Operation Missing Mask

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Masking the Solution for BGA Rework

Figure 1: BGA site designed for test socket application.

Every now and then an error will occur at the most inopportune time; in this case it was in the design phase of the printed circuit board. 

When circuit boards are in the proto-type phase it is not uncommon for sockets to be used at BGA sites to prove out functionality. Design Engineers must layout the BGA sites to meet the requirements of the selected test socket. Many of these sockets rely on a planar site that does not include solder mask covering the connecting circuits and via's (see figure 1). This allows the BGA socket to be uninhibited when mounting onto the board surface while providing direct contact to the BGA pads.

That's all well and good until, you decide it is time to move on with phase II of qualification, or better yet, to the production phase of the circuit board in which you'll be soldering these high ball count BGA's on.

You're probably not surprised that there is an occasional blunder at this point. In this case, someone neglected to re-layout the solder mask layer and that is when the fun began. Have you tried soldering a BGA component onto an unmasked site lately? I can assure you, fun it is not!

The primary purpose of solder mask at a BGA site is to isolate solder wetting to a specific area, the BGA pad. Solder mask is used to isolates the pad from the conductor, or from the circuit leading to a via or plated hole.  Without the solder mask, the molten solder will bleed away onto the connecting circuits, or into the attached vias. Solder joint opens and shorts are a guarantee.

The obvious key to preventing this would have been to re-layout the mask layer along with any other changes deemed necessary after the phase I testing (see figure 2). 

Figure 2: BGA site with connecting circuits and via's covered with solder mask.

Since that was not done, solving the missing mask issue can be approached in a few different ways, but the fact is, it must be confronted.  Here are a few options:

Option One:
Scrap em out - Big money boards plus big money parts equal a major ouch to the pocketbook. 

Option Two:
Re-socketize - use the same socket that was used through the phase I development.  This is probably not a practical solution, but it can be pursued. These sockets tend to be very expensive and the durability of the connection can be a concern when being used outside the controlled proto-type lab environment. 

Option Three:
The most common approach that we utilize is reworking the solder mask at the site.  This can be accomplished in a few ways. The most common method is to manually apply the solder mask, or an equivalent high temperature epoxy to the site.

This technique takes a steady hand, and the use of a variety of tools, such as a controlled dispensing system, or fine tipped artists brush, along with tape or a template to shield the solder mask from reaching any undesired locations, such as the BGA pad. 

Figue 3: BGA site after rework with partial mask coverage to connecting circuits.

Prior to applying the mask there are some critical preparation steps that must be completed to ensure the new mask will adhere properly to the surface and create the wanted barrier.

These steps include burnishing the location upon which the mask is going to be added, typically the circuit or via connected to the BGA pad.This surface must also be thoroughly cleaned to eliminate any contaminates and optimize the adhesion of the mask to the surface.

Complete coverage of the site is not always necessary, the key is creating the isolation barrier and this can sometimes be accomplished with a much smaller application of mask at the connecting circuit while leaving the adjacent area clear (see figure 3). 

Another option recently introduced to the industry that may serve as a solution is an adhesive backed stick-on mask template. Standard BGA patterns should be readily available, but if it is an unusual pattern, or newly introduced component type then lead-time for custom patterns can be a concern. 

As is the case with many reworks applications, new techniques are continually evolving.  Go to the Guidebook Section of our web site to see a list of the latest repair and rework procedures for this and other applications.

Several members of the Circuit Technology Center team contributed to this feature story.