Paste-in-hole processes are somewhat more complex than traditional SMT from a process control standpoint, but with good engineering practices and the properly optimized stencil, it can be a very controllable process for many users.

The key is doing some math on the front end and determining how much paste will be needed to fill the space between the barrel of the through-hole and the lead of the component.

Since paste is generally about 50-55% metal by volume, you will generally find that you need to overprint the pad area to ultimately deliver enough solder to the solder joint. Various stencil design techniques will work; contact your solder paste and/or stencil supplier for more detailed pointers.

As for picking the right paste for paste-in-hole applications, many standard solder pastes may drop into this type of process without any issues. However, there are two additional solder paste performance requirements for paste selection in paste-in-hole versus traditional SMT.

These include coalescence behavior and anti-dripping behavior. The coalescence behavior becomes more of an issue than usual due to the likelihood of overprinting onto the solder mask on the top side of the board. The anti-dripping requirements stem from the fact that some pastes can drip off the bottom of the component lead upon heating, resulting in less solder than expected for the solder joint.

This can be avoided by selecting a solder paste that was designed not to drip in a paste-in-hole application.

Brian Smith General Manager - Electronic Assembly Americas, DEK International Mr. Smith has been supporting customers in the electronics assembly industry since 1994. His expertise is focused on solder paste printing and reducing soldering defects. He holds a BS in Chemical Engineering and an MBA in Marketing. He has authored several papers in trade magazines and at industry conferences. He is an SMTA Certified Process Engineer.

To ensure proper plated hole fill during the screen printing process you must have the proper volume of solder paste. Here's an example of the formula you can use:

H=Hole diameter
D=Lead diameter
T=Board thickness
L=Width of lead in the X direction (For square lead)
W=Width of lead in the Y direction (For square lead)
Pi =3.14

Hole Volume (HV)=(3.14)(H/2)(H/2)(T)
Lead Volume (LV)=(3.14)(D/2)(D/2)(T) (For round leads)
Lead Volume (LV)=(L)(W)(T) (For square or rectangular leads)
Annular Ring Area(RA)=(3.14)(L/2)(L/2)-(3.14)(H/2)(H/2)
Solder volume (SV)=HV-LV
Print Volume (PV)=(2)(SV) (50%)
Print Area (PA)=(F)(PV)/stencil thickness
F=Inspection Factor

.7=no fillet
.9=fillet on both sider
0.8=fillet on primary side
1.0=large fillet on both sides

Joe Karcewski Product Manager, APS-Novastar, LLC Joe Karcewski has been a Process Engineer for 16 years in the industry. He is a certified IPC-A-610 trainer and is presently working at APS-Novastar as a Product Manager for Soldering Systems including Selective Soldering systems.

The best solution for the Pin-in-Paste process is the ProFlowR enclosed head system.

The main reason for this is that ProFlow allows independent control of paste pressure and, therefore, the ability to control the amount of paste fill for the through-hole apertures.

If you are unable to use ProFlow, then a 45 degree squeegee can increase the paste pressure. But, if the board is thicker than 1mm, several print stokes will be required, which could degrade the SMT deposits.

Standard solder paste should be used for this application.

Clive Ashmore Global Process Manager, Dek Printing Machine Mr. Ashmore is responsible for the Global Applied Process Engineering group for DEK. Clive specializes in all aspects of manufacturing engineering, with special emphasis on mass imaging technologies.

Same paste should work. You can calculate amount of paste required knowing 4 items: pin dimension, board thickness, hole size, and annular ring size. Paste to solder shrinkage is usually about 50%.

Paste volume can be achieved in 3 ways: Overprint, Step-up Stencil, through-hole paste fill.

Good reference paper is in SMT Magazine Nov and Dec 2006 "Intrusive Reflow for Lead Free paste."

Bill Coleman Vice President Technology, Photo Stencil For over 18 years, Dr. Coleman has been the vice president of technology for Photo Stencil, working closely with customers to understand their printing requirements. His efforts have resulted in several new stencil products.

The use of an off-line x-ray inspection system post paste that offers oblique angle views without tilting of the actual board to inspect a representative sample of boards will allow you to see the amount of paste that is located down the holes prior to reflow, non-destructively.

The same test can also be used post reflow to quickly confirm that the resultant pin-in-paste hole fill meets the IPC 610D recommendations of a minimum of 75% fill.

Dr. David Bernard Product Manager, Dage Precision Industries Mr. Bernard has been the X-ray Systems Product Manager at Dage for over 5 years and have been involved in all aspects of x-ray inspection and test for printed circuit board assembly applications. Prior to this, Dr. Bernard was working with radiation measurement instrumentation.