CHANGE RECORDS Ed./Rev. Date Verified and approved by Description Writer 1 12/31/2015 WY / LLR Initial Document FS 2 02/26/2016 FS / LLR - 1.2. : new applicable document added : validation of the mounting of 3D PLUS memory stacks on pcb - 1.3. : new document in reference added : 3M 2216B/A Gray added - 3.2. : new adhesive added : 3M 2216B/A Gray WY TABLE OF CONTENTS 1. GENERAL... 3 1.1. Purpose and scope... 3 1.2. Applicable documents... 3 1.3. Documents in reference... 3 1.4. Definitions... 3 2. PROPOSED SOLUTIONS FOR UNDERFILLING... 4 2.1. Recommended solution (1): Basic underfilling... 4 2.2. Alternative solution (2): Underfilling through holes in board... 4 2.3. Alternative solution (3) : Underfilling limited by a H spacer... 4 3. MODUS OPERANDI FOR UNDERFILLING (SOLUTION 1)... 5 3.1. Toolings... 5 3.2. Adhesives... 5 3.2.1. Choice of adhesive... 5 3.2.1. Preparation of mixture... 5 3.3. Preparation of toolings and modus operandi... 6 3.3.1. Preparation of the dispense tip... 6 3.3.2. Estimation of quantity of glue to dispense... 8 3.3.2.1. Inputs to define:... 8 3.3.2.2. Outputs to define: dispense parameters... 8 3.3.2.3. How to evaluate the dispense parameters:... 8 3.4. Underfilling of the module on the board... 9 3.5. Just a few information about solutions (2) and (3)... 11 Doc. N :3641-0841-2 Page 2/11
1. GENERAL 1.1. PURPOSE AND SCOPE This document aims to describe the process for underfilling a SOP module mounted on a board. This process is used at 3D PLUS for assembly verifications. In the following chapters, the 3D PLUS module 3DSS24G08VS3626 RTIMS-FLASH is chosen as example. 1.2. APPLICABLE DOCUMENTS AD1 : Assembly Recommendations - Manual Reflow - SOP Packages - ref: 3300-1300 AD2 : Assembly Recommendations - Automatic Reflow - SOP and BGA packages - ref: 3300-1301 AD3: Validation of the mounting of 3D PLUS memory stacks on pcb Ref : 3300-1303-1 1.3. DOCUMENTS IN REFERENCE RD1 : Datasheet of Hysol Eccobond 285 with Catalyst 11 RD2 : Datasheet of 3M 2216B/A Gray 1.4. DEFINITIONS Stand-off : distance between the lead and its pad The presence of a stand-off is mandatory for stress relaxation in the solder joints after module assembly. The stand-off is recommended to be between 50µm and 150µm Doc. N :3641-0841-2 Page 3/11
2. PROPOSED SOLUTIONS FOR UNDERFILLING 2.1. RECOMMENDED SOLUTION (1): BASIC UNDERFILLING PROS : The solution is used for assembly verification procedure CONS : Some space around the part is required on one side of the part 2.2. ALTERNATIVE SOLUTION (2): UNDERFILLING THROUGH HOLES IN BOARD PROS : This solution eases the process of underfilling CONS : This solution limits the routing access on both sides of the boards holes 2.3. ALTERNATIVE SOLUTION (3) : UNDERFILLING LIMITED BY A H SPACER PROS : Similar to the recommended solution, but this solution lower the risks of underfill overflow «H» spacer CONS : This solution limits the access and visibility for visual inspection just after assembly Note 1 : the H spacer has to be mounted onto the board before module assembly Note 2 : The thickness of the {spacer + bond} has to be strictly lower than the gap between board and module body. Note : only the recommended solution (1) will be described in the following chapters. Doc. N :3641-0841-2 Page 4/11
3. MODUS OPERANDI FOR UNDERFILLING (SOLUTION 1) In this document, it is important to note that before any underfilling, the module is mounted on the board according to the assembly recommendations for manual reflow (3300-1300) or for automatic reflow (3300-1301) available on the website. The modus operandi explains how to perform a constant width and well-centered underfill between the module leads, thanks to a discontinuous process the underfill is the sum of several dots of adhesive dispensed at regular pitch. 3.1. TOOLINGS Time/Pressure-controlled dispenser with adjustable timer Low volume syringe : (10cc or 30cc for example) 38mm-long flexible dispense tip Precision balance (+/- 0.01g) 3.2. ADHESIVES 3.2.1. CHOICE OF ADHESIVE The glue used for mechanical reinforcement is the bi-component thermal epoxy adhesive : 3M 2216 B/A Gray (see RD2). It has been qualified with a wide range of 3D PLUS products (see AD3). Eccobond 285 with catalyst 11 seems to be a good alternative, but has only been tested on one 3D PLUS product. This combination of Eccobond 285 / Catalyst 11 is compatible for space applications in term of outgassing, and is well adapted for thermo-mechanical behaviors (CTE, temperature range ) and ease of process (pot life, viscosity ) (see RD1). It is important to note that before any underfilling, the module is assembled on the board according to the assembly recommendations for manual reflow (3300-1300) or automatic reflow (3300-1301) available on the website. 3.2.1. PREPARATION OF MIXTURE - Respect the recommendation of mix ratio from the manufacturer. - Outgas the mixture with a vacuum chamber. - Let the adhesive mixture stand for 30 mn. - Put the mix into a syringe, in order to use the dispenser avoid bubbles as much as possible when putting the adhesive mix into the syringe. - Let the adhesive mixture stand in the syringe for 15 mn at least before any use. Doc. N :3641-0841-2 Page 5/11
