The application of DFMA principles in the manufacturing and assembly of printed wiring boards can manifest in the form of reduced cost and assembly time. It is important to start defining the critical process path during the design stages of the PWB, to avoid potential manufacturing problems encountered during the course of assembly. As an example cited in a previous issue of the Empfasis, the transitions from a Eutectic Tin/Lead to a Lead-Free soldering process is a prime example of the value of a DFMA undertaking.
The list below represents the manufacturing process of a mixed technology PCB with through-hole components on one side, and SMT devices on both sides of the PCB. The insertion of a lead-free paste can change the soldering operations sufficiently to warrant a close examination of the options available in avoiding manufacturing pitfalls. At each interval in the process, there are procedural considerations to be taken into account as part of a DFMA system to define a critical path.
Board prep
• Substrate cleaning process
• Board surface finishes impact wetting of the solder paste onto
the pads of the PWB. This is especially true for Lead-Free solders
• Board drying for hydroscopic materials. Be aware that the
thermal exposure as a result of the solder change will
increase vapor pressure
 Stencil print primary side
• Limitations in pitch
• Correct aspect ratio – should be > 1.5 for good paste release
• Geometry of stencil apertures
SMT placement primary side
• Automated or manual placement
• Consider the residence time of the solder paste prior to reflow.
It may change versus conventional Sn/Pb paste.
SMT reflow
• Reflow ovens. Are they suitable to achieve specified profiles?
The risk factor in using a 5 zone oven versus a 10 zone oven for
the higher temperature profiles, may warrant the procurement
of the higher zone oven.
Assembly cleaning
• May be necessary at this stage
• Additional drying may be needed for moisture sensitive
components
Dispense adhesive on second side
• In some cases this may be avoided, depending on the process.
If the process used is simply two-sided SMT, smaller devices
(< 4 gm/cm2), allow the surface tension of the solder to hold
the component in place.
• An alternate PTH solder application may allow the exclusion
of the adhesive step
• If you use an adhesive, choose one suitable for the higher wave
solder temperatures.
Application of solder paste (alternate to adhesive)
• Reflow secondary side solder paste or cure adhesive
• Be aware of the primary side components. The size of the SMT
component will determine the feasibility of secondary reflow
process; i.e. adding additional adhesive re-enforcement.
Secondary assembly cleaning
• This may be warranted with additional drying to offset
entrapped moisture prior to subsequent soldering
DIPS and axial insertion
• Consider the lead finish for lead-free soldering application
• The MRT rating may have changed due to higher temperature
exposure
• As always, orientation of the components should be considered
• For a limited amount of through hole components, consider
truncating the leads to apply an SMT process
Wave soldering
• For smaller volumes of PTH, strongly consider a selective
soldering approach, which allows more flexibility in
design, and localizes the exposure of the board to the through-
hole components
• Consider the profile changes needed to accommodate the
lead-free alloys, including the type and method of fluxing.
Final assembly cleaning
• Do not mix no–clean fluxes with other flux types that call
for mild cleaning. No-cleans require aggressive cleaning when
the application warrants it.
The range of options and consideration for the mixed technology assembly process will more than likely exceed those that have been listed for this article, and will vary from operation to operation. An extensive examination of each assembly and sub-assembly step will help extract where the cost and time savings can occur. Once the specifics of the assembly process are sorted, it is beneficial to take a broader view of each alternate path.
Table 2-1 shows slight deviations among the three process sequences for the assembly of a mixed technology PWB. The first column indicates a “typical” process flow discussed earlier, as part of the extraction exercise to define options and considerations. The middle column shows a red highlighted cell that substitutes a selective soldering system option for the wave soldering step. This substitution allows SMT reflow for both sides of the assembly in lieu of the adhesive paste on the secondary side. This can occur because the entire assembly is not subjected to the heat of the solder wave, which can potentially cause movement of the SMT devices. Another potential approach, as illustrated in the third column, is to shorten the DIP and axial leads, and incorporate a solder paste in the plated vias where the through-hole devices will be mounted. This can be done concurrently when the solder paste is applied on the secondary side for the SMT devices. The through-hole components can be placed manually on SMT assemblies that have a limited number of PTH devices, not necessarily requiring automated insertion equipment. The entire assembly can be reflowed, thus eliminating the need for applying a wave solder to the PTH components.
Regardless of the manufacturing process, there are benefits that can be derived in utilizing the DFMA principles in the area of cost savings, and product delivery. For more information on training courses, please check our website http://www.empf.org and click on training.
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