|Ask the Experts|
December 22, 2017
Pin-in-Paste Hole Fill
We are about to implement Pin-in-Paste technology. How can we ensure proper plated hole fill during the screen printing process? Will the process require using a different solder paste than the one we use for pure SMT assemblies?
|Expert Panel Responses|
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.
General Manager - Electronic Assembly Americas
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
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.
Global Process Manager
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."
Vice President Technology
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.
Dage Precision Industries
There are numerous tradeoffs to consider when pin in paste hole fill is the goal. First, overprinting can be problematic due to the possibility of mid chip solder balls. Step stencils are more expensive than standard stencils, and will likely result in reduced throughput and material expense due to the need to increase underwipe frequency. One technology which is becoming very popular is the use of solder preforms in tape and reel packaging. The preforms come in standard sizes, such as 0402, 0603 and 0805 to name a few. They are packaged exactly like chip capacitors and resistors. Placement is done after paste printing. Only 20-25% of the preform needs to contact the solder paste. Since the preform is solid metal, a preform as large as four times the volume of the paste volume can be added and still achieve perfect reflow results.
Global Product Manager
Readers may be interested in the free ebook on "Pin In Hole Intrusive Reflow Design and Assembly" at http://www.pihrtechnology.com The most common problem with the process is the hole to lead ratio, at the design stage its easy to change and it gives design engineers more tracking space. Changing the hole to lead ratio on existing design is easy too as its not going to impact reliability either.Bob Willis, Bobwillisonline.com, UK
Going into the reality of pin-in paste requires a little more than applying paste into the hole. There are usually unforeseen issues which occur after the pin in paste operation. The stencil can be designed to prevent inadequate hole fill by forming a cross pattern with the paste around the top side of the hole. The flux will stay mostly on the top of the pcb under the part instead of flushing out to the bottom pad. The flowed solder will only take what is required to do the job. Making the use of expensive preforms is not necessary. People are asking for advice or suggestions as to which way to perform their particular task at hand. They may not be aware of the pitfalls of following this advice. I know I would prefer to also know what I shouldn't do as well as what I should.Mark A. Maheux Sr., Sr. Manufacturing Engineer, Honeywell Life Safety, USA
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