The Pin in Hole Intrusive Reflow process is not new, the process dates back over ten years when presentations started to emerge at major conferences in Europe and the USA. Screen printing or dispensing paste onto the printed board, inserting the component terminations manually or automatically and reflowing parts seemed like a simple solution to engineers rather than hand or wave soldering. In Japan the process was referred to as multi spot soldering widely used by Sony on their portable CD players, all be it with modifications to the process generally accepted in USA and Europe.
The interest in selectively soldering through hole terminations on predominately surface mount products has lead to many suppliers offering dedicated selective soldering systems. These use either solder gets, solder baths or robotic soldering systems. Each process means dedicated machines for the process rather than the simplicity of using existing production equipment, screen print, placement and Reflow. Another alternative has been the use of selective soldering pallets in conjunction with wave soldering masking bottom sided surface mount parts. The jigs are expensive, need cleaning, and need the wave soldering process conditions to be modified to gain the best performance.
The limiting factor of PIHR to some companies has been justification of an odd form assembly system for one or more parts. The interim solution was to manually assemble through hole parts, which is common. But now that selected pick and place systems have been developed to handle the full range of surface mount and through hole on a single platform, there is renewed interest in intrusive reflow. Automatic assembly brings with it quality improvements and economic advantages to the user if parts can all be processed by the same equipment.
The key to the process is to provide enough solder paste to make a joint which is ideally the same volume as joints produced by hand or wave soldering. The volume of solder paste that can be printed is defined by correct connector design, distance between lead terminations and the stencil thickness, which is often defined by other components on the board. The ability of the printing process to fill the hole with paste also helps to achieve the ideal result rather than simply relying on paste on the surface of the board.
Ideally the design of the board should not change, but using the minimum hole to lead ratio for automatic assembly reduces the volume of solder to fill the plated through hole. Normally the hole to lead ratio is the pin size plus 0.010". Care needs to be taken in specifying the solder resist if printing over the mask is to be undertaken as resists are not necessarily designed to have solder paste reflowed on their surface. Most solder resists are not an issue, but care needs to be taken that they are correctly cured as it can lead to solder balling.
When terminations are inserted into the printed board some solder paste will be displaced on the tips of the pins. Trials show that very little paste is lost on the pins, in fact it reflows back along the pin and capillaries in to the plated through hole during reflow. Ideally the pin length protruding from the base of the board should be less than 2mm. People have experiences problems with paste displacement during insertion where the body of the component contacts the paste. This is simply poor connector design for the process or incorrect aperture design in the stencil.
In the case of reflow, care needs to be taken to profile the board correctly. Traditionally the largest QFP or, in recent times, the BGA may have been the coldest part of the assembly. Now the through hole parts often have the largest mass so they must be one of the parts checked when setting up a profile. The temperature under the connector, often where the solder joints are located, will be the cold spot buy as much as 15-20oC. With correct design of the connector this can be minimized with any final differences in temperature overcome with changes to the profile to obtain the ideal solder joint.
With the impending legislation in Europe, lead free processes have been in the forefront of people's minds. Recently Harting has been co-operating with National Physical Laboratory (NPL) Teddington, London on a lead-free soldering project. NPL is running a DTI project in collaboration with industry to evaluate the solder joint reliability of various lead-free solders.
The test board figure 1 was design to feature a 96-way edge connector using Pin-In-Hole Intrusive Reflow process. The design of the connector features and the stencil apertures were provided by Harting based on their own production trials. NPL also included changes to selected stencil apertures to widen the experimental results. The project consisted of 145 boards with mixtures of the following lead-free alloys SnAgCu, SnAgBiCu, SnCu, SnPbAg and a standard tin/lead control. The test board consisted of a wide selection of surface mount component as well as the connector. Figure 2 shows selected through hole joints after reflow soldering with 100% fill and positive solder fillets. To date the samples have been through thermal cycle testing of 1500 cycles of -55oC +125oC with dwell times of 40 min with no failures. The final report on the NPL project will be presented at a future NPL meeting with further details on their web site http://www.npl.co.uk/npl/ei in the next few months.
The initial results of the NPL study confirms previous results on lead-free trials and conventional tin/lead solder paste alloys that there is no difference in the reliability of through hole reflow and conventional soldering processes.
For further information on Pin In Hole Intrusive Reflow, visit Bob Willis web site www.bobwillis.co.uk or come and discuss your pin in hole design projects or process concerns free at Productronica stand 205 hall B5.
Bob Willis authored the first report on the technology and process introduction for the SMART Group and has produced two interactive CD-ROMs on the technology as well as the first training video on design for pin-in-hole reflow process.
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