3.3. PREPARATION OF TOOLINGS AND MODUS OPERANDI 3.3.1. PREPARATION OF THE DISPENSE TIP The purpose of this preparation is to ease the repeatability of underfilling, in term of positioning and quantity. Original tip In order to keep the end of the tip centered between the lead rows: - Put a Kapton tape bent flat on itself at the end of the tip. - Symmetrically cut the tape a little shorter than the gap between both rows (for example on : 3DSS24G08VS3626 RTIMS-FLASH : gap = 24.5mm cut ~ 23-24mm). Tip with Kapton Tape This way, the end of the tip will remain centered between the lead rows. In order to create a constant width underfill (also to avoid any extra adhesive on leads). - Add many marks at regular pitch (1mm for example) with indelible pen. Tip with marks Doc. N :3641-0841-2 Page 6/11
In order to ease the flow on a large width (case for wide gap between lead rows over 15mm : (this is not mandatory) - Make 2 small chamfers at the end of the tip to allow that the adhesive spreads more widely. End of the tip with 2 chamfers In order to ease the insertion of the tip between the board and the module : - Pinch the tip in order to squeeze it under the module. (Tips used at 3D PLUS have a 1.6mm diameter step required if the diameter is greater than 1.2mm). <1.2mm Flattened tip Doc. N :3641-0841-2 Page 7/11
3.3.2. ESTIMATION OF QUANTITY OF GLUE TO DISPENSE 3.3.2.1. Inputs to define: - The regular dispensing pitch: p (1mm is recommended). - The gap between module and board : e = 1.35mm (stand-off is included). - The expected area covered by the underfill : s (400mm2 for 3DSS24G08VS3626 RTIMS- FLASH). Note : for the area, a margin between underfill and lead has to be considered to avoid any contact between leads and underfill A good practice is to define the margin at >2mm for a <7.5mm gap, >3mm for a 7.5-14.5mm gap, >4mm for a >14.5 gap. - The Density : d (2.27 for Eccobond 285 + Catalyst 11). - The Estimated length of the underfill line : L ~ module length in row direction (28mm for 3DSS24G08VS3626). 3.3.2.2. Outputs to define: dispense parameters - The number of pulses to do for each step/dot of underfill : n. - The Pressure on the dispenser : Pr. - The Time/Tempo : Tp. - The rest time of the mixture before dispense: Rt. 3.3.2.3. How to evaluate the dispense parameters: - Calculate the volume to dispense : V = s. e. = 540mm3 for 3DSS24G08VS3626 Or the mass to dispense M = s. e. d. = 1.22 g for 3DSS24G08VS3626 - Define the dispense parameters Pressure and Timer/tempo by iterations: o For a given set of parameters : Pressure Pr: pressure set on the dispenser Time Tp: tempo set with the timer button o Count how many pulses Np are needed to reach the mass M thanks to a precision balance If Np is a multiple of L / p : the set of parameters is correct If Np is too far of a multiple of L / p : iterate on a new set of parameters adjusted to approach a multiple. (Note : the multiple should be 1, 2 or 3 in order to let the underfilling process more comfortable and less risky for the operator) - And this multiple is n, the number of pulses for each step/dot of the underfill - Note the rest time of the mixture Rt, in order to perform the underfilling in a range of 1H around this rest time. (beyond this range, the viscosity changes and it may affect the result on the underfill) Doc. N :3641-0841-2 Page 8/11
3.4. UNDERFILLING OF THE MODULE ON THE BOARD After the rest time Rt of the mixture : - Set the dispense with Time Tp and Pressure Pr - Verify that the tip is filled with adhesive all along and that its end is clean - (1) Insert the tip between the board and the module, until its end appears on the opposite edge. Then (2) note the mark position on the tip, right to the insertion edge. 1 2 1 2 1 - (3) Before underfilling: o Go 1 Step back for narrow underfill (gap between lead rows <= 7.5mm) o Go 2 Steps back for medium underfill (gap between lead rows : between 7.5mm and 14.5mm) o Go 3 or more Steps back for large underfill (gap between lead rows : > 14.5mm) (Note that this step back depends on the pitch of marks too) 3 Doc. N :3641-0841-2 Page 9/11
- (4) Do many pulses, until the adhesive appears on the edge of the module. 4 3 4 3 4 - By iterations, until the end of the tip gets out: 3 o (5) : Go 1 step back o o (6) : do n pulses Repeat steps (5) and (6) until the end of the tip gets out 6 1 5 1 - (7) End of the underfilling Doc. N :3641-0841-2 Page 10/11
3.5. JUST A FEW INFORMATION ABOUT SOLUTIONS (2) AND (3) For solution (2) : - The number of holes through the board must be defined by the customer - The principle for defining the number of pulses or the quantity of adhesive can be similar to the principle presented above. For solution (3): - The principle is the same, but the Kapton tape is not necessary, and the quantity will be injected in 2 halves, from both edges. Doc. N :3641-0841-2 Page 11/11