More Freedom of Choice - Exclusion Fixtures
by Finlay Buchan
Director of Design
Datum Dynamics USA / Datum Dynamics Ltd. (UK)
Following Terry Morgan's article in June's EP, Finlay Buchan describes how to evaluate if problem boards can be flow soldered.
Introduction
Whilst the last decade has seen the continuing rise of surface mount technology used in the production of PCBs, pin through hole technology still dominates as the preferred method of fixing connectors and many magnetic components. This has given rise to densely populated double sided mixed technology boards which can be problematical to wave solder. One solution to this set of difficulties is to enclose the solder side SMT components whilst allowing solder to flow around the pin through hole (PTH) components.
Accurate Exclusion Fixtures (also known as Selective Wave Solder Fixtures) can now be manufactured using the PCB design data linked to mechanical computer aided design and manufacturing techniques. However, the success of this type of fixture requires adequate clearance around the pins to be soldered. This article presents methods for estimating suitable clearances, which ideally would be used in PCB design, or more likely to determine the likely success of an exclusion fixture.
Enclosing fixture Description
An enclosing fixture is one which fully encloses all (or nearly all) the solder side surface mount components whilst leaving apertures for the pin through hole components to be soldered using a wave process. Figure 1 and figure 2 show part of such a pallet in plan and section views. Because of their complexity exclusion fixtures must be designed and manufactured using the same data that describes the board. Please do not ask for exclusion fixtures if you do not have access to the board data (gerber & drill files).
Alternative Processes
An alternative wave soldering method for such a board would be glue printing or dotting of solder side SMT components prior to wave. However the move towards more complex active components is now starting to rule out such processes. Mini wave soldering can be used if the number of PTH components to be soldered is small and the variability in soldering can be tolerated.
Advantages
Reduced production time / capital employed arising from the elimination of the Glue Dot/Stencil, mini wave, stencil dip, vertical soldering processes.
Increased product quality by eliminating shorts and thermally damaged components.
Increased Board Density - More complex and thermally sensitive SMT devices can be populated on the solder side.
Elimination of board bowing, which reduces stress related joint failures.
More chance of the finished product mating up with backplanes and cut-out apertures in a chassis.
Reduced board clamping requirements compared to open style fixtures.
Disadvantages
Higher fixture costs - exclusion fixtures normally cost around 50-200% more than "picture frame" fixtures. This is caused by the large amounts of time required to design and machine the fixtures.
Longer Procurement - the you longer design times can mean having to wait up to a week for the initial fixture (shorter manufacturing times are possible from Datum Dynamics - but remember designers have a life too - well, some have!).
Enclosing Design Rules
(fig 1.)Solder Side Components Located Parallel to direction through wave.
Note that the clearance required between the pin land and SMT pad can be made quite small, as the solder does not have to flow "under" the component pockets.
(fig 2.)Solder Side Components Located Perpendicular to direction through wave.
Note the larger separation required due to the solder having to flow "under" component pockets.
This graph shows the effect of SMT component height and placement, on the required separation. This directly leads to the following:
PCB Design Implications
Keep large (height) components away from PTH areas.
Leave the leading and trailing areas around PTH components as clear as possible.
DON'T put any components within 3mm (0.12") of any PTH components.
DON'T put all PTH components in line along one edge of a board - leave some space to allow bridges to be fabricated to support masking in the centre (center) of the board.
Data required to prepare or evaluate exclusion fixtures:
Gerber Data:
Ideally RS274x (embedded aperture list) - if not, DO NOT forget aperture list.
Topside Gerber:
Silk Screen - so we can see what is to be soldered.
Solder Paste - so we can see what not to put board fixings on top of.
Bottom Gerber:
Silk Screen - so we can guess what you want to re flow.
Solder paste - so we can avoid putting material where your components are.
Solder Mask - so we can avoid leaving you with large icicles on ground planes.
Drill Data
Normally Excellon - so we know where plated and non-plated through holes are.
Don't forget the tool list!
Placement Data
Bottomside placement data with accurate component heights (as required for placement machines).
Conclusions
Enclosing fixtures are not the solution to all wave soldering problems - but with a half decent board design - a little luck - and a decent wave solder fixture supplier you can push defect rates to a minimum. As a first off - if you're a board designer - send your design to finlay@datumdynamics.com we will have a look, suggest changes and if we manufacture the wave solder fixture we will keep doing this for as long as you like.
Wave Solder Fixtures, RoHS and Syntracote
Datum Dynamics USA has risen to the challenge of RoHS and is the only company to develop a coating for wave solder fixtures that overcome the problems of resin burnout and structural deformation caused by the increase in the melting point of Lead Free Solder to circa 275°C. This coating, Syntracote, is exclusive to Datum Dynamics has been specially formulated and tested. We have been so impressed with its performance that a patent has been applied for. Please follow the Syntracote link for more information.
Pneumatic Press and Heat sink Attachment
Over the last year Datum Dynamics has developed a solution to the problem of maintaining the correct force for the correct length of time when attaching heat sinks to components. We have produced a range of Pneumatic Presses and control systems to ensure that the process is accurate, repeatable and easy to use in a production environment. Please follow the Pneumatic Press link for further details.