PARTNER FOCUS FEATURE
EXPERIENCES AT CSL As mentioned in the introduction, Circuit Solutions offer Design for Manufacturability to their customers and this plays an important part in the support policy of the company. Some examples of DFM experiences
demonstrating the type of issues that Circuit Solutions have identified and resolved ahead of building new products for customers: The first example demonstrates a
problem that Circuit Solutions experienced with the solderability of a component during the assembly of a prototype board. This was due to the design of the printed circuit board where a surface mount device was being fitted. This device was an 0201 diode ESD7381, where the main issue was centred around the terminations of this part being beneath the body of the component and not on the end caps as most passive SMT components are. When reflowed in the oven after placement, there was too much solder volume on the solder joints. This resulted in the diode floating on the meniscus of solder creating a weak solder joint. To overcome this problem, Circuit
Solutions proposed widening the pitch between the 0201 pads and increase the pad dimensions on the printed circuit board Gerbers. Also reducing the thickness and aperture diameter thus giving a reduction of solder volume by 48%. The pitch of the aperture openings was also widened to encourage the solder not to wet too far under the diode. The solder mask opening was increased to 1.00mm to allow excess solder to wet down the track reducing the solder height further. This action was implemented and
subsequently resolved this issue. The second example (on the same
board) was a via placed within the component pad on the printed circuit board. This was a pad for a surface mount 0402 resistor and the issue would have resulted in the solder fillet on this joint being reduced in size such that it would not conform to the required specification of the IPC610 standard. The solder would flow down the barrel of the via during reflow process leaving insufficient solder on the solder joint. To overcome this issue, the proposal to the customer was to resin fill the via and plate over during the manufacture of the printed circuit board. This suggestion was implemented with
the solder joint forming correctly on this PCB. A third example is ‘tombstoning’ of
/ ELECTRONICS
surface mount components, which is not an uncommon problem. This is where one side of the surface mount component is attached to the solder pad and the other terminal flips up at 90 degrees, only soldering to one terminal. The example Circuit Solutions experienced was with an 0508 capacitor where one terminal of the component was connected to a large pad area which formed part of a ground plane. To control the solder wetting during reflow the pad was outlined with solder resist leaving an area for the solder terminal. During the reflow process this pad absorbs more heat than the opposite small pad and hence has a slower cool down rate. As the board enters the cooling zone in the reflow oven, the small pad cools down first and begins to solidify, whilst the solder on the large pad is still liquid. The solder on the small pad contracts as it solidifies pulling the component upright and as the large pad is still liquid it has only the meniscus force to counteract it, so the part lifts partially or even completely out of the solder on the large pad. The suggested remedy to this problem was to get the
solder at each end of the component to solidify at the same rate so that all the forces are equalised across the component. To correct this, we proposed a change to the pad layout of the 0508 footprint, reducing the area by adding thermal relief to the large pad. This action was implemented and removed the problem of tombstoning on future builds.
X-RAY INSPECTION For new product introduction, X-Ray inspection has been a great benefit at Circuit Solutions. This equipment enables inspection of any components that have hidden connections on the underside of the component body. These will include BGAs, QFNs and connectors, confirming
Figure 3 DFM solutions
Figure 2: X-ray inspection
that the solder joint meets the required specification including connectors where the consistency and penetration of solder volume down the barrels of the plated through holes would otherwise be unknown.
DFM SOFTWARE SYSTEMS There are several DFM software programs available that can automate much of the manual work in identifying potential design or manufacturing issues. These offer intelligent analysis and are particularly beneficial when used ahead of new product introduction for electronics manufacturing. Generally these systems operate by
importing PCB CAD design and BOM data. They then integrate with a parts library to simulate assembled boards. The virtual assembled PCB is then checked against a list of good design practice criteria, the results are analysed and categorised before the program compiles a report recommending areas that need investigation. Circuit Solutions works closely with
their customers by carrying out DFM during new product Introduction. They also carry out DFM with existing products particularly where there has been a revision change made to the board. Circuit Solutions believe the key benefits of carrying out DFM for their
customers are: • Helping to achieve the best timescale for new product introduction
• Achieving the lowest product cost • Attaining the best and most
consistent quality control during manufacture
Ciruit solutions
www.circuitsolutions.co.uk
ELECTRONICS | DECEMBER/JANUARY 2021 29
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