Pin In Hole Reflow Assembly

All text Copyright Bob Willis, EPS

Surface Mount Technology (SMT) is now part of mainstream electronic assembly with virtually all market sectors benefiting from the use of SMT. One problem that has always proved a problem to design and process engineers is existing through hole parts where no direct equivalent SMT part is available.  

It is possible to hand solder conventional through hole components after surface mount assembly and soldering operations are complete. This however, is time consuming and may leave more flux residues on the surface of the joints causing a problem to in circuit test. Many through hole parts are used for direct contact during test and residues can quickly clog test pins in a no clean process.

A range of selective soldering equipment is available to either semi or fully automatically solder through hole leads. This does of course require capital expenditure on equipment. One method of soldering all surface mount and through components in a single operation is Pin In Hole Reflow (PIHR) or Intrusive Reflow soldering.

Possible Assembly Stages

There are three alternatives to adding solder paste for through hole components prior to inserting parts:

Stencil Printing
Double Stencil Print
Paste Dispensing

Stencil Printing

Solder paste is printed into through holes and over the pad surface during normal surface mount paste printing. The size of the aperture in the stencil is adjusted to allow paste to fill the hole, cover the pad and fill the resist aperture on the board. Component insertion is then conducted either manually or automatically by robot prior to the whole assembly passing through the soldering process.

Double Stencil Print

As it may be difficult to obtain the volume of solder for a through hole components if lead to hole ratio is excessive a double print operation is possible. In this case an initial printing operation is conducted to force paste into the hole only. A second printing operation is then conducted to add additional paste to the hole, pad and resist window. This increases the solder volume in the joint. This operation does require two printers and two stencils.

Paste Dispensing

An alternative to printing is dispensing the paste in to the through hole which can overcome all the problems of metal volume. Today due to the increased speed of dispensing, the throughput would not be a problem for an in line application.

In each case the goal to through hole reflow is to achieve the required solder fill and guarantee the reliability of conventional components. The desired solder joint is a fillet visible on each side of the board after reflow. This would be the same criteria applied to wave or hand soldering operations to either IPC, IEC or British Telecom standards. However, it may not be possible to obtain a positive fillet above the surface of the board. Often the solder fillet remains flush with both sides of the circuit board but it is as strong and reliable as a full connection.

The following process parameters affect the solder joint formation, each need to be know to calculate the volume of solder required for each joint. In some cases design modification can be made to improve manufacture and the final joint quality.

Diameter of component contact
Diameter of plated through hole
Thickness of circuit board
Diameter of pads 

Component compatibility needs to be confirmed with the suppliers. Today may connectors, sockets and some active parts many be assembled by reflow. The main problem that may occur is distortion of the parts causing them not to sit flat to the board surface. Stress cracking may also be seen on the corner of parts due to stresses build up during their manufacture.

Care needs to be taken over the reflow process to eliminate voiding in the joints. As so much paste is confined in a hole rather than on the surface of the board the evaporation of volatiles from the paste can cause a problem. This is particularly true when connectors are mounted on the top side of the board during reflow. In this case volatile gasses find it difficult to escape from the through hole. The profiles of most reflow systems may need to be re-examined as most of the through hole parts like connectors, pin grid arrays and sockets can have a marked effect on local board temperatures. 

For further information on EPS services Email Bob Willis or visit his Internet Home Page.

Email: eps@bobwillis.co.uk
Home Page: http://www.bobwillis.co.uk