
PCB to the necessary flatness tolerances to run on normal production lines.
TTems had previous experience in DFM and DFT issues and manufacturing of flexi-rigid
PCBs; however these were the first to be manufactured in volume.
At the initial manufacturing stage, the PCBAs are manufactured in panel of 4 assemblies. It is the rigidity of the panel during the solder-paste application process that was to prove problematic. The first build showed a high number of manufacturing defects.
TTems set up and coordinated a problem solving team lead by Dave Whippey, quality manager at TTems, which included representation from the customer and the PCB supplier.
It transpired that the rigid panels were not of uniform thickness, with variations
up to 0.012 inch. The ramification of this was that the gap between the solder
paste stencil and the board was at places up to 0.006 inch. This led to the
deposition of solder paste being inaccurate which in turn created solder balls
and short circuits.
Whippey comments: “The issue was resolved by improving the PCB design
and the bare PCB manufacturing process, reducing the thickness variation from
0.012 to 0.005 inch.”
Satherley adds: “We also found that the flexi sections were not sturdy
enough to hold the rigid areas horizontal during reflow, allowing solder to
spread (flow towards the centre of the PCB) and creating short circuits.
“ Pip support were introduced by the PCB manufacturer into the rigid
area of the circuitry to hold it steady within its frame. This appeared not
sufficient to provide an accurate paste deposition, crucial at this first
stage of production.”
To provide the required support to the panel and improve repeatability, TTems developed purpose-build machined nests. In addition, TTems used all the parameter available through its state-of-the art stencil-printing system. The MPM Acella Speedline has the highest specification in-line solder-paste stencil-printing system for high volume production. Stencil cleaning after every panel was introduced and every parameter checked and fixed.
Whippey recounts:” In parallel to concentrating our attention on the flatness and sturdiness of the PCBA, we had to take on the challenge of reducing the formation of minute solder balls. Despite their small size being permitted under the highest workmanship specification (IPC-A-610 Revision D Class 3 Acceptability of Electronic Assemblies), the solder debris that was formed during reflow could cause reliability problems. The issue was raised with the relevant standards body to review the permitted size.”
However, the customer needed the ‘workmanship standards’ bar
to be raised. TTems took up the challenge and explored every possibility that
might lead to that goal.
Michael Satherley explains: “The solder paste stencil was redesigned,
introducing changes in thickness (steps) to correspond to those in the PC
panel: On the bottom-side stencil, the step on the PCB side of the stencil
ensure that the stencil mates with the change in the PCB thickness during
the screen-printing process.
This was also true for the top-side of the PCB. However this was made more
complex as the customer required a 0.006 inch paste deposition on one of the
custom-device. Therefore an additional step up of 0.001 inch was added around
the devices to alleviate the reliability concerns. The apertures in these
sculptured stencils are not laser-cut, but created by electroforming to reach
the given thickness at a controlled rate and minimise the paste seepage under
the stencil reducing potential solder bridges.
To achieve the zero-solder debris assembly required by the customer, an additional
chemical wash process was introduced at stage two of the manufacturing process.
A high-level relationship
To solve such engineering issues, the EMS provider needs to maintain a constant high-level dialogue and a focus on its obligation of continuous improvement. To play its part in supporting quality, lead-time and cost objectives, TTems has adopted lean six-sigma and EFQM principles.
Commercial manager Simon Titman emphasises the importance of the company’s programme management strategy “The aim of our cross-functional team approach is to ensure that product delivered to our customer is manufactured to the highest quality at lowest cost.
“TTems has worked with this customer since 2003, and in September 2005
we became its supplier of choice for manufacturing PCB assemblies, as well
as a participant in its Supply Chain Excellence Programme. We have developed
the capability to deliver PCB assemblies in accordance with the customer’s
stringent quality requirements and within target cost.”
The customer’s purchasing manager concludes: “TTems continues
to provide valuable resource to us and we look upon its capabilities as an
extension of our own. Our alliance agreement, and its participation in our
Supply Chain Excellence Programme demonstrate our commitment to work together
strategically to achieve product improvement.”
Ends
